eHANDBOOK - Control Global

eHANDBOOK

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TABLE OF CONTENTSMake sense of sensitive level 5

Radar offers increasing opportunities to improve control at a reasonable price.

Differential pressure level in a purged tank 10

Hydrostatic and head pressures can lead to confusion.

Wireless sensor sweet spots 15

How battery life interacts with update rate to define the most practical applications.

Measuring oceans and ice 17

Accuracy and precision support predictions and balance the sea level budget.

AD INDEXABB Measurement Products • abb.com/level 14

Endress+Hauser • go.endress.com/us/terminal-management-capabilities 4

Krohne, Inc. • optiwave.krohne.com 9

Vega Americas • www.vega.com 2

eHANDBOOK: Level Measurement, Part II 3

www.ControlGlobal.com

http://abb.com/level

http://go.endress.com/us/terminal-management-capabilities

http://optiwave.krohne.com

Jon Eide Process Consultant for Terminal Management

Gain workflow continuity and streamline processes.

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http://go.endress.com/us/terminal-management-capabilities

Greg: It’s not well recognized that

a sensitive level measurement

can be extremely valuable. For

inventory, a fraction of an inch change in

level in a large-diameter tank is a lot of

material. For closure of material balances,

these seemingly small changes in level

can make a huge difference in account-

ing and in process control of some key

unit operations. For crystallizers, evapo-

rators and reactors, tight residence time

control depends on an accurate level

measurement. Many multi-effect evapo-

rators measure and control the product

density in the last stage by manipulating

a product or feed flow. These multi-effect

evaporators depend upon tight level con-

trol to make sure changes in flow in and

out of each effect are equal. For distilla-

tion columns where the distillate receiver

level controller manipulates reflux flow,

extremely tight level control enables much

better temperature control, particularly

through inherent internal reflux control.

The most predominant level measurement

uses a differential pressure (DP) transmit-

ter. The actual level, of course, depends

on density. A second DP whose high and

low connections are always submersed or

a Coriolis meter in a recirculation line can

be used to measure density to correct the

reading, but the DP sensitivity is not great.

The accuracy seriously deteriorates due to

problems and limitations of impulse lines,

purges and capillary systems.

We have a situation for level similar to

last month’s for mass flow, where Cori-

olis meters had an order of magnitude

Make sense of sensitive levelRadar offers increasing opportunities to improve control at a reasonable price

By Greg McMillan



better accuracy, much greater reliability,

and much less vulnerability to installa-

tion problems. The level measurement is

radar. Like Coriolis, radar doesn’t depend

on density (and thus associated changes

in composition), and while the hardware

cost is higher, the installation cost and

especially the maintenance cost is less. If

you include the improvement in process

monitoring and control, the return on

investment is a no-brainer.

To help us get sensitive to sensitive level

measurement, we gain the insight and

knowledge offered by Jeff Blair, offer

manager for level, Schneider Electric.

Jeff: Thank you for inviting me to Con-

trol Talk, Greg. It’s an honor to be here.

I enjoyed reading your material along

with items from Hunter Vegas and many

others.

Most of the free space radar (FSR) and

guided wave radar (GWR) applications we

see work flawlessly and get rave reviews.

However, a small number of radars are

misapplied or not installed properly, so

we’ve been on an educational tour to help

teach the operators, technicians and engi-

neers about proper installation procedures

and how to get the best signal return. It’s

for reasons like these that there’s been a

trend for manufacturers to offer additional

field services (including startup services)

for radar. FSR and GWR are actually easier

to commission and use than most instru-

ments, as long as the end user follows

proper vendor instructions and/or has a

rep or vendor either onsite or on speed

dial/Facetime/Skype to help.

Greg: What equipment and process

application details do you need to know

to correctly specify and install FSR

and GWR?

Jeff: When selecting a radar measure-

ment, the best thing to do is to gather the

criteria of process temperature, process

pressure, expected dielectric constant

(Dk), material requirements, available tank

connections and economics. Next, work

with your favorite representative or man-

ufacturer to help narrow the choice of

applicable radar models. They can guide

you on available models and also help

select special models that can perform

interface measurement (GWR) or tolerate

high temperatures and pressures for appli-

cations such as steam drum measurement.

