1-s2.0-S0957582015001792-main

7/25/2019 1-s2.0-S0957582015001792-main

1/10

Process Safetyand Environmental Protection 9 9 ( 2 0 1 6 ) 110

Contents lists available at ScienceDirect

Process Safety and Environmental Protection

journal homepage: www.elsevier .com/ locate /psep

Effective utilization of tracer gas in characterization

of underground mineventilation networks

Guang Xua,, Edmund C. Jongb, Kray D. Luxbacherb,c, Harold M. McNaird

a Western Australian School of Mines, Curtin University, Kalgoorlie 6430, WA, Australiab Virginia Center for Coal and Energy Research, Blacksburg, VA 24060, United Statesc Virginia Tech, Department of Mining and Minerals Engineering, School of Engineering, Blacksburg,

VA 24060, United Statesd

Virginia Tech, Department of Chemistry, Blacksburg, VA 24060, United States

a r t i c l e i n f o

Article history:

Received 23 June 2015

Received in revised form 15

September 2015

Accepted 1 October 2015

Available online 22 October 2015

Keywords:

Tracer gas

Underground mine ventilation

Gas chromatography

Evacuated gas sampler

CFD modeling

Experimental optimization

a b s t r a c t

Tracer gases are an effective method for assessing mine ventilation systems, especially

when other techniques are impractical. Based on previously completed laboratory and field

experiments, thispaper discussessomecommon andchallenging issuesencountered when

using tracer gases in underground mines. The discussion includes tracer release methods,

sampling and analysis techniques. Additionally, the use of CFD to optimize the design of

tracer gas experiments is also presented. Finally, guidelines and recommendations are pro-

vided on the use of tracer gases in the characterization of underground mine ventilation

networks. This work has informed the practical use of tracer gases in mines, and this body

ofknowledge is expected to contribute to more efficient and more common use of tracer

gasesby mineengineers, which willallowforbetter characterizationof mineventilationsys-

tem andimproved safety. Thefindings canalso be used when using thetracer gastechnique

in the evaluation of atmospheric environment and air quality investigation in buildings.

2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction

Ventilation is a fundamental to the engineering of under-

ground mines because it has considerable effects on health

and safety. Measurements of airflow underground are usu-allycarriedoutusingtraditional instrumentationsuchas vane

anemometers,hot-wireanemometers, pitottubes, and smoke

tubes.However, these methods arenotpracticalunder certain

circumstances in an underground mine. Examples include

the measurement of recirculation and leakage, flow in inac-

cessible zones, and flow with very low velocities. Sometimes

traditional instrumentation fails to provide accurate results.

For example, in a study that investigated jet fan effectiveness

in dead headings, tracer gas results were found to be more

accurate than theresultsofsmoke tubes (Timko andThimons,

1982). Therefore, tracer gas techniques are a valuable tool for

Corresponding author. Tel.: +61 8 90886113.E-mail addresses: [email protected], [email protected](G. Xu).

accurately measuring airflow in situations where traditional

methods cannot be employed and providing information to

characterize underground mine ventilation.

The tracer gas technique is a useful and versatile tool

for studying mine ventilation systems with a long history ofapplication. The Bureau of Mines (Thimons and Kissell, 1974)

conducted a series of tracer gas tests using sulfur hexafluo-

ride SF6 and proved the usefulness of tracer gas techniques in

measuringrecirculation, air leakage, airflowin large cross sec-

tion, lowflow velocity, andtransit air time. Grenier et al. (1992)

used tracer gases to analyze the spread of dust in a fluorspar

millingplant. Theresults indicatedthat tracer gases behave in

the same physical manner as respirable dust and can be used

to find patterns in dust movement within a ventilation sys-

tem. Tracer gas has been accepted bythe mining industry as a

viableventilation survey tool. More examples that used tracer

http://dx.doi.org/10.1016/j.psep.2015.10.0010957-5820/ 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
http://www.sciencedirect.com/science/journal/09575820http://www.elsevier.com/locate/psepmailto:[email protected]:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_7/dx.doi.org/10.1016/j.psep.2015.10.001http://localhost/var/www/apps/conversion/tmp/scratch_7/dx.doi.org/10.1016/j.psep.2015.10.001mailto:[email protected]:[email protected]://crossmark.crossref.org/dialog/?doi=10.1016/j.psep.2015.10.001&domain=pdfhttp://www.elsevier.com/locate/psephttp://www.sciencedirect.com/science/journal/09575820

7/25/2019 1-s2.0-S0957582015001792-main

2/10

2 Process Safetyand Environmental Protection 9 9 ( 2 0 1 6 ) 110

gas to investigate various ventilation problems can be found

in Timko and Thimons paper (Timko and Thimons, 1982).

Compared to the uses of tracer techniques in under-

ground mine, these techniques are wildly used in the

atmosphere studies and environmental monitoring applica-

tions (Vanderborght and Kretzschmar, 1984). Pekney et al.

(2012)useda perfluorocarbontracer thatwasinjected together

with CO2 to monitor the leakage of the carbon sequestrationtechnology. Connan et al. (2011) evaluated two computation

models for the prediction of atmospheric dispersion by com-

paring them to a SF6 tracer study. Belalcazar et al. (2009, 2010)

used tracer gas and CFD modeling to study the road traffic

emission factors (EFs), and showed that it is possible to esti-

mate the EFs from tracer studies.Jayaraman et al. (2006) used

tracer gas in an indoor space experiment for the purpose of

evaluating the accuracy of different CFD models. As most of

these studies can also be related to the problems in the mine

environment, it is evident that the tracer gas technique has

great potential in the research of mine safety and health.

An ongoing research project that involves the selection of

novel tracer gases for mine ventilation, the development of

a methodology to use tracer gases and computational fluid

dynamics (CFD) modeling to analyze, predict, and confirm the

underground ventilation status together with the location of

the damage, and finally validate the developed methodology

in thelaboratoryandinthefield are currentlybeingconducted

at Virginia Tech. Details of this work have been published in

severalforums (Jong et al., 2012; Pattersonet al., 2010; Xu et al.,

2013, 2015).

