All too often, when doing a CS entry or rescue course, we are surprised by the number of entry supervisors who do not understand the major limitation of a combustible gas meter… the LEL Correction Factor. EVERY meter in use today will have this limitation! This is how it works…
Most combustible gas detectors these days literally measure the contaminant by combustion at a catalytic detector. Older style meters used the Wheatstone bridge method. The heat produced is used to measure the “explosivity” of the contaminant in the air. Different compounds produce different amounts of heat when they are burned. So the meters respond differently to different chemical mixtures in air. Combustible gas meters can ONLY be expected to respond accurately to the gas for which they were CALIBRATED. To measure other gases with the same meter, consideration MUST be given to the gas’s and detector’s specific properties. Most manufacturers have responded to the need to estimate more accurately the concentration of other gases by providing “correction factors” which allow a calculation of percent LEL from the measured level. Some flammables have a correction factor as high as 3.0!!!
Here it is in simple terms…
You calibrate your meter(s) using a calibration gas with Methane. Now if you are measuring the atm in a space and the flammable gas in the space is actually Methane, then the meter will be accurate in its reading. However, if the gas you are looking for is Hexane, we have a HUGE problem! The meter will pick up the Hexane, but the meter reading will be WAY OFF. To be exact, it is off about 2.3 times. That is right; you must multiply your meter reading by 2.3 to get the actual percent by volume of Hexane in the space. Let’s say, for example, your CS entry program allows up to 10% of the LEL for entry (e.g., OSHA’s LEL Limit). The supervisor does his/her initial atm check and finds the LEL meter reads 7%; below the 10% allowed by OSHA, right? NOT SO FAST! Using the correction calculation of 2.3, you send workers into a space at 16.1% of the LEL of Hexane, well over the 10% OSHA allows in 1910.146.
There is a very easy way to eliminate this problem without making all your CS entry supervisors safety professionals – throw out the OSHA limit of 10% and lower your allowable LEL% limit to 3%. The supervisor can approve the permit if the meter reads an LEL of 0-3%. Any reading above 3% on the LEL sensor requires the safety professional to assess the hazards of the space, using their knowledge of the “correction calculation.” Each user of a gas meter must READ the meter owner’s manual, as each meter will have different correction calculations with the different calibration gases that can be used for the meter. Here is the link for the table for those that use Industrial Scientific meters:http://www.indsci.com/training_details.aspx?id=1564