All application parameters are important

to make sure the correct radar is selected

and that it performs for the lifecycle of

the process. Besides ensuring a Dk of 1.1

or higher (1.5 preferred), the other most

important thing to consider when select-

ing radar is to ensure proper selection of

the cable, rod or coaxial probe (GWR) or

antenna (FSR). The reason is that those

items and their design and dimensions



have a large part in determining the over-

all maximum measurement length, beam

angle (FSR only), and temperature and

pressure limitations. A close runner-up to

the antenna selection is the proper choice

of the physical installation location (i.e.,

what distance from the tank wall, ensuring

location is away from center of tank and

any filling nozzles or streams) on top of

the tank, vessel, sump pit or stilling well.

The goal of proper physical location of

the radar is to eliminate potential obsta-

cles, and give the radar the best possible

chance at return signal. There are fewer

constraints and restrictions when choos-

ing an installation location for GWR as the

microwave energy always stays within a

radius of the probe.

Greg: What are the installation require-

ments you need to carefully address for

FSR and GWR?

Jeff: The beauty of both types of radar

is their simple installation techniques

compared with other types of level instru-

mentation. There are no requirements for

process tubing runs that are often required

for DP level applications. Regarding GWR,

pay attention to ensure you have enough

overhead clearance to install a rod or

coax unit. It’s no different than ensur-

ing overhead coverage when installing a

capacitance probe or a magnetostrictive

instrument. And some of the probes come

in segments, so several short pieces may

be assembled together rather than a long

10- or 20-ft rod. Cable antennas for GWR

may need to be secured to the bottom of

the tank. If that’s the case, there may be

some potential for confined space permits.

FSR is a bit simpler because it is non-con-

tact; there’s no probe or cable that extends

down into the liquid.

Once those items are accounted for,

both FSR and GWR are simply screwed

into their respective process connections

with the face oriented in accordance with

instructions.

The goal of proper physical location of the radar is to eliminate potential obstacles and give the radar the pest possible chance at a return signal. There are fewer constraints

when selecting GWR as the microwave energy always stays within a radius of the probe.



Installing radar when the tank is empty is

advised. This allows the radar software

to take a snapshot of the tank and iden-

tify any obstacles (e.g. baffles, ladders)

that may be interpreted as false level

reflections. The snapshot or tank mapping

procedure allows the user to blank out

potential false echoes. It’s worth noting

that high-frequency (80 GHz) types of

FSR have a narrow radar beam that’s

often able to avoid seeing obstacles and

potential disturbances, eliminating the

need to map them out.

Greg: What do you need to know to cor-

rectly install FSR and GWR?

Jeff: In radar, the measurement is inferred

from distance, like how level is inferred

from head pressure when using DP level.

The data needed are three pieces of

information:

1. The tank height—from the process con-

nection of the radar to the tank (where

the radar signal generates) to the

tank bottom.

2. The 4 mA or 0% level—this should nor-

mally be located at an offset from the

very bottom of the tank.

3. The 20 mA or 100% level—this should

be located below the flange and allow

room for the radar dead space. Each

type of radar requires a small amount of

buffer space either below the process

connection or the horn (FSR only).

Greg: When would you use ultrasonic

instead of radar level measurements?

Jeff: Cost-conscious consumers often

chose ultrasonic. Ultrasonic transmitters

using sound waves were about half the

cost of radar, and like radar, they perform

flawlessly if they’re applied and installed

properly. Now, in general, the average

price of process FSR and GWR have come

down, so that the price delta between

radar and ultrasonic closed. Price being

nearly equal, most users opt for radar as

it can handle higher temperatures and

pressures. Now, ultrasonic measurements’

primary advantage is that its unaffected

by Dk. One of the drawbacks to ultrasonic

used to be the buildup of condensation

on the transducer face. Recently, there’s

been some advancement in that field

where some ultrasonic measurements are

better able to shed condensation droplets

to make a reliable measurement.



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Q When using a differential-pressure

(DP) level transmitter, are the high

and the low pressure ports reversed if

the tank is purged with nitrogen? Purg-

ing is needed because the tank contains a

hydrate inhibitor.

R. REVISH

[email protected]

A In all wet leg-type level installations,

the top connection on the tank is

connected to the high-pressure side of

the DP cell, and the bottom one to the

low-pressure side, because the wet leg is

always full of liquid (a constant head pres-

sure plus the pressure of the head space

or nitrogen purge), while the hydrostatic

head at the bottom connection varies and

is always less than the one on the wet

leg side (the level in the tank is changing,

while the level in the wet leg is constant

and always higher). Naturally, in such

installations, the DP transmitter has to be

reverse-acting.

