RAE Systems’ LEL sensors can detect various combustible gases and vapours, each with different response levels. Because these sensors use a diffusion barrier to regulate gas flow to the catalytic bead, they are most sensitive to high-diffusivity compounds. Consequently, they are much more responsive to small molecules like hydrogen and methane than to heavier substances like kerosene. The optimal way to calibrate sensors for different compounds is by using a standard of the gas of interest. However, Correction Factors (CFs) have been established to allow users to quantify numerous chemicals using a single calibration gas, typically methane or pentane.
Oxygen Requirement and Matrix Effects:
LEL sensors need oxygen for combustion and are unsuitable for environments with less than 10 vol% oxygen. This is the safe limit for up to 100% LEL of most chemicals, depending on the combustible gas concentration. For instance, RAE Systems LEL sensors show minimal or no oxygen dependence down to about 5 vol% oxygen for 10% LEL methane. When an LEL sensor is moved from air into pure nitrogen, a transient response occurs that decays to the background reading over a few minutes. This happens because the reference bead takes time to adjust to the slightly lower thermal conductivity of nitrogen. Similar transient responses can occur with other inert gases. Humidity and temperature generally have minimal effects on the sensor response, with temperature increases decreasing the response by less than 4% between 0° and 40° C, and increasing relative humidity decreasing the response by about 2% between 20% and 90% RH.
Methane Sensitivity Changes:
- Calibrate the unit with methane to read in methane %LEL equivalents, then manually multiply the reading by the Correction Factor (CF) to get the %LEL of the target gas.
- Calibrate with methane, then either call up the CF from the instrument memory or select the CF first and then calibrate with methane. The unit will then read directly in %LEL of the target gas.
- Calibrate with methane but input a “corrected” span gas concentration. For example, to read in isopropanol LEL units, apply 20% LEL methane but enter 52 (20 x 2.6) for the span gas concentration.
Correction Factors for New and Used Sensors:
Correction Factors apply to new sensors, but as sensors are used and lose sensitivity, the response to methane may decrease faster than for higher hydrocarbons. Consequently, CFs will gradually decrease, and methane calibration might overestimate the %LEL of other gases, making methane calibration the safest approach. RAE Systems LEL sensors generally maintain stable CFs in lab tests but may vary under special-use conditions. Calibrating with other vapours like propane or pentane can help avoid CF changes, but this might underestimate methane while accurately measuring higher hydrocarbons. If methane is confirmed absent, propane or pentane calibration is suitable.
Correction Factors When Calibrating with Non-Methane Compounds:
To obtain CFs for other span gases, divide the methane value in the table by the methane value for the span compound. For example, to find CFs on the n-pentane scale, divide the values in the LEL CF column by 2.1. In RAE Systems instruments with selectable span and measurement gases, this calculation is done internally. Just enter the span and measurement compounds, and the unit will automatically calculate and apply the new CF.
RAE Systems by Honeywell have put together a correction factors for combustible (LEL) sensor chart. Complete the form below to obtain this chart.
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