Don, I know that it is one of your crusades, and a good one too, but the idea that only a single tank was contaminated kinda sticks in my craw.
It is actually quite common and likely it was the only tank that had a concentration which was high enough to cause symptoms at that depth, pressure, and time, but if one actually checked the entire run of tanks before and after the suspect tank you would find lower concentrations that may have only resulted in sub-clinical or no symptoms.
At the Montreal fire service in the 1990's the firefighters were complaining of decreased exercise tolerance and early fatigue. A young toxicologist researcher was called in who looked at the constellation of symptoms and decided to test a few scba tanks for CO and sure enough the first ones at one station had levels in the range of 0 to 30 ppm. The testing was expanded to other stations and tanks were found with concentrations as high as 250 ppm. These were young healthy firefighters so a tank at 1 atmosphere with only 10 ppm would not cause problems but those with CO concentrations in the 50 ppm to 250 ppm range were the ones causing early exertional fatigue. It was not until about 70 tanks were tested that the full range of contamination was identified with concentrations from 0 to 250 ppm CO. I seem to recall about 50 tanks had some level of CO contamination out of several hundred.
The suspect "electric" compressor was identified and a CO monitor was installed to measure CO concentrations in real time. For the first three hours under load there was no CO produced but at the 3 hour mark under full load the compressor would start to produce CO. This is the problem with any sort of air testing in that it is rarely done at full load and under worst case scenario conditions. A single grab sample after starting the compressor under no load will not detect the contamination. There were multiple factors identified which lead to the CO production which included:
1. poor quality mineral oil with low autoignition point
2. poor compressor install with inadequate ventilation which lead to overheating
3. narrow remote intake which create sub-atmosperic pressure at the intake and compressor overheating
4. excessive moisture in the compressed air which prevented Hopcalite from converting the CO to CO2.
Most of this is detailed in this paper:
Carbon monoxide and water vapor con... [J Toxicol Environ Health. 1997] - PubMed - NCBI
There was another CO incident described in Dive Training magazine a few years back where only two tanks were affected. The first diver was much more symptomatic as he had breathed his tank down to 500 psi the day before while the second affected diver still had half a tank before the compressor burned the lube oil. The concentration of CO in the first diver's tank would have been much higher. The rest of the divers whose tanks were filled earlier in the run were not checked, however they did not show any symptoms of CO poisoning and these asymptomatic diver tanks may have had no CO or only low concentrations.
The article states that the tank was sent to the lab and upon speaking with the author the incident tank did show high levels of carbon monoxide. The compressor was examined and the 4th stage piston had seized up.
Please note that this diver was resting on the bottom at only 20 feet for a short period of time before he began to experience symptoms.
August 2008 Volume 18 Number 8
In these sorts of CO contamination incidents until one actually measures the CO concentration in the the other tanks it is not possible to say only one tank is affected.
