Acceptable CO Levels?

[betti154]

Guys,

Just wondering what people consider an acceptable level of carbon monoxide (CO) in a filled a scuba tank?

I was under the impression that anything over 3ppm was unacceptable and should be rejected. This is based on hearsay and doco from the analyzer company (analox in this case, Uk standard I believe).

We've been diving OC in Mexico for a week and have seen analysed CO of 2ppm. Today we hit 7-8ppm consistently for all 6 cylinders we got filled. We rejected them, but all the shop's other 32% cylinders were in same range, so we switched to air as they were all 2ppm (presumably filled a different day). Our analyzer was calibrated, and provided the same values as the shop's analyser.

After our dive the shop in question still had all their 32% cylinders (20-30 11L alloys) in the ready pile and as far as I could tell were going to rent them out to the next punters that came along. They stated that it's normal, and that whilst working in Madagascar they'd seen figures up to 12ppm and had no problems there so shouldn't be an issue here.

I understand there is a depth implication here, so let's assume a MOD of 40m for these tanks in question. Alarm bells start to ring in my mind, but I've very little experience with CO.

Your thoughts?

Regards, Damien.


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[gschaut]

Depending on WHO'S standards you are using, the upper acceptable limit for CO is 2ppm, 5ppm or 10ppm.

But the simple fact is that a PROPERLY operating Hopcalite catalyst on an air compressor can draw the CO level down to below 1ppm.

So ANY CO reading above 1-2ppm would indicate improper filtering on the compressor.

The primary causes of problems with Hopcalite catalysts are excess temperature (>200c) and possibly hydrocarbon (oil vapor) contamination.

In any case, I would personally be very suspicious of any compressor putting out more than 2ppm CO.

[scububa]

Is there anything out there that would indicate the monitors read different for the same level of CO in different levels of O2?
(i.e. is there any thing about higher oxygen that could bias the sensor?)

[gschaut]

Is there anything out there that would indicate the monitors read different for the same level of CO in different levels of O2?
(i.e. is there any thing about higher oxygen that could bias the sensor?)

The only reference I can find to false CO readings is from... Hydrogen. Some types of CO sensors will read hydrogen as a false CO reading of about 1/10th the hydrogen concentration. i.e.- an atmosphere containing 100ppm Hydrogen would give a false reading of 10ppm CO where no CO is present.

The paper suggests that the most common source of free hydrogen contamination in the air is.... Human flatulence.

Let the jokes begin...

[kwinter]

From the Analox EII CO manual:

2.6 Maximum allowable CO exposure levels
The maximum allowable CO exposure levels for air diving vary in the standards set form one
country to another, so you may want to consider referring to the applicable safety standards
for your county to determine your maximum allowable CO exposure level.
However, Analox advises that the maximum allowable CO exposure levels should be to that
stated in the British Health and Safety Executive (HSE) DVIS9(rev1) document (Diver’s
breathing air standard and the frequency of examination and tests (Diving Information Sheet
No 9 (Revision 1)).
This states that: Carbon monoxide content shall be as low as possible but not exceed
3 ppm. (For air diving to 50metres)

I recently had a situation where my booster started leaking and I tested the gas and got 7 ppm for CO. I dumped the gas and flushed the tank. I see no reason for high O2 content to alter the CO reading.

[Rhino]

The O2 content effect I've seen is with an Analox CO Analyzer showing - (negative) CO levels. This has happened more than once, and been as high as -12, but only when analyzing 100% O2. Responding to my initial concerns, someone at Analox (I forget who, but she identified herself)hypothesized that it may be the O2 clean materials used in the valve that could cause the false readings. Personally, since the analyzer is not reading CO, and obviously is not detecting negative CO, I don't consider this a problem. Others have expressed contrary opinions.

[Bobby]

I will not accept anything over 2ppm. It is not only the presence of CO that is a possible problem. If oil vapor is the cause there can be very high levels of CO2 as well. The filters, if working properly, will convert CO into CO2. When seeing such high CO readings there can be a lot of CO2 as well which combined with the CO can cause rather bad effects even at shallow depths. IMO you took the best path of rejecting the cylinders and finding alternative gas with low CO readings. Many days of diving stress the body and adding to it with CO and/or CO2 is unpredictable at best.

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[betti154]

The next morning the shop had dumped all cylinders and banks and refilled everything. I'm not sure if something was originally lost in translation, but good too see a proactive position. Talking to the owner they take it quite seriously and don't accept anything about 2ppm.

The shop's original position of 6-8ppm being acceptable was surprising, as they have a great setup, their own CO analyser, relatively new compressors, what appears like more than adequate filtration and an active log of filter changes (no where new due for change either).

All good in the end, but good example of the need to analyse for CO. thanks Bobby!

regards, ds