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Thread: Lower Explosive Limit (LEL)

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    Default Lower Explosive Limit (LEL)

    Wow, been slow here lately. Anyways, I have a question regarding LEL's. So first lets look at the composition of normal breathing air.


    78% Nitrogen and 21% Oxygen. Now let's say we have a space that contains methane at a 100% of its LEL or 5% by volume. Would the methane just displace the O2 or would it also displace the N2? What I am unclear about is, would the 5% methane just displace the O2, and our Gas indicator would read 5% less O2 or 15% O2?

    If yes, why would the methane not displace N2 as well?

    I'm confused...
    Thanks

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    Any gas that would be introduced would displace the components of the air pretty much equally.
    With a 4 gas you may notice a drop in the O2 level. Since we don't have nitrogen meters we don't have a way of measuring that.
    I've measured confined spaces that had our meter reading 100% of the LEL with little to no change in the O2.
    The biggest thing to remember is that 5% of the LEL is a tiny fraction of a gas.
    Methane (for example) has a flamable range of 5 - 15% gas in air (flamable limit) So what that means is that at 100% of the LEL you have 5% gas in air You might drop from 20% to 19% O2

    Hope that helps
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    Quote Originally Posted by ADSNWFLD View Post
    Any gas that would be introduced would displace the components of the air pretty much equally.
    This.

    Under the scenario of 5% of something being in a space, there is a displacement of 5% of the O2 (5% of 21) and a displacement of 5% of the the other 79% of stuff.

    You won't see a drop of 5% in your O2 reading you'd see a drop of 1%.
    I used to be DCFDRescue 2. Forum changover locked me out.

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    Thanks a lot guys. That clarifies things. It would be nice to have a N2 sensor on our gas meters.

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    Default Oxygen level

    Quote Originally Posted by MichaelXYZ View Post
    Thanks a lot guys. That clarifies things. It would be nice to have a N2 sensor on our gas meters.
    Good morning Michael.

    Assume for a minute that when performing an atmospheric test of a confined space, the oxygen level drops from 20.9% to 19.9%. According to OSHA, as long as it doesn't go below 19.5% it is acceptable to make an entry.

    The drop in oxygen can be from two things. It was either consumed (rust, corrosion, hot work, mildew, slime, etc.) or it was displaced by another gas. Again, assuming that is the worst, oxygen displaced by another gas, the atmosphere could be well above the IDLH with just a 1% drop in the oxygen level. 1% of any gas = 10,000 ppm.

    Since oxygen accounts for approximately 21% of the gas mixture and nitrogen accounting for approximately 78%, a displacement of 1% oxygen also means that we have displaced 4% of the nitrogen since there is 4 times as much nitrogen than oxygen. This now means that a 1% drop in the oxygen level = a 5% total gas mixture drop which now equals 50,000 ppm of the displacement gas.

    Ventilate and wear appropriate respiratory protection.

    Mike Dunn
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    Default Gas detection

    Since we are on the subject of gas detection..........

    There is no monitor on the market that will tell you everything you want to know about the atmosphere in a confined space. A standard 4-gas meter (any brand) measures the oxygen level, the lower explosive limit, carbon monoxide and hydrogen sulfide because those are the 4 most common gases found in confined spaces. There are literally thousands of gases used in industry but our most common monitors only detect 4 of them.

    If the oxygen level drops the monitor doesn't tell you why. It could be because of a nitrogen leak (the #1 killer in industry), it could be carbon dioxide or lots of other non-flammable gases. The meter is preset by the factory to alarm at 19.5% and 23.5%. Unless the user has the ability to change the alarm levels it presents a problem in the maritime industry where high oxygen is 22%, not 23.5%. A new generation meter has an accuracy on the oxygen side of plus or minus 0.5%.

    The LEL meter is designed to measure the % of flammable test gas up to the LEL. Most monitors are calibrated with either pentane or methane and they read the test gas fairly accurately. They do not necessarily read flammable gases other than the test gas accurately. The maker of the monitor usually provides a chart indicating what the actual concentration is when measuring a gas other than the test gas. For example, a monitor calibrated with methane may have a 2.5 conversion factor when measuring xylene. An LEL reading of 40% on the monitor would be multiplied by the conversion factor to get an actual concentration of 100% which means we now have an explosive atmosphere. A new generation meter on the LEL side has an accuracy of plus or minus 5%.

    The CO sensor is supposed to measure the amount of carbon monoxide in the atmosphere and is commonly displayed as parts per million (ppm). The CO sensor is easily faked out and many other gases (hydrogen, acetylene, alcohols, solvents) can cause it to indicate CO when there is no CO present.

    The hydrogen sulfide sensor can also be faked out but those gases that fake it out smell like H2S (for those who still have a sense of smell).

    I do contract work (teaching and rescue standbys) in industry all over the world and atmospheric monitoring is universally the most screwed up part of the confined space entry job. Workers don't know how to calibrate the monitors, they don't know how to interpret the readings, they don't know how to properly perform an atmospheric test or they are just plain lazy.