The focus of this paper is to provide some useful findings

for the use of tracer gas based on authors experience and

practice. As this research progressed it was evident that there

are few resources in the literature that provide the practical

aspects ofconductingtracer gasstudies inmines.Someessen-

tial aspects of the tracer gas technique are discussed as well

as new findings and recommendations, and studies in the lit-

erature are referenced as well. Using CFD modeling to design

tracer gas experiments is also presented in this paper. Some

modeling examples are provided to illustrate how CFD can

help to determine the optimized tracer release and sampling

locations, the release rate and duration, and eventually help

to achieve desired results. Tracer gas experiments are time

and resource consuming in underground mines, the findings

and recommendations provided in this paper can be used by

other researchers and industries for the design of tracer gas

experiments more efficiently with less trial and error.

2. Tracer gas techniques

2.1. Choices oftracers

Sulfur hexafluoride (SF6) is a widely accepted standard tracer

gas that has been used in mine ventilation studies. It was

first used in the building ventilation system sixty years ago

(Dick, 1950) and has been widely used for ventilation analy-

sis both in buildings and underground mines (Kennedy et al.,

1987). SF6 is non-toxic (Lester and Greenberg, 1950), and the

Occupational Safety and Health Administrations (OSHA) Per-

missible Exposure Limit (PEL) andtheAmerican Conference of

Governmental IndustrialHygienistss (ACGIH)ThresholdLimit

Value (TLV) for SF6 is 1000ppm (OSHA, 1993). The amount

of SF6 released to a mine is generally much less than either

the PEL or TLV limit. SF6 can be detected accurately using

gas chromatography (GC) in concentrations as low as parts

per trillion. It is also odorless, colorless, chemically and ther-

mally stable, and not found in the natural environment. It is

not measurably adsorbed on sandstone and coal. These are

all desirable properties as a tracer gas (Thimons and Kissell,

1974). There are a wide range of applications of SF6 in under-

ground mines,including the study of ventilation recirculation,

fan effectiveness, and flow rates estimation (Thimons et al.,

1974;Thimons andKissell, 1974), themeasurementof airleak-

ageand dust control effectiveness (Timkoand Thimons, 1982),

themeasurement of turbulentdiffusion (Arpa et al., 2008), the

study of methane control (Mucho et al., 2000), and the appli-

cation of SF6 to characterize underground mine ventilation

systems after emergency (Xu et al., 2013, 2015). Some alter-

native gases have also been used as mine ventilation tracers,

such as nitrous oxide and helium. However, because they are

not easily detected, large amounts need to be released thus

causing transportation problems and difficulty in achieving

stable flows (Kennedy et al., 1987).

It has long been realized that multiple tracer gases can

add flexibility to ventilation surveys in many ways. Multiple

gases notonly allowthe release of different tracers atdifferent

points without increasing the number of collected gas sam-

ples, but also provide more information because the source

of each tracer in one sample can be identified and air flows

in different zones can be investigated simultaneously with

multiple tracers. Once a tracer is released to the mine, it may

take days to weeks for the tracer background to be reduced

to a level that will not affect the next test. However, if mul-

tiple tracers are available, a different tracer can be used to

conduct another test right after the previous test. Although

the advantages are apparent, the use of the multiple tracer

technique is still not common in underground mines. One

key requirement for identifying other tracer gases is that the

gas should be measurable by the same method being used

for SF6, which is the most commonly used tracer gas. SF6 is

commonly analyzed by GC, so it would be better if the other

tracers were able to be analyzed by the same GC method.

Using thesame GCmethod, theadditional tracers shouldhave

similar sensitivity to SF6 as well as be able to be separated

from SF6. Kennedy et al. (1987) investigatedsixFreons that are

promisingcandidatetracer gases.They found that only Freon-

13B1 (CBrF3) and Freon-12 (CF2Cl2) were within two orders of

magnitude of the sensitivity of SF6 when analyzed using a

GC with an electron capture detector. The other four Freon

gases were several orders of magnitude less sensitive. CBrF3and CF2Cl2were tested in the field and it was concluded that

they perform well as mine ventilation tracer gases and are

comparable toSF6. Battermanetal. (2006)usedhexafluoroben-

zene (HFB) and octafluorotoluene (OFT) for indoor ventilation

tracers. However, those two tracers were not simultaneously

analyzed with SF6. Have used propane (C3H8) as a tracer gas

to evaluate the ventilation system of a full face tunnel bor-

ing machine, but pointed out that it cannot be used in coal

mines (Stokesand Stewart, 1985). Patterson, (2011) researched

the selection of novel tracer gases that can be used in mines

together with SF6. Freon 14(CF4), C3F8, and PMCH(C7F14) were

tested on different columns using various GC methods. CF4and C3F8 were found to have much less sensitivity than SF6on the tested columns, with similar retention times to SF6.

PMCH was reportedto beanappropriatetracer ifused together

with SF6. A GC protocol was developed that can be used to

analyze SF6 and PMCH concurrently on an HP-AL/S capil-

lary column. One drawback of the protocol is the long 18min

7/25/2019 1-s2.0-S0957582015001792-main

3/10

Process Safety and Environmental Protection 9 9 ( 2 0 1 6 ) 110 3

Fig. 1 Different flow meters.

analysis time,which process canpotentiallybe optimizedand

shortened.

In summary, as for the choice of tracers in underground

mines, SF6 is no doubt the best choice. CBrF3, CF2Cl2, andPMCH (C7F14) can be used together with SF6. CBrF3and CF2Cl2have both been successfully used in a mine. PMCH is a novel

underground mine tracer gas. The authors research group is

developing method to apply it as additional tracer that can be

used together with SF6.

2.2. Tracer gas release technique

Accurate and precise release of tracer gas is critical to con-

ducting a rigorous study; there must be high confidence in

the mass or rate of gas released to the system in order to

achieve meaningful analysis of results. In the application of

mine ventilation, the release methods also need to be reason-

ably portable and easy to operate. There are two commonly

used tracer gas release techniques: pulse-injection, which is

based on the injection of a short duration of tracer gas, and

constant-injection, which is based on the continuous injec-

tion of tracer gas. Pulse-injection is easier and quicker to

deploy, butrequires a large numberof measurementsto define

the resultant curve. Continuous injection is advantageous in

allowing evaluation of airflow stability (Stokes and Kennedy,

1987).

