Analyzing multiple nitrox tanks

[yle]

you do NOT recalibrate between tanks, certainly not based on what they were saying. You calibrate once, and you wait until the reading stabilizes, that may very well take 15+ seconds depending on the o2 sensor, but you have to be patient.

True. The whole issue would be avoided if people would slow down and give sufficient time for the gas in the tank to fully replace any residual gas in the analyzer. So you're right in that the real issue isn't "recalibrate or don't"; the real issue is to take the time to analyze properly.

Unfortunately some people (like the guy that analyzed my 8 tanks, and probably more that day) get complacent (because they do it every day and never have a problem) and rush.

 

[yle]

Recalibrating your analyzer between tanks of nitrox will cause your reading to be lower than actual. The analyzer takes awhile to get the higher O2 air flushed out and if you calibrate it at that point it accepts that high O2 air as 21% lowering subsequent readings.

I think we mean two different things by the word "calibrating". It sound like you think it means "turn the little knob until it reads 21".

To me, calibrating means verify that your instrument provides the correct reading for a known sample.

If you just blindly turn the little knob until the analyzer shows 21, then I agree with you: the analyzer's subsequent readings will be wrong. But that's not what calibration is. Anyone that doesn't understand the difference really shouldn't be handling the measuring instrument...

 

[tbone1004]

@yle calibrating does not mean wait for it to stabilize, calibrate means adjust an instrument until it reads what you want it to read. What you described is turn the knob until it reads 21%, that is how you calibrate O2 analyzers. Waiting until it stabilizes back to 21 is not calibrating, it's just waiting. There is 0 need to wait for it to come back to air though, all you have to do is go from tank to tank and wait for it to stabilize. Doesn't matter if the gas in the analyzer is atmospheric or what is in from the previous tank, the gas you are trying to analyze has to fully displace what is inside the analyzer anyway, so you just have to hook it up and wait long enough for the reading to stabilize. No refractory period is required. If it was, rebreathers wouldn't work since they use identical sensors and technology. The sensors read in real time, just have to make sure the right gas is over them

 

[KenGordon]

A couple of other points to keep in mind:

Sometimes the analysers used by the dive ops are not working properly.
Sometimes the gas is not completely mixed.

I thought the second was typical dive op bs until I saw it happen.

 

[tbone1004]

A couple of other points to keep in mind:

Sometimes the analysers used by the dive ops are not working properly.
Sometimes the gas is not completely mixed.

I thought the second was typical dive op bs until I saw it happen.

if PP blending it's a very real thing. If it's banked it should not be

 

[tursiops]

The most egregious example of not understanding calibration and analyzers I've ever seen was at a popular resort that had Nitrox tanks out for customers to pick up and analyze/log, and an analyzer mounted on the wall with a whip to attach to the tank. A fellow and his wife walked up, took a tank each, and he attached one to the analyzer, and turned the gas on. He asked his wife, "What percentage do you want?" She responded, "We're not going deep, how about 36%?" "OK," he said, and he then adjusted the analyzer calibration knob to show 36, turned the tank off, removed the whip, and handed her the tank. I am not making this up.

 

[boulderjohn]

As I reported in a recent post, I performed a scuba review for a very experienced diver who had all her own high end equipment. We started with equipment setup, of course, and when she turned on her air, she looked at her air integrated computer and said, "OK, the computer is analyzing the mix and sees that it is 32%." It took a long time for me to convince her that her computer was not analyzing the mix. At some point in her past diving she had understood how her computer works, and she had set it to match the 32% mix she was using on that dive. For however many dives she had done after that, she had assumed her computer was doing the analyzing and assumed that she was diving 32% on all her dives, not matter what she actually had in the tank. It is possible she really did have 32% or something close to that, but there is no way of knowing.

 

[RandomGuy1]

The most egregious example of not understanding calibration and analyzers I've ever seen was at a popular report that had Nitrox tanks out for customers to pick up and analyze/log, and an analyzer mounted on the wall with a whip to attach to the tank. A fellow and his wife walked up, took a tank each, and he attached one to the analyzer, and turned the gas on. He asked his wife, "What percentage do you want?" She responded, "We're not going deep, how about 36%?" "OK," he said, and he then adjusted the analyzer calibration knob to show 36, turned the tank off, removed the whip, and handed her the tank. I am not making this up.

Wow. Just wow.

 

[RainPilot]

The most egregious example of not understanding calibration and analyzers I've ever seen was at a popular resort that had Nitrox tanks out for customers to pick up and analyze/log, and an analyzer mounted on the wall with a whip to attach to the tank. A fellow and his wife walked up, took a tank each, and he attached one to the analyzer, and turned the gas on. He asked his wife, "What percentage do you want?" She responded, "We're not going deep, how about 36%?" "OK," he said, and he then adjusted the analyzer calibration knob to show 36, turned the tank off, removed the whip, and handed her the tank. I am not making this up.

As I reported in a recent post, I performed a scuba review for a very experienced diver who had all her own high end equipment. We started with equipment setup, of course, and when she turned on her air, she looked at her air integrated computer and said, "OK, the computer is analyzing the mix and sees that it is 32%." It took a long time for me to convince her that her computer was not analyzing the mix. At some point in her past diving she had understood how her computer works, and she had set it to match the 32% mix she was using on that dive. For however many dives she had done after that, she had assumed her computer was doing the analyzing and assumed that she was diving 32% on all her dives, not matter what she actually had in the tank. It is possible she really did have 32% or something close to that, but there is no way of knowing.

Mind. Blown.

I am occasionally embarrassed by the behaviour of others in my profession but nothing makes me cringe like what fellow dive instructors allow through.

 

[boulderjohn]

Mind. Blown.

I am occasionally embarrassed by the behaviour of others in my profession but nothing makes me cringe like what fellow dive instructors allow through.

I am quite sure that in the case of my student, she once knew exactly what she was doing. She probably had had instruction that was just fine. She was also not stupid. With the passage of time we forget about the things we were taught.

When I went to college, I was a chemistry major who was pretty much of a whiz with math. I switched out of that completely. Today my knowledge of both subjects is sadly rudimentary.