    Every FD responding in to a confined space emergency needs to have the capability to conduct their own atmospheric testing for the safety of the firefighters. Ok, I'm off my soap box now. LOL.

    Mike Dunn
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    So I have a question about correction factors. We calibrate our gas meter with methane, but the published correction factors for our meter are for pentane. Using the CF for pentane to methane is .47

    So if I am calibrated for methane but have pentane CF numbers, and I am measuring pentane how do I apply the CF numbers?

    Did that make sense?

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    Quote Originally Posted by MichaelXYZ View Post
    So I have a question about correction factors. We calibrate our gas meter with methane, but the published correction factors for our meter are for pentane. Using the CF for pentane to methane is .47

    So if I am calibrated for methane but have pentane CF numbers, and I am measuring pentane how do I apply the CF numbers?

    Did that make sense?
    Yep, it makes perfect sense. To get a reading as accurately as possible you need the proper conversion chart for gases other than the test gas. For example, if calibrating with methane (pretty common) you need a chart to indicate conversion factors for gases other than methane. If calibrated for pentane you need a chart for gases other than pentane.

    Note: This chart only works with the gas the monitor is actually calibrated with. It may be wildly inaccurate if you calibrate with methane and then read the conversion factors as if it was calibrated with pentane.

    If you don't have the proper chart for your meter, check first with the manufacturer (most of them have data online you can access) or call the manufacturer and find someone there that can help you out.

    As always, no meter will tell you everything you want to know about that atmosphere in a confined space. If in doubt....pack out.

    I tried putting a couple of different conversion charts on here but the formatting was all screwed up and I don't have the computer skills to make it look good. If you are interested, send me a fax # or an address and I'll send it to you.

    Mike
    Last edited by rsqman; 05-10-2013 at 08:18 AM. Reason: correct chart

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    Well I had called the gas indicator tech support. It was the tech guy that told me we had to use pentane gas for calibration. After going to their website I found this LEL CF chart.
    http://www.indsci.com/services/train...l-correlation/

    So that chart answered my questions.

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    Quote Originally Posted by MichaelXYZ View Post
    Well I had called the gas indicator tech support. It was the tech guy that told me we had to use pentane gas for calibration. After going to their website I found this LEL CF chart.
    http://www.indsci.com/services/train...l-correlation/

    So that chart answered my questions.
    Yep, that's the chart I was trying to send you. Have a great weekend and stay safe.

    Mike

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    Quote Originally Posted by rsqman View Post
    Yep, that's the chart I was trying to send you.
    Don't forget to read the fine print on that conversion chart....the part where it says it is accurate to plus or minus 25%!

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    Quote Originally Posted by rsqman View Post
    Don't forget to read the fine print on that conversion chart....the part where it says it is accurate to plus or minus 25%!
    Sure, but since we alarm at 10% LEL we still have a big safety margin yes? What I don't get is the minus 25%

    So if we have a meter calibrated for methane in a pentane environment and we are reading 10% LEL use our CF of 1.9 for a pentane reading of 19% but use the +/-25% we can have -6% up to 44%

    The low end number is not possible, but on the high end we still have 100% - 44% safety margin. Or am I confused?

    This is a really helpful discussion. Thanks

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    Quote Originally Posted by MichaelXYZ View Post
    Sure, but since we alarm at 10% LEL we still have a big safety margin yes? What I don't get is the minus 25%

    So if we have a meter calibrated for methane in a pentane environment and we are reading 10% LEL use our CF of 1.9 for a pentane reading of 19% but use the +/-25% we can have -6% up to 44%

    The low end number is not possible, but on the high end we still have 100% - 44% safety margin. Or am I confused?

    This is a really helpful discussion. Thanks
    Nope, you are not confused yet although it can be a confusing subject. What the + - 25% really means to us end users in the field is expect your monitor to be off as much as 25% or more depending on if it was properly calibrated, the test properly conducted, the proper conversion factor applied, the proper gas detection cartridge used, etc. etc. It could be off on the upper end or it could be off on the lower end or it may even be accurate. Because the gas cloud we are measuring is seldom static (non-moving), the 44% reading we are getting right now may change very suddenly to 100% as the air currents shift the concentrations in the gas cloud around.

    Will ventilation help or hurt? It could reduce the concentration of flammable gas below it's LEL making it much safer for us or it could inject air into a too rich to burn gas cloud making it potentially deadly for us. Don't get too hung up on trying to put an exact number on the meter. Recognize that the meters, calibrations, conversion charts, etc., may be perfect in a laboratory setting but our emergencies seldom happen under laboratory conditions.

    When the alarm goes off for low oxygen, high oxygen, high LEL, high CO, high H2S or high whatever cartridge we use it means danger. Plan your response accordingly and keep everyone safe.

    Mike

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    Quote Originally Posted by rsqman View Post
    Don't forget to read the fine print on that conversion chart....the part where it says it is accurate to plus or minus 25%!
    That 25% is not 25 percentage points. It's 25% of the reading. So, if the reading is 10% of the LEL, you're looking at a range of 7.5-12.5% of LEL.

    Here's the tricky part- what gas are you reading in there? You need to know what gas or gases you're reading to even use the conversion chart.

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