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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 Ben Waller on
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OSHA atmospheric monitor standards require that the LEL sensor is calibrated to 100% LEL at 100% of the cal gas LEL, not 10%.
The 10% of 10% rule of thumb is an urban legend.
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Permalink Reply by Jason Brooks on
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That is what I was trying to say. If your meter reads 19.5% oxygen is a space, then there is something there displacing the oxygen. In most confined spaces, this could be Methane. This is provided your LEL readings were not jiving. We also use pentane for everything except PID's, they us Isobutylene. I was not referring to 5% of the LEL, but 5% by volume.
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Buffalo Bill,
I understand what you're saying, but that principle is for inert gases, not flammable gasses or vapors.
If a flammable gas/vapor is displacing the oxygen, the LEL reading, not the oxygen reading is what's important.
If you introduce a non-flammable/inert gas - say helium - into the space at 5% by volume, then it will displace a little more than 1% of the oxygen by volume, as around 80% of ambient air is made of nitrogen and other non-oxygen gases.
In the methane example, if you introduce 5% methane by volume into a space with a monitor that's calibrated to methane, then your LEL sensor is going to read around 100% of the LEL, which is a long way past "no warning".
If your LEL sensor is calibrated to a mid-range gas like pentane or isobutylene, you'll have a CF around 1.5, which will still going to give you a methane concentration of around 3.3% by volume. That 3.3% is over the 50% corrected LEL level in the OSHA standard, and the LEL sensor should still go into alarm.
In sewers, old well shafts, septic tanks, natural sinkholes, etc, where organic decay is a common factor, the displacement gas very well might be methane.
In industrial spaces - mixing vessels, storage tanks, rail cars, etc. the displacement gas is probably not methane. It is much more likely to be whatever the vessel is placarded for or whatever the MSDS for the cleaning solvent being used in the mixing vessel states.
Bottom line - if the LEL sensor is properly calibrated, it's going to pick up a methane mix of 5% by volume at somewhere between 50% and 100% of the LEL, even with an old-school meter that doesn't self-adjust LEL readings in oxygen-deficient atmospheres.
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This is true for calibration. However, the alarms are set to go off at 10% of the LEL. Not 100%
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Permalink Reply by PFC Harmon on
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The ones that we use at my day job are alot more complex than the ones we use at our fire department, I'm not sure of the manufacturer but they are only reguired to be calibrated every 6 months. If there is a question about the accuracy of one of the units we will get another one to check it with, if its wrong we will send it off. On a side note you were right about zeroing and not calibrating.
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Permalink Reply by FETC on
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JB
Sorry but I need to correct a comment. We have 10 different meters, some of them have methane LEL sensors, some with pentane, some natural gas and others with propane LEL sensors as we have a mix of city gas and propane threats within our response district. Some of the newer / cheaper CGI manufacturers have generic LEL sensors that are able to be calibrated to a specific gas from a calibration tank. These meters with generic LEL sensors are cheaper because they can be used and calibrated to many different gases, but they are not as accurate as a type specific LEL sensor because they are formulated by using a CF factor to begin with.
We have 10 different meters in my department and they are all different with different configurations. The key is knowing your meters. Here is where I would like to clarify a comment.
You stated: "Also, most, if not all, meters are set to say 100% LEL at 10% of the LEL that is was calibrated to." this is wrong.
The industry has driven to have all CGI (non-pid) meters ALARM at 10% of any detectable gas. This alarm is the safety factor that your meter has detected a gas and if it is PENTANE gas in the air and your using a pentane LEL sensor then you are actually at 10% of the pentane LEL. Worst case scenario after using the CF factors for many different manufacturers, I have only seen one or two chemicals with a +CF upwards to 3.0; which in reality is 30% true LEL to that specific gas at a 10% alarm but nowhere near the 100% LEL you stated.
The numbers I stated for CF are to correct the LEL % not the actual atmospheric % of that gas in the air by volume... two very different things that need to be calculated by different formulas.
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http://www.heathus.com/_hc/index.cfm/products/gas/combustible-gas-i...
http://www.heathus.com/_hc/index.cfm/products/gas/gasurveyor-500-se...
Here is what we use, they are made by heath consultants. Ours are also calibrated for methane gas.
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The best point that is made here so far is to know you meters. You are 100% correct by this. The biggest problem is that there are way too many people that only know how to turn them on and walk into a situation with them.
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Brooks,
I stand by my comment that "10% of 10%" is an urban legend for LEL monitoring, as well as for calibration.
There are at least two reasons for this...
1) LEL meters do not necessarily alarm at 10% of the LEL, even for the calibration gas.
Most 4-gas meters are either field-programmable or computer-progarammable to allow the calibration technician to select the LEL percantage at which that sensor will put the meter into alarm. The 10% LEL default is an industry convention, but they are not generally locking into this default. Most meters are shipped with this default, but that doesn't mean that's the percentage to which YOUR meter's alarm is set.
2) Even if the low alarm is set at 10% of the LEL, that assumes that the gas being monitored has a LEL of 10% by volume. That is demonstrably not the case, as with your methane example. In methane's case, the LEL alarm is set at "10% of 5%".
For the 10% of 10% urban legend to be accurate, the LEL for EVERY combustible gas and vapor would have to be 10%.
It is not.
There's another hazard with LEL sensors and combustible gases that have very wide explosive/flammable ranges. Some of these gases are combustible in almost ANY concentration - for example, acetylene and ethylene oxide.
The flame arrestor that protects the LEL sensor (at least in some 4-gas monitors) may not be adequate to prevent the LEL circuit from becoming an ignition source in atmospheres containing those two gases. There may be other gases with very wide flammable ranges where this could be a problem, as well. This is a situation where the "10% of 10%" urban legend can be dangerous.
The good news is that acetylene leaks are usually easy to determine from visual cues like the presence of welding gas cylinders. Ethylene oxide is so flammable that it's been said that "If it's leaking, it's burning".
I appreciate your post and the discussion - it's a good one.
Thanks,
Ben
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Here's an example of LEL sensor CF values and technical notes on calibration from RAE Systems:
http://www.raesystems.com/~raedocs/App_Tech_Notes/Tech_Notes/TN-156...
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Same LEL sensor CF values for the Industrial Scientific, for which I was contracted to deliver some specific M40 CGI meter training for a nearby fire department.
http://www.indsci.com/support/tech/tech.aspx?ID=1564
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Permalink Reply by J Mac on
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OK, here is something else to think about…..
Your 4 gas meter reads 20% O2, 0% LEL, 0% H2S, 0% CO.
This could be a bad sensor or could something that is not flammable, H2S, CO, or a possible cross contaminant be displacing the O2?
It’s only 1% right?
1% = 10,000 ppm
But O2 is about 1/5th of the atmosphere……
5% = 50,000 ppm of an unknown
Hmmmm… Something to think about!
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