Acceptable mix accuracy for Nitrox

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Manatee Diver

Stop throwing lettuce at me!
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Location
Tampa Bay, FL
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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?
 
No, you are not being too picky. +/- 1% the requested blend is what should be expected.

Speak with the owner or the person in charge and tell them you are unhappy or ask them what is going on. If they can't fix it, find a new shop. Filling a tank with nitrox isn't that hard, if they can't do that correctly just image what else they are screwing up.
 
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?
No, but....

Are you seriously pushing MOD and NDL times with, say, 32? If so, it matters and you should get what you ask for with 1% above or below.

If not, it's no big deal as long as you know what's in the tank and plan accordingly. (And you should be checking.) Don't stress about it.

Question for you: Do you fully drain your tank before refilling? They might not be, which might affect things. If my tank is nearly empty, I'll fill Nitrox as an added safety margin since I'm often going up and down with CESA tests or otherwise helping an instructor. However, I almost always end up with half a tank or more left after checkout dives so I refill with air. That gives me a "half-trox" mix that I analyze. I figure it is still a margin of safety.
 
No, but....

Are you seriously pushing MOD and NDL times with, say, 32? If so, it matters and you should get what you ask for with 1% above or below.

I don't know the dive profiles yet, I was told to bring a specific mix by the operator. Since they are no stop dives it will really depends on whose computers we are basing off of. I know with my air consumption once the depth gets below 60-70ft it is typically no stop times that limits my dive times. And above that is often the total dive time allowed by the operator.

Question for you: Do you fully drain your tank before refilling?

They claim they drained them.
 
If you ordered 32% is is not unreasonable to expect 32%
 
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?
Baseline: I am not a trained blender myself, but I do still know something of the topic.

Are they filling while you wait, or are you leaving them overnight? And are they doing partial pressure blending, or are they using a "stick"?

If they are rushing for a "while you wait" service that may be part of the issue. And aside from accuracy issues, my understanding is that partial pressure blending can take a surprising amount of time to mix uniformly within the cylinder and provide an accurate analysis. I suppose the same would occur to a lesser extent if topping off a different mix or even mixing from a blending stick rather than blending with 100% O2.

Or they could just be doing sloppy work. But worth checking the other possibilities.
 
Are they filling while you wait, or are you leaving them overnight? And are they doing partial pressure blending, or are they using a "stick"?

Leaving overnight (typically a couple of days).

No stick. They used to partial pressure off of a rich banked nitrox, but I get the impression that they've switched to completely partial pressure blending as they no longer fill nitrox in tanks without an O2 clean sticker.
 
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.
 
https://www.shearwater.com/products/perdix-ai/

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