Is a Nitrox test at the dive site necessary?

[The Chairman]

I agree with Andy with the one caveat: if the tank is not at room temperature when you tested it, I wouldn't trust the test. I own my own analyzer and usually wait until I'm about to plunge before I measure my tanks. Some shops require that I test it there, but I don't like that. So, I'll fill in their little journal and then recheck before I splash. I usually adjust my mixes on my PDC at the same time.

@Stoo ... I think that's a great idea and might just require it for private classes. Elena and I will be starting to require our students have a PDC for OW in the not too distant future and that falls right in line with that thinking.

 

[BoltSnap]

I do it when picking up the Tanks from the fill station and before diving (they are usually not on the same day) since with the wide range of temperature change the % may change. I also use different brand analyzers just to make sure. I also test for CO.

 

[diver 86]

There is a certain intersection that we routinely come to that is hard to see approaching traffic from the driver's seat. Even after my wife says it's all clear, I twist nearly out of my seat to check. As the driver, I'm responsible for safety.

 

[Steve_C]

I dive my own set of HP100s. I personally analyze them when they are filled. They are labeled although the requested mix is always the same 30%. But it varies some on testing. I am the only one that dives the tanks. They are also labeled with my name and initials so they are not confused with other HP100s ( a popular tank in this area).
I choose my mix with a mod of 1.3. I do not retest when I get to the boat or dive site. I am comfortable with this. If there were any chance of a mix up like I had more than two HP 100s (my only other tanks are two AL90s and an AL19 and they are all air) or they were rental with other tanks around I would retest. In the Caymans I tested at the boat.

 

[Kairoos]

I still test mine on the boat, even though I test them at the dive shop. It's a good habit and it doesn't hurt. The shops O2 analyzer could be giving off a bad reading, or maybe I forgot to label my tanks, or something could happen. People make bigger mistakes. I usually set up my gear as soon as I get on the boat, so I have a ton of time to kill before reaching the dive site anyway.

I carry my own tanks and I've never seen anyone else with gray HP117's so I could assume I'm safe to not test them on site.

 

[uncfnp]

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? Pressure, yes. But percentage?

 

[PfcAJ]

I won't leave the fill station without analyzing my tanks.

 

[diver 86]

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? Pressure, yes. But percentage?

It doesn't.

 

[Steve_C]

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? Pressure, yes. But percentage?

If you had a tank partially full at one temp and then you added some more gas of a different mix at a different temp and based your final mix on initial pressures, then there could be some effect. For example the warm gas when it cools will drop in pressure so it is less than you thought it was. Not saying it would be significant but without taking temp into account pressure is not the same as number of ammount..

 

[RJP]

If you had a tank partially full at one temp and then you added some more gas of a different mix at a different temp and based your final mix on initial pressures, then there could be some effect. For example the warm gas when it cools will drop in pressure so it is less than you thought it was. Not saying it would be significant but without taking temp into account pressure is not the same as number of ammount..

The temperatures of any and all gasses in the "receiving" tank are instantly the same for all but the most exquisitely hypothetical/theoretical purposes.