This reminds me of the Fukushima acci-

dent, with which I was involved, and later

wrote a book about how to fix its controls

(www.amazon.com/Automation-Can-Pre-

vent-Next-Fukushima/dp/087664017X).

During the accident, the water level

reading showed the level rising when it

was dropping because the temperature

around the overheated reactor caused

the condesate in the wet leg to boil off.

The Fukushima installation is shown in

Figure 1.

In my correction of this level control loop,

I added a second DP cell (∆PT in Figure 2)

Differential pressure level in a purged tankHydrostatic and head pressures can lead to confusion



that is continuously detecting the height

of the level in the wet leg (A in Figure 2).

If, for any reason, the height of this refer-

ence leg is dropping, the amount of drop

is added to the the output signal of the

level transmitter (LT in Figure 2).

Naturally, in your application—if you’re

sure that the wet leg will always be there

and will always be of constant height—you

don’t need the correction I made in Figure

2 for the Fukushima reactor. In your case,

you can just reverse the pressure taps or

use a reverse-acting transmitter.

Béla Lipták

[email protected]

A It’s very difficult to help you with the

minimal detail you provided. However,

if we review some general details of level

measurement, maybe you can understand

some of the difficulties in using differential

pressure to measure liquid level.

In Figure 3, the pressure at the top of the

tank must be presented to the DP level

transmitter (LT) through an impulse line.

The impulse line is not empty. It may be

filled with a non-corrosive fluid designed

for this purpose, or it may be filled with

condensate of the fluid in the tank being

measured. For that reason, I labeled this

tubing as “wet leg” in the drawing. Like-

wise, the pressure at the bottom of the

tank is also presented to the LT via its

impulse line (not labeled in the figure). We

know the pressure due to the liquid level

in the tank is higher at the point of mea-

surement (D) than the pressure at the top

of the tank (P1) in the drawing. However,

the LT sees the pressure at the top as the

CORRECTED INSTALLATIONFigure 2: A second DP cell (∆PT) continuous-ly detects the height of the level in the wet leg (A). If the height of this reference leg is dropping, the amount of drop is added to the output signal of the level transmitter (LT).

A

Steam pressure

Wet leg actual level

Reactor∆PT ∆

LT

Σ

+ +

+

-

-

Wet leg referenceA

∆L = lost wet leg

Correct level

HP

LP

FUKUSHIMA INSTALLATIONFigure 1: The arrangement of the level trans-mitter (LT) in the reactor vessel at Fukushima, according to Dr. Ritsuo Yoshioka, president, Japanese Functional Safety Laboratory.

LT



weight of the fluid in the wet leg plus P1,

while the pressure at the bottom is the

sum of the weight of the tank fluid (head)

plus the weight of the fill fluid in the

bottom pressure sample line plus P1.

From this analysis, you can see that the

pressure at LT at the wet leg (top of tank)

will actually be higher than the pressure

at LT from the bottom of the tank. This is

due to the pressure head from the wet leg.

Many instrument mechanics are confused

by this, and connect the top and bottom

pressure taps in the reverse positions on

the LT, which is what I would guess has

happened in your case.

Since the only thing you’re interested in

is the level in the tank, even though the

connections are reversed, the difference

between the HP and LP taps will still be

a correct measurement of liquid level.

Sometimes we’re just lucky.

For others, note that using a HART or

Foundation fieldbus smart level trans-

mitter will allow you to do a wet leg

compensation when the DP appears to be

negative because the LP is connected to

the wet leg and the HP is connected to

the bottom of the tank.

Dick Caro

[email protected]

A To measure level in a closed tank

that’s purged with nitrogen, you

need to follow these steps.

1. Determine the span of the measurement:

high point minus low point (in meters,

inches, feet, cm, etc.).

2. Since you’ll be using a purge that adds pres-

sure onto the measurement element, you’ll

need to know the following: pressure (in bar,

psig, etc.) and if the pressure will be con-

stant or variable.

3. Next, you’ll need to define the type of con-

nection on the tank—to know how the DP

cell is to be connected (tubing, seals, wet

leg, etc).

4. Then, identify the location of the transmitter

versus the datum or low level point of mea-

surement of the tank (in meters, inches, feet,

cm, etc.).

