Mixing Nitrox with a Partial pressure system

[Superlyte27]

I recently became a believer.
I always thought the guys rolling tanks were idiots. Give me a break. Really? Roll a tank on the ground? LOL, what a joke.
Then, a few weeks ago I filled 6 tanks with 36% via PP method. Immediately after filling I analyzed all 6 tanks. None of them were at 36% or even close. I decided I would roll 1 out of the 6 tanks for 3 minutes.

I analzyed all 6 tanks again less than 5 minutes after tanks were filled. The tank I rolled was spot on at 36.2%, None of the other tanks were within 2%. The next morning, I again analyzed all 6 tanks. They were all at 36% +/- .3

Now, i'm never in a hurry, so rolling not really an issue, and I have my own analyzer, so I can analyze at the site, not the shop. But If I were in a hurry and didn't have an analyzer, rolling your tank is an option to get an accurate reading at the dive shop/your house.

 

[Searcaigh]

It doesn't look familiar.

Ok, that was the one I "dived" in back in 1995, had a nice big dome lens on the front end, but saw fewer fish than fingers on my hand

Sorry for the hijack

 

[Dive the World]

I would like to know the general steps in mixing a 30% EAN from the point of connecting the tank to the system to final analysis. As i have looked this up it seems a tank filled in this fashion needs to "settle" before analysis. I have seen it suggested a tank needs to be "rolled" after at some point then analyzed. If either of these is the case if I let my 30% EAN filled tank sit in the closet a month or so is the mixture going to be different than when it was filled? ANother question, if the tank does neet to "settle" before analysis, how long does it need to settle? If the target mix is 32% and I analyze it 2 days after the fill and find the mix to be 26% what can the shop do to correct the problem? Thanks in advance.

DiveLvr

1st - get proper training in blending before doing this for your self.

Fairly simple thing to do, connect hose from 100% tank to Scuba Tank. open valves slowly and go for it.

Ofcourse you need to know if tank is empty, or already has something in it - in which case how much and what mix. Ad 02 as desired. I always add 1 to 2Bar extra O2*. I will explain further down why.

No matter you do, all tanks to settle after filling, one way or another. As filling a tank requires pressured air, which causes heating of gas, which causes expansion, and therfore you may think you have 220 but after cooling could end up with 200 or even 190!!! depending.

So if don't refill in a pre-mixed nitrox situation, you would end up with the right mix but might not be as much as you had hoped for as far as pressure is concerned.

In the event of partial mix, you will add air and same thing as above will happend, once tank cools the pressure in the tank reduces and would give you a lower pressure and there for a different mix. 1% difference is not a concern.

*I always add 1 to 2Bar extra O2* as it is a lot easier and simpler and cost effective. For example, if you wanted to have a 36% and end up with 40% all you have to do is let a little gas out and fill it back with air and you will get your 36%.

However, in your scenario, if you are aiming for 36 and get a lower mix such as 30 - the shop will have to likely drain the entire tank and do it all over again. And that generally could be a loss of 34 Bar of O2 and that has a $$$ amount to it.

Hope my 2cents helped.

 

[PfcAJ]

and I have my own analyzer, so I can analyze at the site, not the shop. But If I were in a hurry and didn't have an analyzer, rolling your tank is an option to get an accurate reading at the dive shop/your house.

Having had a few instances where mixes were WILDY wrong (like 50% in a 190 bottle, should be 18/45...) than what I asked for, I highly suggest analyzing before leaving the shop. Could be the difference between going diving and staying topside.

 

[Superlyte27]

Having had a few instances where mixes were WILDY wrong (like 50% in a 190 bottle, should be 18/45...) than what I asked for, I highly suggest analyzing before leaving the shop. Could be the difference between going diving and staying topside.

I have an automatic nitrox mixer now (aka rebreather)
Also, I fill my own tanks. I usually get what I ask for