Don't Underestimate the Importance of LEL Correlation Factors Your team's safety depends on selecting the right correlation factor, calibration gas, and LEL sensor for your application. By Scott Jubeck

of determining the performance parameters of an instrument by comparing it with measurement standards."

Once calibrated with a particular gas, the instrument will display LEL values assuming all gases in the environment are that one specific gas. If a sensor calibrated to methane detects ethane, the instrument will display LEL values assuming it is

~ truly methane. Many members of the safety community under­"' stand this challenge and have procedures in place to account for ~ f2 this sensor limitation. (Please note that this article refers to LEL ~ IR sensors, not gas-specific sensors like methane IR. LEL IR sen­ill sors are sometimes called Hydrocarbon IR sensors.) ~ Correlation factors are used to translate instrument readings ~ from the units of the calibration gas to the units of a second de­,. sired gas. For example, correlation factors can be used to trans-

late readings on a pentane-calibrated instrument to equivalent units of ethane.

The next challenge is that most members of the safety com­munity were trained on the industry-standard CB LEL sensor technology. This training leads to typical alarm levels of 10 per­cent LELand 20 percent LEL because most CB correlation fac-

~ tors are relatively dose to the reading on the display, typically

Four of your team members are hard at work, side by side two times or less. The logic is the combustible gases in the at-at your facility. Each is wearing a brand-new gas detec- mosphere will not exceed 40 percent LEL (20 percent LEL high tor that was calibrated and bump tested only a few hours alarm multiplied by a correlation factor of 2), assuming a worst-ago. The problem is that two of the instruments are in case scenario. Even assuming a +25 percent error in the reading,

high alarm, one is in low alarm, and the fourth is not in alarm the user is still below the lower explosive limit and has time to

at all. Which instrument is defective? The two instruments in retreat to a safe location. high alann? The one instrument in low alarm? How about the WJ.th the continuing emergence of lR technology, many us-instrument not in alarm at all? Are they all broken? Actually, the ers assume the sensor performs in the same way as catalytic bead instruments may all be working perfectly.... technology, even though the detection technology is very dif-

Many members of the safety community have spent a lot of ferent Not only is that assumption incorrect, but also it can be time recently debating which Lower Explosive Limit {LEL) sen- very dangerous. Calibrating CB and IR with the same gas often sor technology is superior: the industry-standard catalytic bead leads to dramatically different results. The table below identifies (CB) technology, which has been in use for more than 50 years, correlation factors for CB and IR sensors, standard calibration or the newer infrared (IR) sensor technology. Much has been gases, and common gases found in the field. written about the strengths and weakness of each technology and can easily be found by your favorite search engine. This ar­ticle will discuss the intersection of correlation factors, calibra­tion gas, LEL sensor technologies, and their impact on ensuring every worker returns home at the end of his shift.

Let's start with a quick summary of the sensor technolo­gies. Both CB and lR sensors are designed to protect against the combustion of gases in the atmosphere, not the detection of one combustible gas specifically. In other words, an LEL sensor does not know whether it is detecting pentane, methane, or propane, but it only recognizes that one or more combustible gases are in the atmosphere. For this reason, both CB and IR sensors must be characterized through calibration. The McGraw-Hill Science and Technology Encyclopedia defines calibration as "the process

28 Occupational Health & Safety I MAY 2014

. C:.Aiytlc Beed LEL Sensor Infrared LEl Sensor

GasBelns C:.llbratlon GIS CaHbratlon Gas Sampled

Methene Pentane Methane Propane Acetylene 1.3 0.7 0.0 0.0

Butane L7 0.8 3.0 1.0 Ethane L3 0.7 6.S 1.7 HI!XIne 2.9 1.4 0.9 0.8

Hydrogen 0.9 o.s 0.0 o.o Methene 1.0 o.s 1.0 3.0 Pentane 2.0 1.0 1.S 0.9 Propane 1.6 0.8 3.8 1.0

• CDtrtlarlon /D<ton-""' bosftl on rnrlng by lndusltfol Sdtnll/k uslllfllnduSirlol Scmrrlfl< /nslruiJU!niS. CllfffiiDikw /DCIOIS trK1F Wll)' by monufoCIUfflr.

The formula to translate your instruments readings to the actual concentration in the atmosphere is: Instrument Reading

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(% LEL) = Gas Being Sampled {% LEL) divided by Correlation Factor.

The following examples were devel­oped to prove the importance of the com­bination of correlation factors, calibration gas, and LEL sensor technology in ensur­ing the safety of your team.

I • 1 11 , ' ·q

lO X Ill O! r:,l•th.ln o " •I •• ; , • ,h .,1111 r. 10 "' •" ''

s.n- CF lnstrumant Pentane-calibrated CB LEL sensor o.s 2'"' Methane-callbroled CB LEL ""'sor 1 1'"' Propone-allbnlled IR LEI. sensor 3 n. Methane-calibrated IR LEL sensor 1 I""

sensors, when calibrated with methane, are very accurate in their detection of methane, as expected. A pentane-cali­brated catalytic bead LEL sensor will read high because the correlation factor is 0.5. On the other hand, a propane-calibrated infrared LEL sensor will read low at 3 per­centLEL

This example highlights the impor­tance of two best practices: 1) Calibrating to the known hazard in the atmosphere will lead to the most accurate results. If methane is the known hazard, calibrate

In this basic example, both CB and IR your LEL sensor to methane. It doesn't

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matter if you are using a CB or IR sensor. Correlation factors are never perfect and will introduce error into your readings, so calibrating to the known hazard leads to the most accurate results. 2) Catalytic bead LEL sensors calibrated to pentane typically have correlation factors of less than two and often close to one. For that reason, many gas detection manufactur­ers recommend pentane calibration for CB sensors because the instrument will reduce the error between the real gas con­centration and the instrument reading.

