sense / nonsense of CO monitoring

[wstorms]

Hi all,

I am wondering about the sense / nonsense of CO monitoring. Obviously CO in your gas is bad, but at what value? When would you breath a gas and when wouldn't you? I tried to come up with it myself, but the extremely tight tolerances puzzle me:
- CO is natural, 5-10 ppm isn't unusual. Low concentration like that (at atmospheric pressure) is ok
- at depth however, the same concentration can become a problem, so it's a good idea to filter as much as possible (all clear so far)
- ANDI defines 2 ppm CO as the max for a breathing gas (I haven't found other norms from other agencies), so I am considering 2 ppm as the max safe amount
- Even CO sensors specifically for diving purposes like the DE- OX SAFE have tolerances of 2-4 ppm or more, so potentially twice the max level. A deviation +/- 1% isn't unusual
- sensors are usually least reliable at the extreme ends of the scale, so there is even extra uncertainty if you want to measure so close to 0 ppm

So it would easily be possible to have 4 ppm in your actual gas, but still measuring 0 ppm. If you can have double the maximum amount of CO, but still have a "safe" reading, how valuable is the measurement? Do you simply consider a reading of any CO as a "no go" for the gas?

 

[tbone1004]

The filters should be taking all CO out. If the sensors see anything, they should be generating current, so if you have 0ppm on the sensor, it should be 0ppm. If you have any reading, then yes 1ppm could well actually be 4ppm, and because of that, many people have 0 tolerance. There are some compressors that may not have chemical filters to convert CO, like a stock Rix compressor, where anything that is in ambient air will continue to the final gas, so you have to be cognizant about whether you have a 0-tolerance for CO or not. Much of this will also depend on what bump gas you use to calibrate. Similar to nitrox, you don't want to calibrate your nirox analyzer on 100% O2 when you are trying to test for EAN32. Conversely, you don't want to calibrate on air when you are trying to analyze 100% O2.

 

[wstorms]

Did not know that 0 actually means 0 in this case, good to know, thanks!
So 0 ppm can actually be considered reliable, anything else is a knockout? I guess having a chance to catch bad gas is better than not having that chance at all...

 

[tbone1004]

Did not know that 0 actually means 0 in this case, good to know, thanks!
So 0 ppm can actually be considered reliable, anything else is a knockout? I guess having a chance to catch bad gas is better than not having that chance at all...

all depends on your personal tolerance. If I'm doing less than 100ft dives, my tolerance is a lot better than 100m dives.

 

[wstorms]

I understand what you are saying, but for me, having a potential value 3 times higher than the max (I guess max is slightly grey as well) would be too much for comfort, especially since all compressors where I might fill should filter it out to start with.
I'm playing with the idea of building a raspberry pi based gas lab. More because of covid boredom than any actual practical reason. So far O2 is pretty easy to read, I found a sensor that can detect 2.5 micron particles, will look for a suitable CO sensor as well. He might be too much over my head for a DIY project. Although I am not sure I trust my own engineering skills enough to rely on a simple setup like that, it sure is fun to tinker

 

[kelemvor]

CO is natural, 5-10 ppm isn't unusual.

I've often heard that. However, I've been regularly checking my tanks for CO for about 2 years now. Maybe 3. I've only once got a reading of 1ppm, and never anything higher. I think the "5-10ppm is normal" is an urban legend or maybe just outdated thinking.

When I got the 1ppm, I told the shop owner and he promptly rebuilt his filter stack or whatever compressor owners do. He cleaned my tanks, refilled, and warned other customers who had recently bought a fill. All the things I'd expect of a responsible fill station. I was pleased.

My risk tolerance is 0ppm without question. I've tested in Roatan (Anthony's key fill station) and Cozumel(merry-d), and a variety of shops in FL and never got a 1ppm or greater CO reading.

If they can get fills right in Honduras, fill stations anywhere should be capable.

 

[tursiops]

I've often heard that. However, I've been regularly checking my tanks for CO for about 2 years now. Maybe 3. I've only once got a reading of 1ppm, and never anything higher. I think the "5-10ppm is normal" is an urban legend or maybe just outdated thinking.

When I got the 1ppm, I told the shop owner and he promptly rebuilt his filter stack or whatever compressor owners do. He cleaned my tanks, refilled, and warned other customers who had recently bought a fill. All the things I'd expect of a responsible fill station. I was pleased.

My risk tolerance is 0ppm without question. I've tested in Roatan (Anthony's key fill station) and Cozumel(merry-d), and a variety of shops in FL and never got a 1ppm or greater CO reading.

If they can get fills right in Honduras, fill stations anywhere should be capable.

I tested in Mexico (Villas de Rosa) this January. 0 ppm. Owner was pleased....said "good to know!"

 

[rhwestfall]

As someone posted elsewhere, there was a good article in DAN (First Quarter 2020) addressing the effects of CO, and the result of density changes. Helped me define "acceptable limits".

 

[Wookie]

I had CO-Clear analyzers on my air compressors, with a range of 0-10 PPM and an accuracy of + 1 PPM. They usually ran at about 0.1 to 0.2 ppm, and the alarm was set to 0.5 PPM. I never had an alarm. You use the hopcalite in the filter to make the CO zero and the CO monitor to warn you if the hopcalite goes south, IMO.

 

[wstorms]

I've often heard that. However, I've been regularly checking my tanks for CO for about 2 years now. Maybe 3. I've only once got a reading of 1ppm, and never anything higher. I think the "5-10ppm is normal" is an urban legend or maybe just outdated thinking.

I meant in regular, uncompressed, unfiltered natural air. It seems to be backed up as a common value in populated areas by the local governments environmental department