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Potential dangerus meter situation
I recently sat in on a lecture by a well known Hazmat Expert. The topic was tactial meters. Here is a scenario that really made me wonder about our equipment. If for some reason your LEL sensor was not working correctly, your meter could allow you to enter a space in a truly dangerous situation. Long story short, your meter's low oxygen alarms is set to go off at 19.5% oxygen. Room air is typically 20.9% oxygen. Methane has a LEL of 5%. If you displace 5% volume in a space, your oxygen would read 19.9%. This could potentially put you at the LEL of Methane with no warning. Moral of the story is...no your meters and make sure that they are working before you use them.
Replies to This Discussion
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Permalink Reply by FETC on
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Good subject JB. Some people are shown how to turn em on and thats a shame. Calibrate them per the manufacturers specs... Another tip to resolve your concern, I never monitior an atmosphere without the use two different meters at the same time. This includes straight CO calls as well.
I will one up your post and will ask the Nation do you know what type of LEL sensor you have in your LEL meter? If you don't that is a shame... If you do, where is your CF- Conversion Factor Chart to convert your meters LEL reading to a known source chemical and obtain a true ACCURATE LEL reading?
Because sometimes our lack of education can make us feel safe but in reality are in a true 100% LEL environment spelling KABOOOOM!
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Permalink Reply by Kimberly A Bownas on
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You should be calibrating your meter outside the residence before entering it, if I am correct. At least that is what I remember from Haz-Mat refresher class.... Good topic though J. it is always nice to be reminded that sometimes meters do fail and all precautions should be taken.
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Permalink Reply by PFC Harmon on
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My day job is for a natrual gas utility and we respond to about 10 co and gas leak calls a day. We have our monitors calibrated every 6 months.
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yes, and you should hold the end up high in the air. Also it should not be around smokers or vehicle exhaust.
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Permalink Reply by Joe Stoltz on
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I believe our Draeger XAM3000 4-gas detector has a catalytic bead sensor in it, calibrated for methane gas. The meter reads in per cent LEL, meaning a 100% reading tells us the atmosphere is at the lower explosive limit (by volume) of a methane-air mixture.
What FETC is alluding to is the fact that this type of sensor sensor will read differently for different gases and hydrocarbon vapors. For example, a gas meter calibrated for methane will read 50% if your methane-air mixture is 2.5%. For other gases, the meter reads 50% when the ACTUAL LEL is as shown:
Acetylene - 135% LEL actual (ka-BOOM)
Hydrogen - 62% LEL actual
Propane - 94% LEL actual
Toluene - 124% LEL actual (ka-BOOM)
The moral of the story is that you have to know what type of product you are dealing with in order to correctly (and safely) interpret the reading. Our meter is calibrated for methane although we have about a 50-50 split between houses heating with natural gas versus propane gas.
As Kim and others point out, the unit must be turned on outside the structure, upwind of it and any running vehicles so the unit is properly set up during its power up sequence. Some also have a fresh air calibration that can be performed if the accuracy of the meter is in doubt.
We carry the calibration kit right in the rig just in case we really question the results.
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Excellent post Joe. This is why I come here to network with great people!!! Thanks for taking the time to jump in and relay the info I was trying to expound on!
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Permalink Reply by Jason Brooks on
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If your meters are only calibrated every 6 months and you use them daily, there is a problem there. Most manufacturers suggest monthly calibrations. This is for us and we don't tend to use them that often.
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You are not calibrating every time, you are zeroing to a clean environment. This is a big difference and goes back to the previous post about most people not knowing their meters. I am not bashing you, just reminding that there is a difference.
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You should double check your information. Most meters use Pentane as the calibration gas. But, if you are using Methane as your gas, then at least you are aware of that also. The LEL of Pentane is about 25% of that of Methane. Also, most, if not all, meters are set to say 100% LEL at 10% of the LEL that is was calibrated to. If you are truly at the 100% LEL mark, you are already way to deep. I also believe that the use of Pentane is gentler to the sensors, thus extending the sensor life.
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Permalink Reply by Ben Waller on
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There is a big difference between percent by volume and percent of LEL.
Oxygen sensors read percent by volume. If you're reading 19.5% on the oxygen sensor and the sensor is accurate and in calibration, then there really is 19.5% oxygen in the ambient air in the space being monitored.
LEL sensors, on the other hand, read in percent of the LEL. Let's assume that you're reading the gas to which the sensor is calibrated...say a methane-calibrated sensor reading methane. Methane's explosive range is 5% to 15% if at least 12.1% oxygen is present. http://www.landfill-gas.com/html/explosive_limits.html
If your methane sensor reads 5% LEL, then you're reading 5% of the 5% LEL.
That is a safe level for entry. The largest conversion factor (CF) for the widest discrepancy between the cal gas and the direct-reading gas is 3. Multiply the 5% LEL by 3, and you get an acutal LEL of 15%. That is well below the 50% LEL CF listed by OSHA for safe entry into confined spaces and other flammable atmosphere hazards.
Several atmospheric monitor companies print CF tables. ENMET, Industrial Scientific, RAE Systems, and GasTech don't list a CF of greater than 3.
A good rule of thumb is to calibrate your LEL sensor with cal gas in the middle of the CF range. Pentane and isobutylene are good choices for this. Those will give you a CF of less than 1.5 in either direction, regardless of what gas you're actually sampling.
My department has simplified the CF issue by calibrating our LEL (and PID) sensors with pentane and isobutylene, respectively. Using the max CF of 3 and a 10% LEL reading on the meter as the max safe entry level, that means that our personnel don't enter an atmosphere with more than 30% of the real LEL present. In other words, we use a safety factor that is 20% safer than the OSHA standard and that requires no conversion factor calculation by personnel during an emergency.
In the original post example, 5% LEL of the 5% listed LEL of methane is 0.25% by volume. If you are reading 19.5% oxygen and 5% LEL in the ambient environment, there's not enough methane in the atmosphere to explain the difference. That doesn't mean that the atmosphere is safe to enter, just that there's something else displacing most of the oxygen that's missing from the ambient air, as long as there's at least 12.1% oxygen present.
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