Tracer gases can be released in a controlled manner using

various methods. For pulse injection, a known mass of tracer

gas in a balloon or syringe can be used and injected to the

mine. Or directly release tracer gas from a pressurized con-

tainer and determine the weight loss of the container after

release. For example,in a studyconductedby Widiatmojo etal.

(2015) using tracer gas to investigate theairdispersion charac-

teristic in underground mine ventilation, tracer gas (SF6) was

releasedin balloons, andthemass of tracer gaswas measured

by the weight difference after and before the balloons were

filled. An example of using a syringefor pulse injection can be

found (Stokes and Kennedy, 1987). However,it is hard for these

methods to release in a controlled rate. Flow meters and per-

meation tubes serve as more accurate controlled rate tracer

gas release methods, and they can be used for both pulse and

constant injection.

Soap bubble flow meter (Fig. 1A), rotameter (Fig. 1B), and

electrical flow meter (Fig. 1C) are commonly used flow meters

that can be used to measure and to control the flow rate.

The soap bubble flow meter measures the flow rate of tracer

release, but accessory instruments are needed to control the

flow rate of gas. For gases contained in a compressed gas

bottle, a two stage regulator is often attached to the bottle.

Capillary tubing may be used to connect the regulator and

the soap bubble flow meter adding more resistance to achievebetter gas flow control. The two stage regulator controls and

maintains the delivery pressure to the flow meter as long

as gas tank pressure is greater than the delivery pressure.

Selecting different capillary tubing and adjusting the delivery

pressure affects the flow rate. The described setup provides

flow stability unaffected by atmospheric pressure (Grenier

et al., 1992).

Variable area flow meters, also known as rotameters,

provide a different set of controls. The design is based on the

variable area principle and indicates as well as controls the

rate of flow. Positioned vertically, the gas flow lifts the float in

the flow tube. For a constant flow rate, the float will be at a

stationary position, which corresponds to a point on themea-surement scale that indicates the flow rate. The flow rate can

bechangedby adjustingthedelivery pressure ormanipulating

the valve on the flow meter. Although a specific meter can be

purchased for a certain gas, the air flow meters are the most

commonly found in the market. The air flow meter can be

used for other gases with a correction factor provided by the

manufacture.

An electrical mass flowcontroller canbe used to accurately

control the release quantityof a tracer. Using a mass flow con-

troller, the flow rate can be adjusted using the digital control

panel. It requires minimal adjustment to achieve the desired

flow rate compared to the other types of flow meters.

The permeation device is a commonly used tracer releasesource for volatile compounds. The basic concept is to seal

a certain amount of tracer in an impermeable tube with per-

meable material at one end (as shown in Fig. 2). The emission

ratecanbe determined byweighingthe prepared tube at inter-

vals of several days until equilibrium is reached. Theemission

rate is relativelystableif the temperature is constant (Mailahn

et al., 1989). Dietz and Cote (1982) described a perfluorocar-

bon (PFT) source in which a known mass of PFT is injected

Fig. 2 Schematic of a permeation device.

7/25/2019 1-s2.0-S0957582015001792-main

4/10


into a fluoroelastomer plug and crimped in a metal shell. PFT

will diffuse from the end of the plug at a known rate that is

inversely proportional to the square root of time for the emis-

sion of the first 5060% of the original amount of PFT.Johnson

et al. (2007) described the preparation of a permeation device

they used for continuous and constant release of SF6. It was

constructed from brass rod, Teflon, a frit, and a swaglok nut.

Although their design was not used in a mine tracer test, the

method can be used for the release of SF6 and other tracer

gases in mines. Batterman et al. (2006) described an updated

methodfora constant injectiontechniquewith miniaturePFT

sources. The method was shown to be reliable for measuring

indoor air exchange rates. The source was a diffusion con-

trolled release of saturated vapor in the headspace of a PFT

liquidcontainer.A diffusiontube was insertedinto theseptum

that sealed a glass vial partially filled with PFT. The emis-

sion rate was evaluated using a Fickian diffusion model and

tested by experimentation. The described sources mentioned

above can be easily modified for a wide range of applications.

For possibly application of the permeation device to mine

ventilationcharacterization,Jong et al., (2015b)designed a per-

meation plug release vessel (PPRV) for the release of PMCH.

The PPRV was made of an aluminum cylinder and plugged

with and oversized VMQsilicone plug. The release rate was

found to be constant, which was governed by a function of

temperature and plug thickness. The predicted release rate

was proved to be accurate though normal variations of baro-

metric pressure and insignificantly impacted by the PMCH fill

level. This design was later tested in both a laboratory con-

trolled turbulent environment (Jong and Luxbacher, 2015) and

an underground longwall mine (Jong et al., 2015a), in which it

was proved to be applicable to underground ventilation appli-

cations including long-termleakage and gob flow monitoring.

Thepermeationdevice hasthe advantagesof rapidsetup, ease

ofuse, anddeployment inpreviously restrictedareas, andthus

has great application potentials.

2.3. Tracer gas sampling methods

There are two categories of gas sampling methods: collecting

samples for laboratory analysis and collecting samples for

immediate analysis (Peach and Carr, 1986). The first category

is commonly used in underground tracer gas studies, which

includes gas sampling bags, hypodermic syringes, and evacu-

ated containers.

Gas sampling bags have been successfully used for a

number of years to collect gas samples and make gas stan-

dards. Gas sampling bags are available commercially and

come in a variety of sizes and shapes, and are made from

a number of materials, such as PVC (polyvinyl chloride) or

Tedlar (polyvinyl fluoride). One needs to pay special attention

to the material of the sampling bag because the sampled

gas may be reactive, adsorptive, absorptive, or diffusive with

the bag materials (Peach and Carr, 1986). There are many

applications of using gas sampling bags for collecting gas and

vapor samples (Schuette, 1967). It usually requires a pump

to inject gas into the bag, so it is not a very convenient tool

for dynamic gas sampling. Kennedy et al. (1987) tested the

tightness of TEDLAR gas sampling bags and no detectable

degradation of gas samples was found in a 24h period. The

storage time is typicallyno more than 24h, so analysis should

be conducted as soon as possible,unless storage experiments

indicate a longer storage time.

Hypodermic syringes satisfy most underground sampling

requirements. Syringes are inexpensive, easy to carry, and

hard to break.However, their short storage time restricts their

use. For example, in one study, gases such as carbon diox-

ide are lost rapidly due to permeation (Freedman et al., 1975).

It is standard practice to fill and evacuate the syringe twice

before drawing so that the gas left in the syringe does not

contaminate the gas sample. The syringe can be sealed witha vinyl cap (Grenier et al., 1992). The tightness of hypoder-

mic syringes have been tested by Kennedy et al. (1987). In the

study, six syringes were filled with certain concentrations of

SF6, CBrF3, and CF2Cl2, and left for 24h. The concentrations

did show a significant reduction, with a loss of 0.51% for SF6,

2.53%forCBrF3, and 6% for CF2Cl2. These syringes wereemp-

tied and refilled with the same standard gas, left for another

24h, and tested again for the tracer gas content. The loss of

tracer was reduced to half of the original percentage after the

first filling. This indicated that the loss was not only due to

permeation through the walls or leaks in the seals but also

due to adsorption of the tracer onto the syringes. Like samp-

ling bags, samples generally should be analyzed on the same

day they are taken.

Vacutainers have long been used for sampling mine gases.

A Vacutainer is an evacuated glass or plastic tube-shaped ves-

sel and is capped with a self-sealing rubber septum. Such

containers are commonly used to take blood samples. The

advantages of Vacutainers are that they are small, light-

weight, economical, convenient, and simple to use (Peach

and Carr, 1986). The Bureau of Mines officially adopted the

Vacutainers fortaking mine gassamplesbecause they arecon-

venient and can obtain consistent results (Freedman et al.,

1975). Freedman et al. (1975) tested the 10ml Vacutainers for

usein mines. In their study, a device was described which can

evacuate up to 56 Vacutainers to a few millimeters of pres-

sure. It was found that for stored gas samples, CO2 shows

substantial loss in concentration over the 41 days due to per-

meation. TheCO concentration levelgradually increased up to

50ppm over time in factory supplied or completely evacuated

Vacutainers. This phenomenon is unexplained. They recom-

mended that the storage of evacuated Vacutainers should not

exceed 1 to 2 months if low levels of CO can cause interfer-

ence. If precise CO2 level is of interest in collected samples,

the analysis should be done within 1 week after samples are

taken.

The 10ml Vacutainer was also chosen as the sampling

method for our experiments. These evacuated containers are

not completely evacuated at the time purchased and a small

amountof gas (air) is still present. However, thecontainersare

evacuated to a fixed anddesignated pressure (Bush, 2009). The

factory evacuation is designed to draw 10ml of blood. Prelim-

inary tests indicate they draw 9.5 ml of water. The left three

Vacutainers in Fig. 3 illustrate this result. It was noted that

the capability of drawing water to be reduce slowly with time.

The actual Vacutainer capacity measured by water displace-

ment is 12.9ml. Therefore, the Vacutainers were re-evacuated

in the laboratory to improve the sampling accuracy. The right

threeVacutainers in Fig. 3 show theresultof laboratory evacu-

ation, whichimprovedthecapabilityof Vacutainersof drawing

12.5 ml of water. This means 0.4 ml of air is left in each Vacu-

tainer, which is 3.1% of the actual capacity. This will make

the measured gas concentration 3.1% less than the actual gas

sample concentration, but it is acceptable for most tracer gas

analysis purposes.

7/25/2019 1-s2.0-S0957582015001792-main

5/10


Fig. 3 Vacutainers factory evacuation and laboratory

evacuation.

Vacuum Pump

Needle

Vacuum Gauge

Ball ValveVacuum Tube

PVC Pipe

Fig. 4 Evacuation apparatus schematic.

The evacuation system used in the laboratory is shown in

Fig. 4. Basically it is a vacuum pump connected to several nee-

dles so that several Vacutainers can be evacuated at a time.

Vacutainers are inerted onto each of the needles through theseptum for about 30s, and need to be pulled off very slowly,

which allows enough time for the septum to re-seal for a

high quality evacuation. Each needle connected to the sys-

temneeds to be replaced after about 5 Vacutainerevacuations

since it will dull and hard to penetrate the Vacutainer septa.

The re-evacuated Vacutainer shelf life was test by water

displacement over a period of 8 weeks. Several Vacutainers

were re-evacuated on day 0 using the above mentioned appa-

ratus. These Vacutainers were used later on different days to

test the shelf life. Its capacity change with time is shown in

Fig. 5. Each dot on this figure is an averaged value of three

Vacutainersthat wasre-evacuatedonday0.Thefigureshowed

that the capacity does not have obvious change in the first 5weeks, but dropped more than two percent from week 6 to 8.

This result suggests that, for a better sampling precision, the

Vacutainers should not be prepared (re-evacuated) more than

one month before taking samples.

95%

97%

99%

101%

103%

105%

0 7 14 21 28 35 42 49 56 63

Percentage

Time (Days)

Fig. 5 Re-evacuated Vacutainer capacity changewith time.

93.8%

96.9%96.6%

97.9%

96.8%

96.5%96.9% 97.1%

90.0%

91.0%

92.0%

93.0%

94.0%

95.0%

96.0%

97.0%

98.0%

99.0%

100.0%

0 1 2 3 4 5 6 7 8

PercentageoftheA

ctualSampleConcentration

Sample Time (s)

Concentration in Vacutainer

Expected Concentration

Fig. 6 Vacutainer sample time.

Although theVacutainer storage time is promising, an offi-

cial time has not been reported in the literature. An attempt

to test its storage time for SF6was conducted during a month

period and no obvious SF6concentration changes were found.However, the method used introduced significant amounts of

systematic errors that are difficult to control and quantify.

The Vacutainer sample time was also studied to determine

how fast the gas sample will fill the Vacutainers. The sample

time is defined as beginningwhen the rubber septum is punc-

tured bya needleendingwhenthe needle is removed fromthe

septum. This is important, especially for dynamic sampling,

because it determines the minimum sampling interval. As in

the previous test, for each test, three samples were taken and

the average concentration was used to compare with other

results. The test results are shown in Fig. 6. Given adequate

time for the Vacutainer to draw sample, the expected concen-

tration in the Vacutainer should be about 96.9% of the actualconcentration, as mentioned before. The 96.9% concentration

level is marked in the dashed line in Fig. 6. As can be seen, the

concentration in the Vacutainer reached the expected level

with sampling times longer than 2 s. Variations exist which

are likely due to error during Vacutainer evacuation and GC

manualinjection.Therefore,for the10 mlVacutainer, the least

sample time is recommended to be 2 s. Dynamic SF6samples

were successfully taken every 5 s during 8min period in our

underground tracer tests. In each 5 s, Vacutainer takes sample

for 3s and 2s are needed for changing to a new Vacutainer.

This is the fast minimum sampling interval we can achieve if

only one people takes samples manually.

2.4. Analysis oftracer gas

There are two main techniques for determining gas concen-

trations: infrared spectroscopy (IR) and gas chromatography

(GC). This paper focuses on the GCmethod which is one of the

most widely used analytical techniques for gas samples due

to its selectivity and sensitivity. A wide range of compounds

can be analyzed through the proper selection of columns and

detectors.

Gas chromatographs need to be calibrated over its

operating range to quantitatively measure a certain gas

concentration. Thus, tracer standards are needed for the cal-

ibration. The standards can be prepared in the laboratory or

purchasedas a certified mixture.To make thestandards in the

laboratory, a gas-tight syringe can be used to inject a small,

known quantity of tracer into a known volume of air or ultra-

pure nitrogen in a sealed container. An example of one of

7/25/2019 1-s2.0-S0957582015001792-main

6/10


Fig. 7 Glass bulb for tracer gas standard preparation.

these containers is shown in Fig.7. With both valves open, thiscontainer can be flushed using ultra-pure nitrogen and filled

with it after both valves closed. To prepare standards for trace

concentrations, a serial dilution technique can be used. Serial

dilutions are completed by withdrawing a measured quantity

of the mixture prepared in the first step and injecting it into

another container. The procedure can be repeated until the

desired concentration is achieved. This method is time con-

suming and errors can be introduced at each stage of dilution.

Although this method can be used to produceuseable calibra-

tion curves, thepurchased certified gas standards can provide

better accuracy and repeatability (Leonard et al., 1984).

Split injection is a widely used injection technique for GC

analysis. It automatically reduces the sample size to preventthecolumnfrom overloadingbyallowingonly a fraction of the

sample enters the column. This fraction is defined as a split

ratio.A split ratio of 20:1 indicates that onepart of the sample

entersthe columnand20 partsexit theGC systemthroughthe

split vent (McNair and Miller, 1998). Theoretically, this means

1/21 of the total sample is analyzed. Again, theoretically, this

makes it possible to compare two results with the same GC

method but with different split ratio. For example, a sample

analyzed with a split ratio of 50:1 could be corrected to 20:1 by

multiplying a factor of 51/21. However, this split ratio calcula-

tion was found to be inaccurate due to the mechanical nature

of the splitting control. Fig. 8 shows the measured results of

the sameSF6sample with different split ratiobutallcorrectedtothe split ratio of20:1. Ascan beseen, thiscorrection method

fails to provide accurate results. Although the accuracy of the

split on different GC machines may vary, it is recommended

that a single split ratio is chosen and that corrections to the

data based on split ratio are avoided.

Since this correction methodis notalways reliable, thesplit

ratio should be held constant for all calibration standardsand

gas samples. A calibration curve is used to quantify the con-

centration in collected samples. To create a calibration curve,

theanalysis concentration range needs to be determined first,

and then start from a reasonable injection amount and split

ratio for the calibration. It is better to inject the highest con-

centration standard first to make sure the column will notbe overloaded. If it is overloaded, increase the split ratio or

reduce the amount of injected sample. Afterward, inject the

lowest concentration standard to ensure that the concentra-

tion is above the detection limit; otherwise, the split ratio

needs to be decreased or a larger amount of sample needs

to be injected. After the injection volume and split ratio are

determined, theseparameters need tobekept thesameforthe

rest of thestandards andtest samples that use this calibration

curve.

3. The use ofCFD model for tracer gasdesign

Tracer gas experiments are time consuming, especially if they

are conducted on a trial and error basis. Careful planning will

ensure meaningful data collected efficiently. CFD modeling is

a powerful tool that can be used for experimental design. This

tool cansubstantially reducetheeffort that is expended in the

field. In CFD modeling,tracers canbemodeled in severalways.

Thetwomost common methods are the two species transport

model or definition of a user defined scalar to model tracers.

Parameters can be easily changed in the CFD model, such asthe location and the release amount. This provides detailed

information to achieve the desired results, which can help

with the tracer gas experiment design. This section focuses

on the discussion of the determination of some parameters

using CFD modeling in the characterization of underground

mine ventilation networks.

One important parameter is that the expected concentra-

tion in collected samples needs to be within the detection

range of the GC. It is known that the concentration decreases

as the distance between the sampling and release location

increases. If these locations have been determined, the tracer

gas release rate and duration can be used to determine the

concentration level.The release rate is the most sensitive parameter that influ-

ences the concentration. Fig. 9 shows the SF6 concentration

profile results provided by one of the studies conducted by

theauthors. Thescenario modeled was a belt entry connected

with a crosscut entryshownin Fig. 12. Thereleaselocationis in

thecrosscut and is denoted as Release Point1.The sampling

location is 190 m away from the crosscut and is downstream

from thevelocityinlet in thebelt entry. SF6was releasedunder

different rates for 1 min in the CFD model. As can be seen in

Fig. 9, the basic shape of the concentration profile will not

change by changing the release rate, but the profile is taller

under larger release rate. The result can be used to find an

optimized release rate that can produce a tracer concentra-tion profile, which can be reasonably analyzed by GC or other

instruments.

154%

135%

100%

77%

63%53%

0%

20%

40%

60%

80%

100%

120%

140%

160%

SR 10:1 SR 15:1 SR 20:1 SR 30:1 SR 50:1 SR 100:1

PercentageoftheActualConcentration

Split Ratio

Measured Concentration Corrected to 20:1Split Ratio

Actual Concentration

Fig. 8 Same samplemeasured with different split ratio comparison.

7/25/2019 1-s2.0-S0957582015001792-main

7/10


0

50

100

150

200

250

300

0 50 100 150 200

SF6con

centration(ppm)

Time after tracer release

9 L/min

30 L/min

200 L/min

Fig. 9 Tracer concentration profile under different release

rate.

