[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]

Correct

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

 

[Dave Abraham]

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]

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:embarassed:

 

[Dave Abraham]

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:embarassed:

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]

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:frown:

 

[Dave Abraham]

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:frown:

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]

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 :teeth:

Go to

 

[gwilson]

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]

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.

 

[Rydive]

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.

Nothing, but you'd be the first person to put gas in to the cylinder from empty, thus taking the risk that the valve was probably threaded etc, etc.

If you take a cylinder in fully emptied, there is no indication that the valve is properly fitted (except if visible) until it goes bang. I can see where they're coming from.

 

[Hickdive]

How long does it take to check a cylinder is empty, then unscrew the cylinder valve, then screw it back in again? A week or less than a minute? Is it 11 pounds worth of work?

Don't forget that the LDS is question is no more qualified than you or I to undertake the work as they are not IDEST registered. They are charging for work they aren't legally competent for.

Quite simply it is a penalty fee for failing to buy cylinders off them. If they came clean and admitted it people would still be hacked off but would at least have the choice of accepting it or buying their cylinders direct from the LDS. However, to pass it off as some kind of 'safety' concern, like the ludicrous extra fee they charge for filling nitrox into cylinders they haven't cleaned, is absurd.

Besides which what are the chances of someone having a brand new cylinder with a brand new 3/4 bsp valve accidentally screwed into it? None whatsoever because you cannot get new 3/4 bsp cylinder valves, anywhere, at all. There are some websites that list them as stock but when you contact them they say, "er, no, sorry we can't get them." I speak as someone who has a small stock of 3/4 bsp A-clamp cylinders who would like to convert them to DIN and I've spent quite some time searching. Plus cylinder valves now have the thread size on them plus nobody makes 3/4 bsp neck thread diving cylinders so there's no possibility of someone trying to put a metric valve into a new imperial cylinder.

And finally, if you cross thread a cylinder valve it won't sit on the o-ring and it won't seal, not unless you've got one big spanner to apply enough force to drive it down with.

So, presented with a brand new cylinder, with a brand new valve, sold by a known retailer, there is absolutely no good reason to charge a fee. By all means look at the cylinder and valve to see if there's anything dodgy - any cylinder filler should be doing that as routine with every cylinder before they fill it, not just new ones - but taking the cylinder away and 'checking' it for money is nothing but a smack on the wrist for the owner. I wonder what would happen if Trading Standards put a new cylinder in for this check, with a note inside inviting the checker to phone them, do you think they'd get a call?

 

[neilh]

if the cylinder is in test with a valid O2 sticker they should fill it no questions asked.

Wots one of them then? As far as I know there's no such thing

If you take a cylinder in fully emptied, there is no indication that the valve is properly fitted (except if visible) until it goes bang. I can see where they're coming from.

...and yet my LDS prefer me to take mine in empty so they don't have to drain them first before sorting out a Nitrox fill. Each LDS to their own I guess - and it probably depends a lot on how well they know you.

Don't forget that the LDS is question is no more qualified than you or I to undertake the work as they are not IDEST registered. They are charging for work they aren't legally competent for.

Sorry, but I don't see what IDEST has to do with it - I'm pretty sure O2 cleaning isn't covered by IDEST and there is no "legally competent" concept.

So, presented with a brand new cylinder, with a brand new valve, sold by a known retailer, there is absolutely no good reason to charge a fee.

Except that they don't know whether the cylinder and valve were O2 clean when they were delivered to you, whether the valve was installed in the cylinder or whether you installed it in your living room with the kids and dog helping

I can see where they're coming from in that the only way they can be sure at the moment is to clean it themselves.

 

[Rydive]

Steve, I can totally see where you're coming from. However, with regards to the different prices for nitrox fills in cylinders not cleaned by that particular LDS, could it not also be viewed as an incentive or discount for having your cylinders cleaned there rather than as a punitive fine for not doing so? After all the other shop in the area won't fill them at all unless they have cleaned them... as you mentioned earlier. I know which I think is more reasonable.

I think the reason behind both of these practices is the same though, money. But then they are running a business, and I can't imagine that there is that much money to be made when you consider the time, J rental costs, etc, etc, in doing PP nitrox fills. Trying to get people to clean their cylinders in the respective places is a way of bolstering the income. I agree though, it does feel like it's done in a less than up front way. I don't like it either, but I guess that's life.

For what it's worth, I recently had a (new and empty) twinset filled at the shop in question without the stated costs. Granted, there was some discussion.

 

[Dave Abraham]

Steve, I can totally see where you're coming from. However, with regards to the different prices for nitrox fills in cylinders not cleaned by that particular LDS, could it not also be viewed as an incentive or discount for having your cylinders cleaned there rather than as a punitive fine for not doing so? After all the other shop in the area won't fill them at all unless they have cleaned them... as you mentioned earlier. I know which I think is more reasonable.

I think the reason behind both of these practices is the same though, money. But then they are running a business, and I can't imagine that there is that much money to be made when you consider the time, J rental costs, etc, etc, in doing PP nitrox fills. Trying to get people to clean their cylinders in the respective places is a way of bolstering the income. I agree though, it does feel like it's done in a less than up front way. I don't like it either, but I guess that's life.

For what it's worth, I recently had a (new and empty) twinset filled at the shop in question without the stated costs. Granted, there was some discussion.

So what you are suggesting is that I should only be able to get a nitrox fill from the shop that did my O2 cleaning? Not very practical to drive back to my LDS in Bolton when I'm on a weekend in Weymouth and want a fill is it?

 

[Rydive]

So what you are suggesting is that I should only be able to get a nitrox fill from the shop that did my O2 cleaning? Not very practical to drive back to my LDS in Bolton when I'm on a weekend in Weymouth and want a fill is it?

Erm, no that's not what I'm saying at all. If you read my post again you'll see that what I actually said is that I find it more reasonable for a shop to offer a cheaper fill to a customer who has had his cylinders cleaned there, as opposed to refusing to do a nitrox fill at all. After all, like you say, it wouldn't be very practical if you could only get your cylinder filled in one location.

So, in fact, Im saying the exact opposite. Hope that helps.

 

[gwilson]

In typical YD stylee this has gone way of thread.

The 2 main shops we have in Glasgow the Westend/Partick one, will give a Nitrox fill so long as the cylinder/s are in date regardless of whether he has tested them or not. The Southside larger shop will only give a Nitrox fill if they have tested them.

Neither will fill a completely empty cylinder with nitrox or air unless they do a visual inspection.

I'm pretty sure the first mentioned shop is IDEST certified contary to what the second mentioned shop or a third one further west along the Clyde.

Now why cant we express these views and comments on a Scotish based forum without posts being deleted or lock and threats of being banned from said forum.

 

[Rydive]

In typical YD stylee this has gone way of thread.

Look at the name of the thread... I reckon we're bang on!:teeth:

 

[gwilson]

Look at the name of the thread... I reckon we're bang on!:teeth:

Loads of studying going on then ???

 

[Rydive]

Loads of studying going on then ???

Hmm, not really. But this is far more important/interesting/non-legal. And besides, it's imortant to take a brain break... problem is my brains been taking a break for months. :angel: