Correction Factors for Combustible Gas (LEL) Sensors
LEL Correction Factors
Correction Factors (CFs) have been determined that enable the user to quantify a large number of chemicals using only a single calibration gas, typically Methane or Pentane. In our LEL sensors, Correction Factors can be used in one of three ways:
Calibrate the unit with Methane in the usual fashion to read in methane %LEL equivalents. Manually multiply the reading by the Correction Factor (CF) to obtain the %LEL of the gas being measured.
Calibrate the unit with Methane and then call up the Correction Factor from the instrument memory or choose the Correction Factor from the instrument memory first and then calibrate to methane. The unit after that reads directly in %LEL of the gas of interest.
Calibrate the unit with Methane, but input an equivalent, “corrected” span gas concentration when prompted for this value. For example, to read in isopropanol LEL units, apply 20% LEL Methane but enter 20 x 2.6 = 52 for the span gas concentration.
Oxygen Requirement and Matrix Effects
LEL sensors require oxygen for combustion and cannot be used in environments that contain less than 10 vol% oxygen. This threshold is the safe limit for up to 100% LEL of nearly all chemicals, but it depends on the combustible gas concentration. For example, for 10% LEL Methane, RAE Systems LEL sensors show little or no oxygen dependence down to about 5 vol% oxygen. Inserting an LEL sensor from air into pure nitrogen can cause a transient response that decays after several minutes to the background reading. This is because the reference bead takes time to equilibrate with the slightly lower thermal conductivity of the nitrogen. Likewise, other inert matrix gases may cause a transient response.
Humidity and temperature generally have little effect on the sensor response. Increasing temperature decreases the response by less than 4% between 0° and 40° C. Increasing RH (relative humidity) decreases the response by about 2% between 20% and 90% RH.
Methane Sensitivity Changes
The Correction Factors in the table apply to new sensors. As a sensor becomes used and gradually loses sensitivity, the response to Methane may decrease more rapidly than for higher hydrocarbons. In this case, the Correction Factors gradually decrease, and calibration with methane tends to overestimate the %LEL of the other gas. Therefore, Methane calibration is the safest approach. RAE Systems LEL sensors do not exhibit changes in Correction Factors in laboratory tests, but may do so under special-use conditions. Calibrating with other organic vapors such as Propane or Pentane is a good way to avoid Correction Factor changes. The only drawback to this approach is that it is possible to underestimate Methane while still measuring the higher hydrocarbons. If Methane is known to be absent under all circumstances, the use of propane or pentane calibration is appropriate.
Correction Factors when Calibrating with Non-methane Compounds
To obtain Correction Factors for other span gases, simply divide the value on the methane scale in the table by the methane value for the span compound. For example, to obtain CFs on the n-pentane scale, divide all the numbers in the table, column LEL CF by 2.1.
Thus, when calibrating with n-pentane the new CF for ammonia is 1 /2.1 = 0.48. Note that this calculation is done internally in RAE Systems instruments that have separately selectable span and measurement gases. Therefore, in these cases, simply enter the span and measurement compounds (without changing the CFs), and the unit automatically calculates and applies the new factor.
Note: This is a guideline document for calculating correction factors of LEL sensors. The user is encouraged to calibrate the instrument with the target gas for most accurate readings. Refer to the instrument settings and product manuals for information specific to each instrument.
* LEL CF values apply for MultiRAE families, ToxiRAE Pro and other RAE Systems instruments.
These correction factors are applicable for versions 1.64 for ToxiRAE Pro and 1.16 for MultiRAE.
Note: Reported values of the lower explosive limit for jet fuels range from about 0.3% to 0.9% by volume. An earlier third-party test reported a CF for jet fuels of 3.35. However, it has not been possible to confirm this result and it is recommend using a PID as a much more accurate method of measuring LEL for jet and diesel fuels.
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