PURGED VAPOR SPACEFigure 3: Here, A is the measurement span; B is the distance of the minimum level above the connection; C is the distance from the high side connection to the top of the instrument connection and may be filled with fluid; and D is the actual liquid level above the transmit-ter (LT) low side. In wet leg applications, C is connected to the high side and D is connected to the low side of LT.

A

B

Vapor spacein tank

P1Maximumlevel

Minimumlevel

LT

CWetleg

D



Depending on the location of the trans-

mitter to the datum line, type of tank

connection, etc., and with the formulas of

the links below, it will be very clear why

the low-pressure side may have a higher

pressure causing a negative value.

It would appear that you have a differen-

tial pressure transmitter with a wet leg.

That means that the low-pressure side

is filled with liquid to the very top of the

tank connection point. This wet leg will

always have a pressure of the wet leg

height multiplied by the density of the

fill liquid plus the purge pressure on the

top of the tank. This means the pressure

will always be higher than the one being

sensed on the lower connection, which

will vary.

The easiest method to handle this is to

interchange the connection points—then

you’ll be subtracting the level head from

the wet leg head, giving a positive value.

But be careful, since as the level drops,

the value of the signal will increase.

Reference links:

• www.controlglobal.com/assets/

wp_downloads/pdf/mma_070921_

endress_liquidlevelpart1.pdf

• https://instrumentationtools.com/

closed-tank-level-measurement-us-

ing-dp-transmitters

• www.emerson.com/documents/

automation/technical-data-sheet-lev-

el-measurement-pressure-rose-

mount-en-74346.pdf

• www.ibiblio.org/kuphaldt/socratic/sinst/

output/level.pdf

• https://automationforum.co/

open-closed-level-measurement

• https://automationforum.in/t/

what-is-lrv-and-urv-how-to-

obtain-lrv-urv-for-level-measurment-us-

ing-differential-

pressure-transmitter/2396

• https://instrumentationtools.com/

open-and-closed-tank-level-calculations

Alex (Alejandro) Varga

[email protected]



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Untitled-1 1 9/19/2019 9:44:55 AM

http://abb.com/level

Though the majority of instrument

and controls engineers have an elec-

trical background (confirmed by the

surprise people still have when I tell them I

was trained as a chemical engineer and, like

them, “fell” into this profession), we all need

to remember that the reason we are install-

ing all our sensors, control elements and

control systems is to control and manage

the process. The point of this message is

that process dynamics need to be part of

the design process.

With wired devices that are not power-con-

strained, the update rate is decided by the

I/O card and controller. Battery-powered

wireless devices, however, do need to

manage their energy consumption, and the

most common way of doing so is by config-

uration settings of the update rate.

Though update rates for wireless sensor

networks (WSN), WirelessHART and

ISA100.11a can be as short as 0.5 sec., as

the update frequency is increased, there is

an associated exponential decrease in bat-

tery life. As expected, the largest impact

is at the faster update rates that might be

required for closed-loop control. Longer

update periods (beyond 60 sec.) are con-

strained by basic battery life physics more

than the update rate, thus setting the limit

on the slower-update-rate side of the

equations.

So how do we balance the update rate

and battery life? Basic control theory

recommends that the update rate of the

measurement shall be a minimum of three

times faster than the process time constant.

I personally prefer an update rate of six

Wireless sensor sweet spotsHow battery life interacts with update rate to define the most practical applications

By Ian Verhappen



times the process time constant, if possible,

because then I am sure to observe all stages

of an oscillatory process. However, using

the three-times-faster basis for a tempera-

ture loop (where measured temperature

changes with a sensor inside a thermowell

can be 16 sec. or longer, given how much

time is required for heat to penetrate the

thermowell and its mass), the required

wireless update rate would be roughly 5

sec. Since WSN cycles increase by doubling

each time, the closest approximation for

this loop is a 4-sec. update rate.

Industry practice and experience also rec-

ommend the update rate should be four

to 10 times faster than the time constant

of the process for regulatory closed-loop

control, so though it’s at the low end, the

4 sec. update rate would also work in

this example.

Another, non-process-related consideration

in addition to battery life is the impact traf-

fic may have on the network itself, and in

particular, the access point or gateway. One

WSN manufacturer recommends keeping

update rates no faster than 4 sec. since

doing so can impact the total number of

wireless devices that can be put on a gate-

way. Therefore, the 4 sec. update rate for

this example works well by meeting all

three minimum criteria.