0

2

4

6

8

10

12

14

50 70 90 110 130 150 170 190 210 230 250

SF6Concentr

ation(ppm)

Time after release (s)

Release 10 s

Release 30 s

Release 1 min

Release 2 min

Fig. 10 SF6 concentration profile under different release

duration (Xu et al., 2015).

The release duration also influences theconcentration lev-

els in collected samples but is not as sensitive as the release

rate. Under certain circumstances, increasing the release

duration will not necessarily increase the concentration level.

In the same CFD model mentioned above, different tracer

release durations were modeled under the same release rate

(9L/min). As can be seen in Fig. 10, the 30s release increased

the concentration level when compared to the 10 s release.

However, increasing the release duration further, to 1 min

and 2min, did not increase the concentration level. That is

because the concentration reached its maximum under the

fixed release rate and the maximum concentration leveled

out. Although this is not always the case, the CFD results

can provide the data that can be used to pre-determine the

concentration level under certain release duration.

Another important parameter that CFD can help to deter-

mine is the shape of the expected concentration profile. The

farther the sampling location is from the release location, the

broader and flatter the profile becomes. If the release and

sampling locations are determined, longer release durations

can produce broader profiles as well. Fig. 10 shows that the

concentration profile becomes broader as the release dura-

tion increasedfrom 10s to2 min. Generally, a broader profileis

better, especially when the sampling method is not continu-

ous, such as syringeor Vacutainer sampling. A broader profile

allows enough samples to be taken to accurately resolve theprofile. If the profile is too narrow, for example it is only 10s,

but the sampling interval is 5s, only 3 samples maximum

can be taken within the 10s time. This makes it very hard

to accurately depict the concentration profile.

However, in some cases, too broad of a profile will not ben-

efit the tracer experiment not only because it takes longer for

sampling, but also because it weakens the profile characteri-

zation. As indicated from a CFD study presented in (Xu et al.,

2012), under the normal ventilation scenario, there are four

flow paths. If the tracer is released for 1 min, the profile is the

solid line displayed in Fig. 11, which has four peaks and each

of them represent one flowpath. However, if the release dura-

tion changes to 30min, the profile is the dashed line shown

in Fig. 11. It not only requires 30min more sampling time

to capture the entire profile, but the characterization is also

not as obvious as before. It is hard to tell that there are four

flow paths from this kind of tracer profile. In summary, too

short of a release duration may produce tracer profiles that

are hard to capture with certain sampling intervals; too long

of a release duration may weaken the tracer profiles charac-

terization. CFD modeling can be used to check if the release

duration is good enough for certain sampling intervals and to

characterize certain ventilation scenarios.

Changing the location of tracer release point will not only

change the profile peak arrival time but also change the pro-

file shape when the flow feature is complex. For example, in

the 2D CFD model mentioned before and shown in Fig. 12.

Two tracer release points were examined in the model: point

1 is 38m from the door, and point 2 is 2.5m from the door. As

can be seen in Fig. 13, due to the recirculating flow featured

in the zone at point 2, the downstream tracer profile not only

appears later and broader compared to release at point 1, but

the basic shape is also changed. The profile is much broader

and has a longer tail. In this case, the basic tracerprofile shape

is different when released in the same entry but at differ-

ent locations. The CFDmodel can identify these complex flow

zones,which provides information that helps to decide where

to release thetracer andgeta profile that is suitable for certain

purposes.

0

200

400

600

800

1000

1200

1400

1600

1800

0

50

100

150

200

250

0 10 20 30 40 50 60 70 80 90 100 110 120

SF6

Concentration(ppb)forRelease30

min

SF6C

oncentration(ppb)forRelease1min

Flow Time (min)

Release 1 min

Release 30 min

Fig. 11 Tracer profile under different release duration.

7/25/2019 1-s2.0-S0957582015001792-main

8/10


Fig. 12 Tracer release at different locations (Xu et al., 2015).

0

2000

4000

6000

8000

10000

12000

14000

60 120 180 240 300 360 420

SF6Concentration(ppb)

Time (s)

Release 38 m from the door

Release 2.5 m from the door

Fig. 13 Tracer profile under different release location.

The examples shown above demonstrate that CFDis a very

powerful tool as an aid for tracer gas experimental design.

The modeling results canprovide informationfordetermining

some of the key experimental parameters mentioned above.

Using these optimized parameters to perform the actual on-

site experiment can help to avoid the trial and error process

and obtain the desired results efficiently. However, the CFD

model requires much more computational power compared

to other modeling methods, such as network modeling. Using

a high performance computer for the modeling is a solution

to reduce computational time. In addition, starting from a 2D

model can also save time tremendously. Although 2D flow

is totally different from 3D flow, and the 2D model cannot

provide as accurate a result as 3D, a 2D model can still provide

enough information to determine most of the influencingfac-

tors. After those factors have been determined, using a 3D

model to validate the 2D model can increase the confidence

andtheaccuracyof theresults.Finally, in cases where theflow

regime is notcomplex, andthesampling andcollection points

relative to the entry cross-section are not considered critical

to results, network modeling can even be used to understand

expected results. The advantage of network modeling is much

lower computational requirements along with less user skill.

4. Conclusions and discussion

The tracer gastechnique is a preciseand reliablemethodology

for characterizing underground mine ventilation networks.

However, it is time consuming and resource heavy, especially

when it is based on a trial and error basis. The guidelines

and recommendations provided in this paper are based on

the experiences and practices of the tracer gas research con-ducted over the last few years. These recommendations can

help other researchers and industries to reduce the effort in

conducting tracer gas experiments.

Several commontopicsin thetracer gastechniquesare dis-

cussed.As forthe choiceof tracers,CBrF3andCF2Cl2are found

to have been successfully used together with SF6 using the

same GC method. PMCH was also found to be a good tracer

for use with SF6. However, PMCH has not yet been proven in

underground mines.

As for the tracer release techniques, commonly used

flow meters are discussed and compared. Additionally, tracer

release by permeation tube is introduced although has yet to

be used in mines. It may prove to be a promising techniquefor tracer release in mines. These release methods control

the flow rate differently and can be used to serve specific

purposes.

