Completely off track?

[Hickdive]

Check my result please?

I'm not very good with figures and I've not done any kind of course on the subject but I was trying to bend my head round this today. I think that if I have a full (232bar) 12 litre cylinder filled with a 60% nitrox mix and I decant that into an empty 24 litre twin set so that the pressure across all three cylinders is the same and I then disconnect the single 12 and top up the twin set with air to 232 bar I should have a 33.88% nitrox mix in the twins.

Is this correct? If not, where am I going wrong?

I figure that the three cylinders will equalise at 77 Bar of which 60% is O2 i.e. 46.2 bar. I then top up with 155 Bar of air of which 20.9 % is O2 i.e. 32.39 bar so I have 232 bar of gas of which 32.39 bar plus 46.2 bar of which is O2, so that's 78.59 bar out of 232 bar or 33.88%.

Please note this is just me playing around with some hypothetical figures to see if I can figure it out - it isn't something I intend doing.

[Flyboy]

Assuming temperatures remain constant and not quibbling over the odd decimal point rounding error your logic and conclusions are correct
FB

[Dave Abraham]

Originally Posted by Flyboy

Assuming temperatures remain constant and not quibbling over the odd decimal point rounding error your logic and conclusions are correct
FB

Agreed now do it again starting with a 100 cu foot cylinder with 300 bar rating filled with Eanx 60 decanting into a 12 litre 210 bar rated cylinder that starts with 40 bar of Eanx 32 in it and a 15 litre 232 bar rated cylinder that starts with 60 bar of Eanx 36 in it. Just to make it interesting when you have finished I want all the cylinders to be full with a mix of Eanx 34. You will be charged 10x market price for any extra O2 you waste by bleeding off.

Little Fcukers wouldn't get a 100% Gcse pass rate if I set the questions I'll tell yer

[Hickdive]

Originally Posted by Dave Abraham

Agreed now do it again starting with a 100 cu foot cylinder with 300 bar rating filled with Eanx 60 decanting into a 12 litre 210 bar rated cylinder that starts with 40 bar of Eanx 32 in it and a 15 litre 232 bar rated cylinder that starts with 60 bar of Eanx 36 in it. Just to make it interesting when you have finished I want all the cylinders to be full with a mix of Eanx 34. You will be charged 10x market price for any extra O2 you waste by bleeding off.

Little Fcukers wouldn't get a 100% Gcse pass rate if I set the questions I'll tell yer

Look, I had to take my shoes and socks off to get the first part nearly right (ran out of fingers and toes so had to round down the 77.33 recurring pressure) I'm bloody well pleased I even had the principles right, especially as I had Mr Chalk droning on about his boundaries on the phone as I doodled this out on paper

[Dave Abraham]

Originally Posted by Hickdive

Look, I had to take my shoes and socks off to get the first part nearly right (ran out of fingers and toes so had to round down the 77.33 recurring pressure) I'm bloody well pleased I even had the principles right, especially as I had Mr Chalk droning on about his boundaries on the phone as I doodled this out on paper

I usually get my nitrox fills at Capernwray and they insist on draining the cylinders anyway.

So the correct answer is "save a brain cell, drain the lot get them filled correctly and pay the bill" easy really.

[Hickdive]

Originally Posted by Dave Abraham

I usually get my nitrox fills at Capernwray and they insist on draining the cylinders anyway.

So the correct answer is "save a brain cell, drain the lot get them filled correctly and pay the bill" easy really.

It was just a thought exercise, to see if I could break down the concepts and make the calculations since I'm generally rubbish at such things. I'd no intention of doing this in reality, but since I now know my thoughts were right I've set up a little spreadsheet and it doesn't take long to realise after punching a few numbers through it that it isn't a practical exercise.

Mind you, if in reality it was as easy as draining them down and getting them filled and paying the bill I'd be happy.

