I have written a number of articles on the “Limitations of Combustible Gas Indicator, especially about the limitations of the LEL sensors and the “correction calculations”. I thought this piece in OSHA’s Technical Manual was well written and explains the limitation well. So if you did not believe me or understand what I was trying to say, here it is from OSHA.
Application and Principle of Operation
Combustible gas sensors use an oxidizing catalyst such as platinum or palladium. Combustible gas meters measure flammable gas concentration as a percentage of the LEL of the calibrated gas. When possible, to maximize the accuracy of the combustible gas readings, calibrate the instrument with the gas that will actually be monitored. If monitoring for combustible atmospheres other than the reference gas (i.e., calibration gas) consult the manufacturer’s instructions and correction charts/curves to determine a more accurate reading of the true gas concentration. Please note that LEL values for most flammable gases and vapors are a few percent in air (i.e., tens of thousands of ppm) and are NOT appropriate for assessing PEL concentrations of flammable toxic gases.
Calibration
Before using the monitor each day, calibrate it with an appropriate calibration gas as described in the user manual or in consultation with CTC. Follow the instructions in the user manual.
Consult the instruction manual before calibration, and ensure that the calibration gas is introduced at the proper pressure and flow rate, using the appropriate regulator and adaptors. Overpressurization can damage the sensor. For some instruments with an active sampling pump, the pump must be disconnected from the sensor and the span gas flow rate set to match the sampling rate of the pump.
Special Considerations
- Silicone compound vapors and sulfur compounds will cause desensitization of the combustible gas sensor and produce erroneous (low) reading.
- High relative humidity (90 percent to 100 percent) causes hydroxylation, which reduces sensitivity and causes erratic behavior including inability to calibrate.
- Oxygen deficiency or enrichment, such as in steam or inert atmospheres, will cause erroneous readings for combustible gases. Always note the oxygen concentration reading before assessing the LEL reading.
- In extraordinary circumstances, gas concentrations above the Upper Explosive Limit (UEL) may give a false reading indicating a noncombustible atmosphere; be aware that if air is suddenly introduced into such a space, the atmosphere can quickly become explosive.
- Vapors from liquids with flash points above 90°F such as turpentine, diesel fuel, and jet fuel, may not be adequately detected by combustible gas sensors. Use of a photoionization detector may be more appropriate.
- In drying ovens or unusually hot locations, solvent vapors with high boiling points may condense in the sampling lines and produce erroneous (low) readings. Consider taking readings at several different locations around the oven.
- High concentrations of chlorinated hydrocarbons, such as trichloroethylene or acid gases such as sulfur dioxide, will depress the meter reading in the presence of a high concentration of combustible gas.
- High molecular weight alcohols can burn out the meter’s filaments.
Maintenance
The instrument requires no short-term maintenance other than regular calibration and recharging of batteries. Use a soft cloth to wipe dirt, oil, moisture, or foreign material from the instrument.
CLICK HERE to see more of OSHA’s Technical Manual (updated in 2014)