I am currently doing a research project on the purity of divers' breathing gas, and noticed that some standards specify a limit on methane. From my understanding, methane is an inert nontoxic gas that does not pose a hazard unless the concentration reaches at least 50000 ppm. Yet the standards on methane in breathing gas establish a limit of 25 ppm. Can anyone shed some light on the hazards of methane when it comes to diving??
Hazards of methane contamination of breathing gas?
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I can't speak specifically about the methane and the numbers you quote, but I can speak about one concept in general about contaminants in gas while diving.
When a diver descends, the amount of gas going into the lungs increases with depth because, in accordance with Boyle's law, it takes more gas to inflate the lungs fully as pressure increases. Every 33 feet of sea water increases this amount by another atmosphere. Thus, if I am breathing a gas at 132 feet (5 atmospheres of pressure, counting air), whatever contamination was in the gas to begin with is effectively multiplied by 5. A gas that is well within normal limits for contaminants on the surface could be lethal at depth. (The most common concern for this is carbon monoxide.)
When a diver descends, the amount of gas going into the lungs increases with depth because, in accordance with Boyle's law, it takes more gas to inflate the lungs fully as pressure increases. Every 33 feet of sea water increases this amount by another atmosphere. Thus, if I am breathing a gas at 132 feet (5 atmospheres of pressure, counting air), whatever contamination was in the gas to begin with is effectively multiplied by 5. A gas that is well within normal limits for contaminants on the surface could be lethal at depth. (The most common concern for this is carbon monoxide.)
Mr Carcharodon
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They are most likely trying to control contamination from other hydrocarbons indirectly. Methane is the lightest fraction, most volatile, and easiest to detect. The heavier molecules are likely more of a concern since they can build up and ultimately be a source of combustion, but because they are less volatile they are harder to detect in the gas phase.
paddler3d
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It is all about Boyle's Law!
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50,000pppm (5%) has NOTHING to do with the toxicity of breathing methane. That is the lower explosive limit, aka LEL. I.e. above the LEL amount there is enough fuel for it to explode. Above 15% methane is known as the upper explosive limit - >15% methane there is too much fuel and not enough oxygen to explode. These are only measured in normal room air.
Toxicity is a completely different issue. Methane is essentially non-toxic and "just" an asphyxiant but if you are generating it in a compressor something else is going on. Also given the huge temperatures and much higher partial pressures of oxygen in a compressor the LEL is undoubtably lower, how much nobody knows. You would not want methane around in a hyperbaric chamber, mixing with nitrox, or in a compressor of any kind. 25ppm is just a WAG (probably a method detection limit) because methane should easily never be found in breathing gases.
DevonDiver
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Don't feed beans to your compressor operator....
Thank you for everyone's input. I have taken into consideration the effect of increasing pressures on the behaviour of the gas. Most standards on contaminants of compressed breathing air are approximately 1/10 of their usual limit at ambient temperature/pressure. For instance, the ACGIH limit on nitrous oxide at normal pressure is 25 ppm; the limit on nitrous oxide in divers' breathign gas is 2.5 ppm. I guess my question was why is the limit on methane specifically 25 ppm; what scientific basis is there? To have a toxic effect at normal pressure, the concentration of methane would have to reach beyond 30% (300,000 ppm). Even if you account for the pressure difference on land versus under water, the difference between the 25 ppm limit of methane underwater and the 300,000 ppm toxic threshold of methane on land is substantially greater than the usual 1/10 rule... I hope I'm making sense.
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