( N,lfllJllf' fl"/.

oO ')o.. It L of! 1h lf11' 1r1 t ht• r.t rno<.pht rc·! I R"- vol)

sensor CF Instrument Pentane-calibrated CB LEL- 0.7 86% MOihane-callbroled CD LEL semor 1.3 46'1l Propane-calibrated IR LEL sensor 1.7 3~" Mtlhane-callbrated IR LEL sensor 6.5 !I'll

The second example shows a poten­tially dangerous scenario in the field. Some gas detection manufacturers rec­ommend methane-calibration for their IR LEL sensors. Even with 60 percent LEL ethane in the atmosphere, a methane­calibrated IR LEL sensor will display only 9 percent LEL.

This example highlights a third best practice. It is that setting low alarm lev­els are very important for safety. Many industry leaders recommend a 10 per­cent LEL low alarm and 20 percent LEL high alarm levels. In the example above, a methane-calibrated infrared sensor will trigger a 10 percent LEL low alarm only with 65 percent LEL ethane in the atmosphere. Although the instrument will alarm late, the atmosphere is not yet combustible.

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.'S~ II L tt o• o,Hll' U\ t lw 1\trnmphf'rf• !0 1~ .rol) - CF Instrument Pentane-calibrated CB LEL sensor 1.4 IB'Il

Methane-calibrated CB LEL - 2.9 !I'll Propane-allbraled IR LEL sensor 0.8 319l MOihane-caUbraled IR LEL sensor 0.9 2ft

In Example #3, a methane-calibrated CB LEL sensor is now the culprit in alarm­ing late. All three of the other sensors are in alarm, with the infrared sensors in high alarm. Example #2 showed the infrared sensors alarming slowly, while Example #3 shows that catalytic bead sensors also have this limitation at times.

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•,o ,. tlt 1\c.<•tyl!•r ·" rr •he 1"ltrnt • phr·r~

Sensor CF lnstruiMftl Penune-aRbrated ce l.fl sensor 0.7 71" MGthano-callbraled CBI.fl sensor 1.3 3Bl' Propone.callbrated IR l.fliCruor 0 ~

Methane-calibrated IR LEl .. n...r 0 ~

Acetylene is a common welding gas. When using acetylene, it is very impor­tant to understand which type of LEL sensor to use. Some IR LEL sensors will not detect several combustible gases, in­cluding (but not limited to) acetylene, hydrogen, and carbon disulfide. In this scenario, the infrared sensor will not re­spond at all, regardless of concentration. The calibration gas also will not impact the result, as the sensor simply will not detect acetylene. Check with your gas de­tection manufacturer for details.

CB LEL sensors detect combustible gas by burning it With low levels of oxygen, combustion is incomplete or impossible. In atmospheres with less than 10 percent

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oxygen, users should not trust the LEL readings on their instruments because the sensor does not have adequate oxygen to properly detect the hazard. Dilution tubes or IR sensors should be considered when oxygen levels are less than 10 percent vol­ume. Again, ask your manufucturer for details since each CB sensor responds dif­ferently to low levels of oxygen.

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w rth • 10";., Jt>l O • 'r r.f'n - a lnstluiMftl Pentane ... llbrated CBLEL ICn...r 0.8 ~

Mothlne-canbrated CBLEL sensor 16 ~

Propone-callblited IR LELsensor I 2~

Mothlne-caUbrated IR LfliCmor 3.8 5"

Also, in Example #5, the methane-cal­ibrated infrared LEL sensor does not alarm due to the 3.8 correlation factor. This situation was also found in Example #2 The atmosphere actually must reach 38 percent LEL before reaching a 10 per­cent LEL low alarm. Again, this highlights the fact that users must understand that

Circle 36 on caret.

IR correlation factors are often dramati­cally different than CB correlation factors.

Two other considerations should be made when comparing CB LEL to IR LEL sensors: 1) CB LEL sensors may be inhib­ited or poisoned by various chemicals, in­cluding silicone. When these sensors are exposed to silicone, their reaction to com­bustible gases is reduced or eliminated. Many everyday items (like Armor All*) are proven to damage CB LEL sensors. IR LEL sensors are not poisoned by these chemi­cals. 2) Correlation factors for CB LEL sensors may change over time as the sen­sor ages. This is particularly important for methane detection when calibrating with a gas other than methane. Periodic exposure of a known concentration of methane gas to pentane-calibrated sensors is recom­mended to validate performance. OK-S

Scott Jubeck is the Global Product Man­ager of Multi-Gas Solutions at Industrial Scientific (www.indsci.com).

MAY 2014 I Occupational Health & Safety 31