suprane
Contributor
2) When should I be alarmed and refuse the tank, at a measurement just different from 0 ? at 5ppm ?
The first thing you need to think about is the fact that for every atmosphere that you descend the partial pressure of all gasses in the mixture you are breathing increases by a factor of one. That is to say that if you are a breathing 21% O2 mix (ie: compressed air) at 10 metres/33 ft it has the same effect on your body as breathing 42% O2. And at 20 meters/ 66 ft it now exhibits the effects of 63% O2. More simply, it is the fraction of the individual gas multiplied by the number of atmospheres of depth.
The same applies for all gasses in the mix. Therefore, as you descend the toxicity of the CO increases correspondingly. The other thing to keep in mind is that with CO it is not just a matter of how much but also for how long the exposure lasts! The longer you are exposed to it the more hemoglobin it has a chance to occupy. So, a long exposure at a moderate level may prove more toxic than a very brief exposure to a higher concerntration.
For healthy adults, CO becomes toxic when it reaches a level higher than 50 ppm (parts per million) with continuous exposure over an eight hour period.. When the level of CO becomes higher than that, a person will suffer from symptoms of exposure. Mild exposure over a few hours (a CO level between 70 ppm and 100 ppm) include flu-like symptoms such as headaches, sore eyes and a runny nose. Medium exposure (a CO level between 150 ppm to 300 ppm) will produce dizziness, drowsiness and vomiting. Extreme exposure (a CO level of 400 ppm and higher) will result in unconsciousness, brain damage and death.
The following figures come from wikipedia.com:
Concentration Symptoms
35 ppm (0.0035%) Headache and dizziness within six to eight hours of constant exposure
100 ppm (0.01%) Slight headache in two to three hours
200 ppm (0.02%) Slight headache within two to three hours; loss of judgment
400 ppm (0.04%) Frontal headache within one to two hours
800 ppm (0.08%) Dizziness, nausea, and convulsions within 45 min; insensible within 2 hours
1,600 ppm (0.16%) Headache, tachycardia, dizziness, and nausea within 20 min; death in less than 2 hours
3,200 ppm (0.32%) Headache, dizziness and nausea in five to ten minutes. Death within 30 minutes.
6,400 ppm (0.64%) Headache and dizziness in one to two minutes. Convulsions, respiratory arrest, and death in less than 20 minutes.
12,800 ppm (1.28%) Unconsciousness after 2-3 breaths. Death in less than three minutes.
I am not aware of any reliable, well tested crtieria that says do not dive a tank with greater than (fill in a number) ppm CO. At this stage of our working knowledge of CO poisoning as applied to diving I think each individual sort of has to determine what degree of risk (within reasonable limits!) that he/she is willing to accept.
I, personally, would be very reluctant to dive any tank with greater than 10 ppm to any depth greater than 3 atmospheres. And I really get pretty uncomfortable when the number creeps above 7 or 8 ppm.
I hope this helps your knowledge and decision making somewhat.
I currently use a ToxiRae 3 analyser to test all of my tanks. And, will continue to do so! It is calibrated with 10 ppm prior to each trip.
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