Temperature is one example of a slow

process. Level measurement, especially

in large tanks, is another. These sorts of

measurements are well suited to wireless

sensing because they can operate with

slower update rates, and when you consider

that large tanks and tank farms are widely

distributed, not having to install cable infra-

structure makes a lot of sense.

With today’s computing power, the smart

people working in our industry have

developed a number of fancy tricks, such

as custom P&ID algorithms for wireless

networks that consider lag, other con-

trol algorithms (such as Smith Predictors,

developed in 1957), or other math to

compensate for the effects of delayed

measurements. (Some would say mask

rather than compensate, especially if

they are used improperly by a person not

understanding and applying first princi-

ples correctly.)

Despite all the advances we have made

and continue to make with our control

systems, it is always good to remember

why we are implementing the applica-

tion, as well as the underlying associated

engineering principles, and basic laws

of physics and chemistry that need to

be followed.

Not everyone makes a good controls engi-

neer, technician or practitioner, however, if

you remember the basic laws of physics and

chemistry, the chances of succeeding going

up significantly.



I’m optimistic about climate change, and

not only because the 2018 United Nations

Climate Change Conference has shown

that mankind is waking up, or because in

March, a 17-year-old girl was able to mobi-

lize an estimated 1.6 million high school

students in 125 countries around the world

to protest the present inaction about

global warming. I’m particularly optimistic

because of the contributions our tech-

nology—the monitoring capability of the

process control profession—are making.

Consider the fact that our scientific model-

ing was able to convincingly and accurately

predict the future and thereby give us

time to fix things. Consider that, based on

measuring only a few millimeters of ocean

level rise and less than a degree of global

temperature rise, our models were capable

of predicting that this process, if left uncon-

trolled, spells disaster for the future. This is

a fantastic achievement!

Here I’ll focus on the accuracy of two

additional measurements: the level of the

oceans and the changes in the mass of the

ice as polar and glacier ice formations melt.

Accurate measurements are important not

only to resolve the debate between deni-

alists and alarmists, but also to establish

the dynamics of the global warming pro-

cess, so we can more accurately predict its

speed, time constants, rate of rise and tip-

ping points.

Accurate measurement of ocean levels

must take into account that both sea and

land experience vertical motion. Land can

move vertically due to glacial and tectonic

Measuring oceans and iceAccuracy and precision support predictions and balance the sea level budget

By Béla Lipták



processes. Sea level is affected by tides

caused by the gravitational forces of the

moon and sun, and by “weak tides” (also

called meteorological tides) generated by

winds. Because these measurements are

on the scale of millimeters, not only accu-

rate detectors but sophisticated models are

needed to correct observed sea levels for the

effects of regular and meteorological tides.

The level models depend on two mea-

surements. One is the relative sea level

(the height of the water relative to the

land), which is corrected for any earth

movements. The second measurement

is provided by satellite altimeters, which

measure the distance between the ocean’s

surface and the center of the Earth. (Some

of the following paragraphs are taken

from a NASA document at https://sea-

level.nasa.gov/understanding-sea-level/

global-sea-level/ice-melt.)

ACCURATE ALTIMETERS

Laser altimetry: By the 1990s, laser altim-

etry from aircraft revealed thinning of the

ice sheets on Greenland’s coastal margins

[Abdalati, et al., 2001] and later surveys,

using NASA’s Airborne Topographic

Mapper, showed a further increase in that

thinning [Thomas, et al., 2009]. NASA’s

P-3B is a four-engine turboprop equipped

with radar and laser altimeters (Figure 1).

Researchers use highly sophisticated air-

borne instruments on these retrofitted

aircraft to measure annual changes in the

thicknesses and movements of the ice.

Satellite altimetry: A new NASA satellite,

Ice, Cloud and land Elevation Satellite

(ICESat2) was launched in September 2018

(Figure 2). It’s expected to gauge future

changes of the Greenland and Antarctic

ice sheets at an accuracy of the width

AIRBORNE TOPOGRAPHIC MAPPERFigure 1: NASA's P-3B four-engine turboprop is equipped with radar and laser altimeters to measure annual changes in thicknesses and movements of ice. Source: www.nasa.gov/mis-sion_pages/icebridge/instruments/p3b.html

SATELLITE MEASURES SHEET ICEFigure 2: NASA's ICESat2 (cloud and land ele-vation satellite) accurately measures ice sheet thickness at an accuracy of the width of a pencil. Source: www.click2houston.com/news/national/nasa-to-launch-laser-device-into-space-to-mea-sure-earths-polar-ice



of a pencil. In the Arctic,

NASA’s Icebridge opera-

tion studies the effect of

the polar ice on the Earth’s

climate. ICESat2 measures

the time it takes for laser

beams to travel from the

satellite to Earth and back.

