Acceptable mix accuracy for Nitrox

[ChuckP]

Someones gauges and or math are wrong. Odds are the tank doesn't need emptied to get the mix you want - it's simple math that apps, spreadsheets and programs do for you. Most simple nitrox top offs / blends can be calculated in your head and come out closer than 2%.

I just used a Palm O2 analyzer today and unless that one was broke, I don't know about them - way to fidgetdy....

With a good analyzer, nitrox blends are incredibly simple and easy to get within 1% really.

 

[chillyinCanada]

Who's analyzer are you using? Who's calibrating?

 

[clownfishsydney]

I make my own nitrox using oxygen stick. I can get it to within 0.25% of my aim every time. Even when I want to change a fill from (say) 26% to 32% I can get it within 0.5%. Not hard to do. While 30% or 34% may not matter too much, if you want 26% to dive to 45 metres, you do not want 28%!

 

[scrane]

In the past the local shop where I got nitrox from has always been pretty spot on, typically hitting the mix at +/- 1% which is the range I've gotten from other shops.

Lately it has gotten a lot worse, now increasingly it is +/- 3%. And when I raise my eye brow they start arguing with me about MOD and NDL times. I figured it wasn't worth my time and just left with my tanks.

Am I being too picky in wanting it +/- 1% of my requested mix?

Within reason (+/- 2%) I take what they give me and enter the correct value into my computer. You are using a dive computer, aren't you?

 

[tursiops]

Bear in mind that the oxygen analysers have a margin of error in their readings. Furthermore, most people calibrate their analysers to air without adjusting for temperature and humidity, which would also result in inaccurate analysis of o2 percentage.

In addition, partial pressure blending requires pressure measurements, which can be in error. If you drain a tank empty before blending, then you need to make two pressure measurements (of the O2 and then the full tank) and and one O2 measurement (of the filled tank). Starting from a partially full tank, you need and additional pressure measurement and an additional O2 measurement. And the O2 analysis needs to be done after some time, to allow the gases in the tank to mix. Lots of places for accumulated errors to creep in.

Here is the Analox chart for calibrating an O2 analyzer using air; you need to set the analyzer to a number that is less than 20.9, especially in warm, humid air. Some of the analyzers won't even let you set the calibration number, they just assume 20.9 when you "calibrate." Those will be reading high.

 

[Manatee Diver]

Who's analyzer are you using? Who's calibrating?

Their analyzer, and they are calibrating it each morning. It is an Analox Trimix analyzer connected to a flow rate limiter. And I don't think it is off, because when I've had mix down the mixes at a dive charter the results they got on their tester was similar to what I got at the shop's tester.

As as far as I know it is the one that they are using for mixing as I don't see any other analyzer attached to their fill station.

 

[Andy in Gap]

If it's your local shop I would talk to them when you drop the tanks off. If it has to be exact, let them know. If it's not close enough, make them fix it. A couple percent is meaningless for some dives and critical for others. I usually tell my guy if I'm using the tanks on a boat dive that it has to be right on and no short fills. Part of paying double the price of an air fill for Nitrox is the hassle of getting the mix correct.

 

[tridacna]

Part of doing Nitrox fills is the guessing game at the end. Easy enough to get the initial mix right. (Stick or partial). It's the final top off (usually with air) that makes the game interesting. Took me a few tries to get that right. Always better to do a slightly rich mix then wait a day or so for it to mix and cool down. A good tech will be able to top off with air to the final pressure and end up with the right mix.

 

[Manatee Diver]

Always better to do a slightly rich mix then wait a day or so for it to mix and cool down. A good tech will be able to top off with air to the final pressure and end up with the right mix.

Unfortunately even in the past, when it was within my +/- 1%, it was always too lean of a mix rather than too rich of a mix.

 

[tridacna]

Unfortunately even in the past, when it was within my +/- 1%, it was always too lean of a mix rather than too rich of a mix.

Too lean is just incompetence by the tech. We have no Haskell booster so increasing the mix is kinda difficult.