There are several tracer gas sampling methods that have

been used in mines. Both gas sampling bags and hypodermic

syringes have a short storage time. Samples taken using these

methods should be analyzed within 24h. The material of the

sampling bags could affect its sampling capability for certain

gas samples. It was reported in the literature that the sample

loss inthehypodermicsyringesaredueto permeation through

the walls, leaks in the seals, and adsorption of the tracer onto

the syringes. Vacutainers have the longest storage time com-

pared to other sampling methods. However, substantial lossof CO2in stored samples was reported due to permeation. CO

concentration levels were foundto be graduallyincreasedover

time in evacuated Vacutainers in one instance. Storage time

and sample time for SF6 have not been officially reported in

the literature. An attempt to test its storage time for SF6 was

conducted during a month period and no obvious SF6 con-

centration changes were found. However, the method used

introduced significant amounts of systematic errors that are

difficult to control and quantify. The minimum sample time

of the Vacutainers is found to be 2s, however, the minimum

practical sampling interval for one person sampling manually

is 5s.

GC is a commonly used method for tracer analysis. The

preparation of gas standards in the laboratory was described,

which can be used to calibrate a GC when a certified gas mix-

ture isnotavailable. Thecorrection methodfrom oneinjection

split ratio to another is discussed and found to be inaccurate,

thus it shouldbe avoidedunlessexperimentsshowit isaccept-

ably accurate. A procedure was proposed for generation of a

calibration curve. These guidelines can be used to save time

and guarantee an accurate GC result.

CFD modeling can help with the tracer experiment design.

Examples are presented which illustrate how CFD modeling

helps to determine the important factors in a tracer exper-

iment, such as the release rate and duration, the expected

concentration profile, and the release location. After these

parameters areoptimized in theCFD model,the trial anderror

process canbe reduced andthe desired results canbe obtained

more efficiently. However, for a large scale 3D CFD model, the

computational time is considerable. Many of the parameters

7/25/2019 1-s2.0-S0957582015001792-main

9/10


can bestudied usinga 2Dmodel tosave time. Butthe2D model

cannot provide as accurate result as the 3D model because 2D

flow is totally different from 3D flow.

The aim of this work is to provide a practical guide for

people with technical expertise who want to apply tracer gas

techniques to mine ventilation. Although many impressive

applications of tracer gas techniques are available in the lit-

erature, few, if any sources are available that provide a guide

use of the method. This method can allow for characteriza-

tion of the ventilation system from an assessment of leakage

to the efficiency of designs for gas and dust dilution, and

provide engineers with additional tools for improving health

and safety. As tracer gas techniques are wildly used in the

atmosphere studies and environmental monitoring applica-

tions, this work can also be useful in these areas.

Acknowledgements

This publication was developed under Contract No. 200-2009-

31933, awarded by the National Institute for Occupational

Safety and Health (NIOSH). The findings and conclusions inthis report are those of the authors and do not reflect the offi-

cial policies of theDepartment of HealthandHuman Services,

nor does the mention of trade names, commercial practices,

or organizations imply endorsement by the U.S. Government.

References

Arpa, G., Widiatmojo, A., Widodo, N.P., Sasaki, K., 2008. Tracer gasmeasurement and simulation of turbulent diffusion in mineventilation airways. J. Coal Sci. Eng. (China) 14, 523529.

Batterman, S., Jia, C., Hatzivasilis, G., Godwin, C., 2006.Simultaneous measurement of ventilation using tracer gastechniques and VOC concentrations in homes, garages andvehicles. J. Environ. Monitor. 8, 249256.

Belalcazar, L.C., Clappier, A., Blond, N., Flassak, T., Eichhorn, J.,2010. An evaluation of the estimation of road traffic emissionfactors from tracer studies. Atmos. Environ. 44, 38143822.

Belalcazar, L.C., Fuhrer, O., Ho, M.D., Zarate, E., Clappier, A., 2009.Estimation of road traffic emission factors from a long termtracer study. Atmos. Environ. 43, 58305837.

Bush, V., 2009. The Evolution of Evacuated Blood Collection Tubes.Connan, O., Leroy, C., Derkx, F., Maro, D., Hbert, D., Roupsard, P.,

Rozet, M., 2011. Atmospheric dispersion of an elevated releasein a rural environment: comparison between field SF6 tracermeasurements and computations of Briggs and ADMSmodels. Atmos. Environ. 45, 71747183.

Dick, J.B., 1950. Measurement of Ventilation Using Tracer Gas.Heat. Piping Air Cond. 22 (5), 131137.

Dietz, R.N., Cote, E.A., 1982. Air infiltration measurements in ahome using a convenient perfluorocarbon tracer technique.Environ. Int. 8, 419433.

Freedman, R.W., Humphrey, W.G., Craft, R.L., 1975. Use ofVacutainers for Collection of Mine Atmosphere Samples.United States Bureau of Mines, Pittsburgh, PA.

Grenier, M.G., Hardcastle, S.G., Kunchur, G., Butler, K., 1992. Theuse of tracer gases to determine dust dispersion patterns andventilation parameters in a mineral processing plant. Am.Ind. Hyg. Assoc. J. 53, 387394.

Jayaraman, B., Finlayson, E.U., Sohn, M.D., Thatcher, T.L., Price,P.N., Wood, E.E., Sextro, R.G., Gadgil, A.J., 2006. Tracer gastransport under mixed convection conditions in anexperimental atrium: Comparison between experiments and

CFD predictions. Atmos. Environ. 40, 52365250.Johnson, K.A., Westberg, H.H., Michal, J.J., Cossalman, M.W., 2007.The SF6 tracer technique: methane measurement fromruminants. In: Makkar, H.P.S., Vercoe, P.E. (Eds.), MeasuringMethane Production from Ruminants. Springer, Dordrecht,The Netherlands, pp. 3367.

Jong, E.C., Luxbacher, K.D., 2015. An evaluation of aperfluoromethylcyclohexane (PMCH) permeation plug releasevessel (PPRV) in a controlled turbulent environment. Int. J.Min. Sci. Technol. 25, 243251.

Jong, E.C., Luxbacher, K.D., Karmis, M.E., Westman, E.C., 2015a.Field test of a perfluoromethylcyclohexane (PMCH)permeation plug release vessel (PPRV) in an undergroundlongwall mine. Min. Technol. (in press).