Based on that information,

scientists can accurately

calculate the height of gla-

ciers, sea ice, etc.

This is the largest airborne

operation to survey polar

ice, and it’s designed to

plug the gap between

ICESat that ceased func-

tioning in 2003 and the

launch of ICESat2 in Sep-

tember 2018. Researchers

used data from these

yearly surveys to deter-

mine the rate of ice

melting, and found it

was increasing [Thomas,

et al., 2011]. The study

also relied on data from

NASA’S Airborne Topo-

graphic Mapper, as well as

the University of Kansas’

ice-depth sounder, both of

which have made almost

yearly surveys since 1991

of the Jakobshavn Isbrae

glacier in Greenland, and

since 2002, of the Pine

Island glacier in Antarctica.

The early 1980s saw the

first attempt to measure

ice sheet thicknesses using

satellite radar altimetry,

with observations of only

limited parts of Greenland

and Antarctica [Remy and

Parouty, 2009]. The ice

thickness on Greenland is

about a mile, and about

three miles on Antarc-

tica. Since then, however,

altimetry technology has

advanced a great deal

and became an important

means of determining ice

sheet and glacial mass

balance, measuring the

gains and losses in ice

mass. While the primary

measurement is by laser

altimetry (ICESat2), with

high accuracy and a very

small footprint, radar

altimetry was also used

in the European Space

Agency’s (ESA) Cryo-

Sat-2 mission.

Radar interferometry: Sat-

ellite synthetic aperture

radar interferometry tech-

niques measure the speed

at which ice streams move,

as well as the position of

SATELLITE ALTIMETRY SHOWS ICE THINNINGFigure 3 : These photos from NASA's Ice, Cloud and Land Ele-vation Satellite reveal areas of dynamic thinning (red) in Ant-arctica and Greenland (left). Source: https://sealevel.nasa.gov/understanding-sea-level/global-sea-level/ice-melt



the grounding line, which

separates ice grounded

over bedrock from ice

floating on the ocean, as it

breaks off from the same

ice stream. One example of

the power of this technique

was the measurement of

the retreat of Antarctic gla-

ciers [Rignot, et al., 2014].

The researchers used data

from the European Remote

Sensing (ERS) 1 and 2 syn-

thetic aperture radar to

measure the rapid retreat

of the glaciers in West

Antarctica. The study pro-

duced color-coded maps

(Figure 3) of the velocity of

recession for the glaciers,

which showed movement

of the glacial ground-

ing lines.

Satellite gravimetry: The

advent of gravimetric

measurements with the

twin Gravity Recovery

and Climate Experiment

(GRACE) satellites in 2002,

along with more recent

deployment of floating

Argosensors, opened

the way to closure of the

sea level budget—that is,

when the sum of observed

ocean mass and density

changes equals total sea

level change [Leuliette and

Willis, 2011].

GRACE measures changes

in water mass, including

terrestrial storage in the

form of groundwater,

rivers, snow and ice, and

mass changes in the ocean

itself, as well as the move-

ment of water between

land and ocean (Figure 4).

Early attempts didn’t

achieve closure of the sea

level budget for four-year

trend lines [Willis, et al.,

2008, Chang, et al., 2010],

leading to concerns about

possible instrument drift.

However, more recent

efforts led to reports of

closure for more extended

periods, including a NOAA

report, “The Budget of

Recent Global Sea Level

Rise, 2005-2013” by Eric

Leuliette, covering 2005 to

2013.

SEA LEVEL BY SATELLITEFigure 4: The advent of gravimetric measurements with the twin Gravity Recovery and Climate Experiment (GRACE) satellites in 2002, along with more recent deployment of floating Argosen-sors, opened the way to closure of the sea level budget (when the sum of observed ocean mass and density changes equals total sea level change). Source: http://ossfoundation.us/projects/ environment/global-warming/sea-level-rise

50

40

30

20

10

01994 1996 1998 2000 2002 2004 2006

Year

Sea

leve

l (m

m)



http://ossfoundation.us/projects/environment/global-warming/sea-level-rise

http://ossfoundation.us/projects/environment/global-warming/sea-level-rise

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