Is a Nitrox test at the dive site necessary?

[DiveNav]

Ok. I admit physics is not my thing but how does a change in temperature within a closed system (tank) change the percentage of the mix? ......

it does not ..... but the analyzer itself could be affected.
More specifically, if you calibrate your O2 analyzer at temperature X and then analyze a gas at temperature Y you could see some difference in your analysis because the O2 sensors ARE affected by temperature.

The amount of this difference between analysis at temperature X and analysis at temperature Y depends on: delta between X and Y, sensitivity of the O2 sensor to temperature and temperature compensation method (or algorithm).

We always recommend to calibrate the analyzer in conditions (temperature, pressure, humidity and gas flow) similar to the ones of the gas to be analyzed.

 

[kafkaland]

Any word on how the CooTwo performs in the field? I'm in the market for an analyzer, and like the idea of having O2 and CO in one box.

 

[NAM001]

Almost all my dives are shore dives. I also dive with friends, rarely do I dive with an organized shop dive outing.

So I would need to have my own analyzer if I'm to test the cylinder at the dive site.

I will agree that you need your own but 400$ is a lot to spend. perhaps not so much in Canadian dollars. Perhaps you should look for a CO tester while you are at it. Perhaps 400 for the pair.

 

[uncfnp]

...//...The amount of this difference between analysis at temperature X and analysis at temperature Y depends on: delta between X and Y, sensitivity of the O2 sensor to temperature and temperature compensation method (or algorithm).

We always recommend to calibrate the analyzer in conditions (temperature, pressure, humidity and gas flow) similar to the ones of the gas to be analyzed.

So calibrate then analyze. Good advice that I sometimes neglect.

 

[Bigd2722]

Most of the shops that I use call for you to fill out a log book. No big deal. If I am diving the same day I pick up the tank and it does not leave my control I would not worry about retesting. However, if you don't test at the shop or wherever you pick up your tank and you wait until you get to the dive site, you're kinda screwed if the mix is significantly off. Ask me how I learned that lesson.

 

[BoltSnap]

What happens when you test at the shop and at the dive site where the temp is 80+ and then you jump into the cold Atlantic where the temp is 50 degrees?

The fill station where I am isn't the most professional company out there. They fill the tanks sometime too fast and the Tank is still "warm" when I pick it up. I don't trust the initial reading of their analyzer and I don't count 100% on the reading I get then. I ascertain the 100% reading later in a day or two or at the dive site with a different brand analyzer. The readings can be different by 1 - 3 % usually. It isn't much different but it isn't the same.

Sea water temps get to as low as 58 degrees in the winter and the air temperature isn't that much warmer. The summer is a lot warmer but the sea water is warmer too.

 

[Patoux01]

Funny how many people are worried about having their nitrox 2% off...

 

[Dive Right In Scuba.]

The difference in reading you get between a hot fill and a lower temperature can be due to the passage of time. If the gases are not fully mixed when first tested, you will get a different reading later.

 

[Dive Right In Scuba.]

Funny how many people are worried about having their nitrox 2% off...

Yes. And they are probably also afraid that if they accidentally dip below the MOD for 10 seconds they will suffer immediate oxygen toxicity. Things are not that precise, and the standard protocols allow a lot of room for error. We instructors teach those protocols, and we tell people not to violate them at all, but the reality is that you have to more than a little out of line to have a problem on a recreational dive. The dangers are much more pronounced on a technical dive.

 

[Peter69_56]

For all those who think that when you partial pressure fill and dont mix the gas before testing, the difference is minimal, I did an 80 mix a week ago with interesting results.

Finished topping with air, did the first test 35%, rolled it on the grass for a few minutes 57%. Now I am thinking I didn't get this blend that far bad surely, rolled again and swung it up over my shoulder a few times and rolled again, 80.5%. Left it over night and it was stable.

On another occasion recently had a PP fill from a shop, tested after 24 hrs after the fill on pickup, 75%, rolled it around, 80%.

I usually fill, mix and test. Wait 24 hrs and test again. Test prior to loading in my car for use.

I also check all tank pressures prior to loading. Got a suprise the other day after blending a borrowed tank. tested pressure after 5 days and found only 170 BAR instead of the original 210 BAR. Quick check revealed a neck O ring leak.

Rule 1; Never assume anything ever. Murphy is far smarter than you and just waiting for the chance.