YD Scuba Diving Forums banner

1 - 20 of 21 Posts

·
Is fully Mod 1'd up
Joined
·
878 Posts
Discussion Starter #1
Ok so I should know how to do this.... but I dont :(

Can anyone tell me the formula to calculate the mix I will get if I decant air into my twinset?

An example :- I will have a 32% mix for saturdays dive and will take a 15l filled with 200bar air. How do I calculate the new twinset pressure and also the new diluted mix?? I know I will have to do this calc at the end of the first dive when I know my twinset end pressure.

Cheers

Adam
 

·
Registered
Joined
·
3,103 Posts
Have a search for Nigel H on here, and his Blender application. Very easy to use.

Alternatively, if you have Excel (or a pen and a bit of paper) then you can work it out from basic principles - just break down the gas into the separate O2 and N2 components and go from there.

Alternatively, if you're just topping up a mix, you could always go with the approach of just seing what you come out with, as you'll presumably always want to get the strongest mix possible anyway - just top it up and see what comes out.

As a slight side question, is this twin 12's or something smaller? Can't think of many dives you'd do using 32% (so necessarily not that deep) where you'd need a top-up for the second dive - should easily get two hour long dives out of twin 12's, and then you're keeping the best mix for the 2nd dive too.

David
 

·
R.I.P.
Joined
·
5,709 Posts
Ok so I should know how to do this.... but I dont :(

Can anyone tell me the formula to calculate the mix I will get if I decant air into my twinset?

An example :- I will have a 32% mix for saturdays dive and will take a 15l filled with 200bar air. How do I calculate the new twinset pressure and also the new diluted mix?? I know I will have to do this calc at the end of the first dive when I know my twinset end pressure.

Cheers

Adam
Adam,

As Simon said, there isn't enough info to calculate, however, have a look here: http://www.yorkshire-divers.com/forums/tek-talk/55023-nitrox-filling-tables.html

and download the file that Gobfish posted. There is some useful info on there
 

·
His Holeyness
Joined
·
2,198 Posts
Ok so I should know how to do this.... but I dont :(

Can anyone tell me the formula to calculate the mix I will get if I decant air into my twinset?

An example :- I will have a 32% mix for saturdays dive and will take a 15l filled with 200bar air. How do I calculate the new twinset pressure and also the new diluted mix?? I know I will have to do this calc at the end of the first dive when I know my twinset end pressure.

Cheers

Adam
You'll add together all the litres of gas you've got, and divide by the summed cylinder volumes.

e.g.
Lets say the twinset is 12ltrs, and has 100bar in it. Thats 1200ltrs of gas.
There's 3000ltrs of gas in you're 15l.
These will be added together when you decant to give a total of 4200ltrs, but this is distributed across 27ltrs of cylinders giving a pressure of 4200/27 = 155.5bar

You can do the same with the oxygen
Twinset will have 384ltrs of O2, 15l will have 630ltrs
giving a total of 1014ltrs O2
therefore the O2 % will be 1014/ 4200 * 100 = 24%

ColinM
 

·
R.I.P.
Joined
·
5,709 Posts
If you don't have an analyser, top your twins up from the 15 and dive them on air tables, and don't go below the MOD of 32%.... Easy, sorted, and no maths involved.......:)
 

·
Is fully Mod 1'd up
Joined
·
878 Posts
Discussion Starter #11
If you don't have an analyser, top your twins up from the 15 and dive them on air tables, and don't go below the MOD of 32%.... Easy, sorted, and no maths involved.......:)
Thats basically what I do... but just wanted to know. . . .

I dive with twin 10's.

Cheers everyone

Adam
 

·
Driving my truck with my high-heels on...
Joined
·
5,509 Posts
The really simple answer is to have 32% in the 15 as well then there's no maths involved !
 

·
Member
Joined
·
246 Posts
I produced an excell spreadsheet to find the final fill whilst decanting.
It is initially set up to decant a 15 into a set of 12's manifolded, fill each cylinder with manifold closed then equalise the pressure. It will even calculate the final volume so you know whether it's woth decanting or not. You can change the cylinder sizes pressures etc.
As for Nitrox, agree use the same mix, or just analyse afterwards.
PM me with e mail address and i will forward you the spreadsheet.
Regards
Roger
 

·
One Team One Dream
Joined
·
3,388 Posts
why not dive with the 15 as an alternate air source

you may look at tit but at leats you will stay ther longer than the rest on your deco stop
 

·
Registered
Joined
·
254 Posts
Two things you might want to think about.

1. Isolate the twins before you decant, then decant into each cylinder, then open the isolator. You will get more gas in the twins if you do it that way.

2. If going for an air top-up, just think about the equivalent nitrogen depth of the first and second dive. As if the mix is leaner on the second dive, and you goto to the same depth on both dives, you have in fact done a reverse profile for the second dive. As from a nitrogen point of view the second dive will have been deeper. Reverse profiles may be an influence on getting bent.
 

·
Just not enough dive time.
Joined
·
9,135 Posts
Reverse profiles came up a while ago (on Divernet amongst others), I forget the exact 'rules', but from memory (please correct me if I'm wrong) it was felt that if the max dive was less than 30m and there was no more than 12m difference there was no 'clinical' evidence to suggest any greater risk from the dives.

Theory being one thing, practical experience can of course be something else ;)

Matt
 
1 - 20 of 21 Posts
Top