Quiz - Physics - Partial Pressure of Carbon Monoxide

[CosbySweater]

While this doesn't totally answer the question, this was the data I was talking about in the link I posted on the previous page regarding ppCO, exposure times, and effect:
View attachment 578043

That would suggest a maximum partial pressure on a single tank dive would probably not want to be higher than about 0.015% to remain within the "no effect" zone.

If we go off a 132 ft/5 atm dive, that would mean a surface reading of ~.003%(30 ppm) would be about the maximum threshold for a single tank dive on air.

Or to approach it with other numbers from OSHA/WHO:


View attachment 578048

If we instead go off of the WHO 1 hour average limit for 1 hour as our single tank limit. That'd put us at 30 ppm (.003% ppCO) at depth, so a safe surface level reading to be within the WHO 1 hour average limit would be a surface level reading of no more than 6 ppm or .0006%.

This is exactly what I was looking for! Thanks for sharing and thanks for the example. Clearly, the 160ppm I used in my example above was WAY too high.

What standard does the scuba industry use? Do the agencies, the RSTC, or other industry bodies have a recommended standard? Or is the recommended standard just 0ppm?

 

[saxman242]

Or is the recommended standard just 0ppm?

I'd imagine that is the recommended standard, but I don't know what the maximum allowable threshold is (or if there even is a solid number).

 

[DandyDon]

From the beginning we've been taught about the dangers of CO in diving due to the increased partial pressures at depth, but I can't recall receiving any guidance on just how much is too much. Nor can I recall being taught how to analyze a cylinder for CO.

No, Padi and the other agencies, as well as DAN, are still failing in this.

All that being said, I appreciate your answer. Somewhere between 3-10ppm. What number are you looking for when you analyze a tank? 0?

Zero or one ppm is expected as it's very easy to round up a digit even with a well-tuned tester.

That would suggest a maximum partial pressure on a single tank dive would probably not want to be higher than about 0.015% to remain within the "no effect" zone.

Sorry, I did not like the confusing chart, but then I don't like your thinking either. NO! 0.015% is 150 ppm, which is 15 times the maximum allowed. CO toxicity, binding, changes with changes in pressure are all more complicated than that.

This is exactly what I was looking for! Thanks for sharing and thanks for the example. Clearly, the 160ppm I used in my example above was WAY too high.

What standard does the scuba industry use? Do the agencies, the RSTC, or other industry bodies have a recommended standard? Or is the recommended standard just 0ppm?

No and no.

I'd imagine that is the recommended standard, but I don't know what the maximum allowable threshold is (or if there even is a solid number).

As I said earlier in the thread, 3, 5, or 10 ppm depending on which country's regulations are used. Here is an indepth explanationL: Carbon monoxide - Wikipedia

 

[saxman242]

Sorry, I did not like the confusing chart, but then I don't like your thinking either. NO! 0.015% is 150 ppm, which is 15 times the maximum allowed.

I should clarify that my statement about .015% is purely based on that chart. For that chart, a 1 hour exposure at .015% would fall safely in the "no symptoms" range.

I don't think that's the right answer, but based purely on *that* chart it is.

That's why I followed up with the OSHA/WHO numbers and calculated a second time off of those values.

 

[Angelo Farina]

Whilst the partial pressure of CO at 40m depth is definitely 5 times larger than the p.p. at surface, I am absolutely not sure that the effect of CO is proportional to its partial pressure.
Most of us have knowledge about the toxicity of Oxygen, and for this is widely accepted that the toxicity is triggered by its partial pressure. So we evaluate the maximum allowed depth of a gas mixture by ensuring that ppO2 stays below a given threshold, say 1.5 bar.
But the mechanism for the toxicity of CO is very different than the toxicity of O2. CO is toxic because it creates a permanent chemical bond with haemoglobin, so it inactivates it for transporting oxygen in the blood stream. This toxicity is actually at least partially counterfeit by increasing the p.p. of Oxygen (in fact the typical treatment for CO poisoning is hyperbaric oxygen).
This means that, as you dive deep, the larger quantity of oxygen available makes CO LESS TOXIC than at the surface. Of course there is also more CO, but as soon as its quantity is enough for entirely inactivating haemoglobin, the more CO present has no effect on the human body, as all the possible damage is already done.
Instead, the larger availability of O2 dissolved in the blood (instead of being bonded with haemoglobin) can provide oxygen to your brains even if all the haemoglobin has been inactivated.
Considering that the starting point was an already terribly high percentage of CO, my supposition is that breathing such an highly toxic mixture it is better to be deep than at the surface.
Staying deep you still get some oxygen through solution in the blood, while at surface you would will die immediately.

 

[saxman242]

As I said earlier in the thread, 3, 5, or 10 ppm depending on which country's regulations are used.

Not to claim that those aren't correct guidelines to follow, but for the sake of argument, I think it's important to clarify that those regulations are for the "safe" indoor rates for long term exposure, not the time weighted average rates for short term exposure as one would experience while diving.

 

[saxman242]

As an aside, this discussion definitely has me wanting to pick up a CO meter. @DandyDon , I've noticed you mention in the past using a CooTwo. Considering they appear to no longer exist, have you found that sensors are still readily available for yours? I imagine they use generic sensors, but haven't found any documentation on what they use.

 

[DandyDon]

This means that, as you dive deep, the larger quantity of oxygen available makes CO LESS TOXIC than at the surface.

Ha! You have to ascend sometime, then the devil gets his due.

Not to claim that those aren't correct guidelines to follow, but for the sake of argument, I think it's important to clarify that those regulations are for the "safe" indoor rates for long term exposure, not the time weighted average rates for short term exposure as one would experience while diving.

No! Those are maximums by regulations for scuba tanks, varying by country. Or my quoted numbers are. Maybe you are referencing yours.

As an aside, this discussion definitely has me wanting to pick up a CO meter. @DandyDon , I've noticed you mention in the past using a CooTwo. Considering they appear to no longer exist, have you found that sensors are still readily available for yours? I imagine they use generic sensors, but haven't found any documentation on what they use.

Yeah, there is a new servicing company along with a thread on the discussion. There is another unit I like better, but the name escapes me for the moment. There are threads.

 

[justadyver]

I think Dan has an article in there last magazine on this very subject co and partile pressure

 

[saxman242]

No! Those are maximums by regulations for scuba tanks, varying by country.

Gotcha. I've had a hell of a time finding any references to specifically scuba tanks. The searching I did was only able to come up with maximum recommended long term exposure levels. In the US, the max indoor CO level recommended by the American Society of Heating, Refrigeration and Air Conditioning Engineers in a voluntary building industry standard (ASHRAE 62.2) is 9 ppm.

Interestingly, in the US and Canada, consumer household CO monitors are allowed to show "0" for any value under 30 ppm.