Trimix question

[Mark Powell]

Originally Posted by access

I’m thinking of getting trained to dive trimix but am working out the costs involved in getting the mix.
I use a compressor that allows constant flow nitrox mixes up to 40%.
Would I be able to decant helium in to the cylinders and then top off with a nitrox mix?
And is it difficult to calculate what the nitrox mix should be?
Any help appreciated

Yes you can do this. There will be limitations on exactly what mixes you can achieve by topping up with certain mixes but it's certainly possible.

The calclations are not hard but most people these days use a blending program or spreadsheet to do the calcs. On blending courses I give students a couple of the free/trial blending programs that are available as well as a speadsheet that does all the calculations.

[Garf]

DIR gases are a piece of p1ss if you have access to high pressure EAN32

21/35 = 81bar He + top off with 32%
18/45 = 103bar He + top off with 32%
15/55 = 126bar He + top off with 32%


now isn't that nice. Almost like someone thought it all out

[Clare Gledhill]

Indeed Garf is right but rather than think of bar, think per centage.

So 21/35 is 35 per cent of final required fill pressure topped of with 32.

18/45 is 45 per cent, etc. etc.

[NotDeadYet]

Originally Posted by Garf

now isn't that nice. Almost like someone thought it all out

He did think it out and he wasn't even a DIR diver

[nickb]

Originally Posted by Garf

DIR gases are a piece of p1ss if you have access to high pressure EAN32

21/35 = 81bar He + top off with 32%
18/45 = 103bar He + top off with 32%
15/55 = 126bar He + top off with 32%

now isn't that nice. Almost like someone thought it all out

Not just DIR standard gases:

116 Bar => 16/50
139 Bar => 13/60 etc.

Originally Posted by Clare Gledhill

Indeed Garf is right but rather than think of bar, think per centage.

So 21/35 is 35 per cent of final required fill pressure topped of with 32.

18/45 is 45 per cent, etc. etc.

Which is all very nice, but how widely available is pre-blended 32% in the UK guys? Even my LDS (the best in the UK ) doesn't stock pre-mix.

access: If you PM me your email, I'll send you a small spreadsheet where you can enter any combination of Remaining Backgas; He; O2 and top-off mix. (Ideal Gas Laws only I'm afraid, but it's near enough at 230 Bar). It runs very nicely on a Windows PDA too.

[Ahmed Adly]

Originally Posted by access

I’m thinking of getting trained to dive trimix but am working out the costs involved in getting the mix.
I use a compressor that allows constant flow nitrox mixes up to 40%.
Would I be able to decant helium in to the cylinders and then top off with a nitrox mix?
And is it difficult to calculate what the nitrox mix should be?
Any help appreciated

If you use a Stick and an Omni Booster you will be able to achieve ANY mix you want.
The trouble is, that there is no agency that has a class that will cover it
(The "Omni Booster" is for running helium through your compressor)

[access]

Thanks all.
As it seems I can mix trimix with this setup thus having easy access to fills
my next step will be looking in to doing a trimix diving/blending course

[GLOC]

The only thing to bear in mind is how much He pressure you will have in your Js to make it worthwhile. As far as I can remember from a myriad of posts, unless you have a booster, you will have to have around 3 J's to make it worthwhile which isn't a problem if you have cheap rental but in the grand scheme of things, it might be worth just going to a dive centre to get your fills.

HTH

[Steve Jones]

Originally Posted by nickb

Even my LDS (the best in the UK ) doesn't stock pre-mix.

But what more could you want from him, a smile, nice coffee, as he empties your wallet.

Steve

[Steve Jones]

Originally Posted by access

I’m thinking of getting trained to dive trimix but am working out the costs involved in getting the mix.
I use a compressor that allows constant flow nitrox mixes up to 40%.
Would I be able to decant helium in to the cylinders and then top off with a nitrox mix?
And is it difficult to calculate what the nitrox mix should be?
Any help appreciated

I have read the replies and most of them make sense. But all it really needs is for you to understand partial pressures involved in any mix.

I agree that certain software programs make it all very easy. But there is no substitute for a good understanding of the basic maths involved.

Steve