Of the two LDS I can use, one won't fill a completely empty cylinder unless they send it for a 'Compressor compatability test' or, if they do lower themselves to fill it, they charge extra (presumably 'danger money') to put nitrox into a cylinder they haven't cleaned. Whilst the other won't fill anything at all unless they've cleaned it themselves

[Dave Abraham]

Originally Posted by Hickdive

Of the two LDS I can use, one won't fill a completely empty cylinder unless they send it for a 'Compressor compatability test' or, if they do lower themselves to fill it, they charge extra (presumably 'danger money') to put nitrox into a cylinder they haven't cleaned. Whilst the other won't fill anything at all unless they've cleaned it themselves

You need to find a new LDS, if the cylinder is in test with a valid O2 sticker they should fill it no questions asked. As for "only if we've cleaned it ourselves" I've never heard so much crap in my life. I could get them to clean it, go away on a trip and get a fill from a compressor that is not double filtered and then take it back to them for a nitrox fill. They would be none the wiser.

There is however a common application for what you are trying to do. If you do a shallow dive on a rich mix eg nitrox 40 then get an air top up for a deeper dive the following morning. This is a common pattern on Red Sea liveaboards and at Scapa. It's useful to be able to calculate what pressure to leave in your cylinder so that you get the right mix for the deeper dive the following morning.

It's ok to come back from the shallow dive with too much gas as you can drain it down, but most places fill from a J cylinder so may not be able to add O2 if you need to come back with 120 bar and the J cylinder is already down to 80 bar. Pushing gas uphill is tricky unless you have a membrane system.

[Timbobaggins]

Originally Posted by Hickdive

Check my result please?

I'm not very good with figures and I've not done any kind of course on the subject but I was trying to bend my head round this today. I think that if I have a full (232bar) 12 litre cylinder filled with a 60% nitrox mix and I decant that into an empty 24 litre twin set so that the pressure across all three cylinders is the same and I then disconnect the single 12 and top up the twin set with air to 232 bar I should have a 33.88% nitrox mix in the twins.

Is this correct? If not, where am I going wrong?

I figure that the three cylinders will equalise at 77 Bar of which 60% is O2 i.e. 46.2 bar. I then top up with 155 Bar of air of which 20.9 % is O2 i.e. 32.39 bar so I have 232 bar of gas of which 32.39 bar plus 46.2 bar of which is O2, so that's 78.59 bar out of 232 bar or 33.88%.

Please note this is just me playing around with some hypothetical figures to see if I can figure it out - it isn't something I intend doing.

tools

Look under OTHER APPLICATIONS
Find the programme that says Decom Gas Mixer, download using password decom, hey presto a free gasmix programme

Go to

[gwilson]

Originally Posted by Dave Abraham

You need to find a new LDS, if the cylinder is in test with a valid O2 sticker they should fill it no questions asked. As for "only if we've cleaned it ourselves" I've never heard so much crap in my life. I could get them to clean it, go away on a trip and get a fill from a compressor that is not double filtered and then take it back to them for a nitrox fill. They would be none the wiser.

There is however a common application for what you are trying to do. If you do a shallow dive on a rich mix eg nitrox 40 then get an air top up for a deeper dive the following morning. This is a common pattern on Red Sea liveaboards and at Scapa. It's useful to be able to calculate what pressure to leave in your cylinder so that you get the right mix for the deeper dive the following morning.

It's ok to come back from the shallow dive with too much gas as you can drain it down, but most places fill from a J cylinder so may not be able to add O2 if you need to come back with 120 bar and the J cylinder is already down to 80 bar. Pushing gas uphill is tricky unless you have a membrane system.

This particular one will not even fill brand new cylinders that are o2 cleaned unless they "perform" a clean themselves, including fully assembled brand new twinsets. Just a way of making extra cash, the other one up here will fill any cylinders he is happy with BUT does have a sign up saying any completely empty cylinder will not be filled unless you pay £11 for a visual inspection, this is a safety thing as who's to say you have not swapped valves due to damaging one and maybe put a metric thread valve in a imperial cylinder or vis versa, I know this as it was explained to me this morning by LDS owner.

[Dave Abraham]

Originally Posted by gwilson

the other one up here will fill any cylinders he is happy with BUT does have a sign up saying any completely empty cylinder will not be filled unless you pay £11 for a visual inspection, this is a safety thing as who's to say you have not swapped valves due to damaging one and maybe put a metric thread valve in a imperial cylinder or vis versa, I know this as it was explained to me this morning by LDS owner.

Whats to stop me replacing the valve and then decanting a bit of gas from another cylinder so that its not completely empty or even putting an air fill from an auto fill station such as the one they have at Puffin in Oban? Still sounds like an excuse to extract extra cash to me.