Jong, E.C., Peng, Y., Bradley, W.T., Luxbacher, K.D., Agioutantis, Z.,McNair, H.M., 2015b. Development of aperfluoromethylcyclohexane (PMCH) permeation plug releasevessel (PPRV) for tracer gas studies in underground mines.Process Saf. Environ. Protect. 95, 136145.

Jong, E.C., Underwood, S.W., Xu, G., Luxbacher, K.D., McNair, H.M.,2012. A technique for creating perfluorocarbon tracer (PFT)calibration curves for tracer gas studies. In: 14th U.S./NorthAmerican Mine Ventilation Symposium, Salt Lake City, UT.

Kennedy, D.J., Stokes, A.W., Klinowski, W.G., 1987. ResolvingComplex Mine Ventilation Problems with Multiple TracerGases. In: 3rd Mine Ventilation Symposium, University Park,PA, pp. 213218.

Leonard, J.J., Feddes, J.J.R., McQuitty, J.B., 1984. Measurement ofventilation rates using a tracer gas. Can. Agric. Eng. 26,

4951.Lester, D., Greenberg, L.A., 1950. The toxicity of sulfur

pentafluoride. Arch. Ind. Hyg. Occup. Med. 2, 348349.Mailahn, W., Seifert, B., Ullrich, D., Moriske, H.-j., 1989. The use of

a passive sampler for the simultaneous determination oflong-term ventilation rates and VOC concentrations. Environ.Int. 15, 537544.

McNair, H.M., Miller, J.M., 1998. Basic Gas Chromatography. JohnWiley & Sons, Hoboken, New Jersey.

Mucho, T.P., Diamond, W.P., Garcia, F., Byars, J.D., Cario, S.L., 2000.Implications of recent NIOSH tracer gas studies on bleederand gob gas ventilation design. In: SME Annual Meeting, SaltLake City, UT.

OSHA, 1993. Chemical Sampling Information.Patterson, R.R., 2011. Determination of a Novel Mine Tracer Gas

and Development of a Methodology for Sampling andAnalysis of Multiple Mine Tracer Gases for Characterization ofVentilation Systems. Department of Mining and MineralsEngineering, Virginia Tech, Blacksburg, VA.

Patterson, R.R., Bowling, J.R., Xu, G., 2010. Design of anexperimental apparatus for the testing of novel tracer gases inunderground mines. In: 13th United States/North AmericanMine Ventilation Symposium. SME, Sudbury, ON.

Peach, M.J., Carr, W.G., 1986. Air sampling and analysis for gasesand vapors. Occup. Respir. Dis., 4168.

Pekney, N., Wells, A., Rodney Diehl, J., McNeil, M., Lesko, N.,Armstrong, J., Ference, R., 2012. Atmospheric monitoring of aperfluorocarbon tracer at the 2009 ZERT Center experiment.Atmos. Environ. 47, 124132.

Schuette, F.J., 1967. Plastic bags for collection of gas samples.Atmos. Environ. 1, 515519.

Stokes, A.W., Kennedy, D.J., 1987. A real-time tracer gasanalyzeran investigational tool for mine ventilation studies.Min. Sci. Technol. 5, 187196.

Stokes, A.W., Stewart, D.B., 1985. Cutting head ventilation of a fullface tunnel boring machine. In: 21st Int. Conf. Safety in Mines.Mines Research Institutes, Sydney, Australia.

Thimons, E.D., Bielicki, R.J., Kissell, F.N., 1974. Using sulfurhexafluoride as a gaseous tracer to study ventilation systemsin mines. U.S. Bureau of Mines, Pittsburgh, PA.

Thimons, E.D., Kissell, F.N., 1974. Tracer gas as an aid in mineventilation analysis. Bur. Mines.

Timko, R.J., Thimons, E.D., 1982. Sulfur hexafluoride as a mineventilation research toolrecent field applications. U.S.

Bureau of Mines, Pittsburgh, PA.Vanderborght, B., Kretzschmar, J., 1984. A literature survey ontracer experiments for atmospheric dispersion modellingstudies. Atmos. Environ. 18, 23952403.

Widiatmojo, A., Sasaki, K., Sugai, Y., Suzuki, Y., Tanaka, H.,Uchida, K., Matsumoto, H., 2015. Assessment of air dispersion
http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0005http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0005http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0005http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0005http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0010http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0010http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0010http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0010http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0015http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0015http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0015http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0020http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0020http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0020http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0025http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0030http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0030http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0030http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0030http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0030http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0030http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0030http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0030http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref1135http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref1135http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref1135http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0035http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0035http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0035http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0035http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0040http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0040http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0040http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0045http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0045http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0045http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0045http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0045http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0045http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0050http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0050http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0050http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0050http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0050http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0055http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0055http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0055http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0055http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0055http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0055http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0055http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0060http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0060http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0060http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0060http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0060http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0065http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0065http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0065http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0070http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0070http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0070http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0070http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0070http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0070http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0075http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0075http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0075http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0080http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0080http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0080http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0080http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0080http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0085http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0085http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0085http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0085http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0090http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0090http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0090http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0090http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0110http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0125http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0125http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0125http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0135http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0135http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0135http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0155http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0155http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0170http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0170http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0170http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0170http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0170http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0165http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0160http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0155http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0155http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0155http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0155http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0155http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0155http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0155http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0155http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0155http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0155http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0155http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0155http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0155http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0155http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0155http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0155http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0155http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0150http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0145http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0140http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0135http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0135http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0135http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0135http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0135http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0135http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0135http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0135http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0135http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0135http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0135http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0135http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0135http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0135http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0135http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0135http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0130http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0125http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0125http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0125http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0125http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0125http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0125http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0125http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0125http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0125http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0125http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0125http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0125http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0125http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0125http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0125http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0125http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0125http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0125http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0125http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0120http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0115http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0110http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0110http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0110http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0105http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0100http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0095http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0090http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0090http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0090http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0090http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0090http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0090http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0090http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0090http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0090http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0090http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0090http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0090http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref0090http://refhub.elsevier.com/S0957-5820(15)00179-2/sbref

1-s2.0-S0957582015001792-main

Documents

Transcript of 1-s2.0-S0957582015001792-main