Originally Posted By: Caoimh?n P. Connell
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Hi Gents ?
Here is a take from a different perspective.
First of all, heavier than air gases and lighter than air gases will behave exactly the same, they will both perfectly mix in a room and their concentrations will be exactly the same regardless of their intrinsic molecular mix. Heavier than air gases will not settle ? nor will lighter than air gases raise and accumulate in the rafters. Even if you were to carefully fill a balloon with a gas whose molecular weight was say... 30 g/mole and then carefully fill the bottom of the balloon with a another, heavier gas whose molecular weight was, say, 45 g/mole, in very short time, the two gases would perfectly mix and the concentration of the mix would be identical at every point in the balloon. This is true for sealed systems such as bolloons and will occur even quicker in rooms where there are air currents, convection gradients, etc. For all practical purposes, the settling stuff is just myth ? (Having said that it can occur under very special industrial circumstances but not in any housing situations that you will encounter - or if it has occured -
RUN!)
Next ? There are two primary types of CGA on the market ? Pellistor and flame ionization detectors. Neither is capable of identifying a gas and, remarkably, neither is capable of actually telling you what the LEL of gas present is, or how close one really is to an explosion. For the moment, I?ll focus on the pellistor type, since they are the most common. Furthermore, unless one has an oxygen meter, one cannot make ANY statement as to the LEL, or how close one is to the LEL.
The meters I have seen used by home inspectors are "go- no go" meters. They are of little value, under most circumstances, in my opinion, since there is no quantification whatever. They may be of limited use for inspectors whose noses are no longer working, since the mercaptans and thiols are detectable at very low concentrations.
Now, most gases have an LEL of between about 1.5% and 15%. The beauty of a CGA is that the heat of combustion for all gases is
more or less same regardless of the specific LEL, and therefore, oxidizing the gas on a pellistor will produce
about the same amount of heat and, therefore, when applied to a Wheatstone bridge, will result in the same loss of conductance ? giving a reasonably reliable indication of how close one
MIGHT be to the LEL.
However, if one measures even 4% LEL, that would be a HUGE problem in an house since at even 4% LEL, that means there is approximately 880 parts per million in the atmosphere. At this concentration, the odour of thiols and mercaptans (the stinky stuff added to fuel gases) would be very powerful indeed.
Next, as mentioned above, the heat of combust is more or less the same? not really, but close enough for explosion work, where we usually build in HUGE margins of error. The relative response of propane to a CGA calibrated on methane is only about half ? that means that it takes twice the concentration to get the same reading ? thus 4% LEL with a methane calibrated GCA means 8% LEL propane! (Typically, I evacuate at 10% LEL).
The issue becomes more complicated in the case of measuring the LEL in closed quarters or confined spaces or at altitude (I sometimes perform work in homes and workplaces at 13,000 feet). Even at sea-level, a reading of say, 4% LEL propane could indicate either 880 ppm or it could indicate that one has so much propane that the concentration is now above the UPPER explosion limit, UEL, and the propane has displaced the oxygen (which in the case of propane begins at about 10%). So, 4% LEL without the benefit of an O2 meter, is a useless number, and warrants caution ? which is why a CGA should never be used in the absence of an O2 meter, since the reading of the GCA is incumbent on two things 1) concentration of the combustible gas, and 2) concentration of O2.
Finally, the toxicity of most fuel gases are relatively low ? for example, contrary to common belief, methane is an odorless, harmless gas. One could breath an atmosphere of 80% methane and 20% oxygen without any harmful effects (provided there was a little CO2 thrown in to ensure proper blood acid balance). Similarly, one could breath in thousands of ppm of propane without ill effects. But why would you do it, eh?
So, from an industrial hygiene perspective, little machines that go ?click, click, click? are of little use, and warrant less consideration unless you have bothered to correlate the frequency of the clicks to a concentrations of a known gas, at a known altitude, at a known relative response.
But then, I go in to buildings with explosive atmospheres all the time, so how smart could I be, eh?
Cheers,
Caoimh?n P. Connell
Forensic Industrial Hygienist
www.forensic-applications.com
(The opinions expressed here are exclusively my personal opinions and do not necessarily reflect my professional opinion, opinion of my employer, agency, peers, or professional affiliates. The above post is for information only and does not reflect professional advice and is not intended to supercede the professional advice of others.)
AMDG
