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Discussion Starter · #1 ·
Is there any way of working out (or getting a rough idea) of the pressure in a cylinder when decant?

eg: If you have a 15l cylinder with 220 bar of air in it and you decant into a 3l cylinder that has 50 bar of air in it what will be the pressure in each of the cylinders.

If you are decanting from a 15 into a manifolded set of 10 would you get more air in them if you decanted in with the manifold open so decanting into the equivalent of a 20l or decant into each of the 10s then equalise via the manifold?

Ta
 

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Winding up the precious since 2009
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Pretty simply work the amoutn of gas before (Presure x cyclinder size) and add them all up. the divde amongst the cyclinders after wards so in you example:

220 * 15 = 3300
3 * 50 = 150
3300 + 150 = 3450

3450 / (15 + 3) = 192 bar

For the manifold open/close if you run the maths trough you do get a slightly higher end presure doing one side and then the other with the manifold closed then equalising (10'ish bar as I recall) but I don't feel it is worth the additional hassle so just decant with the manifold open.
 

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For some bizarre reason....
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Is there any way of working out (or getting a rough idea) of the pressure in a cylinder when decant?
Nope no method of doing it roughly, I only know the method posted by Elvis of working it out exactly.....

If you are decanting from a 15 into a manifolded set of 10 would you get more air in them if you decanted in with the manifold open so decanting into the equivalent of a 20l or decant into each of the 10s then equalise via the manifold?
Manifold closed - equalise afterwards..... results in more gas being transferred - try running a couple of calcs (or trust me)

-Paul
 

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Winding up the precious since 2009
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Manifold closed - equalise afterwards..... results in more gas being transferred - try running a couple of calcs (or trust me)

-Paul
It does, do the maths and decide for yourself.
 

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Nigel Hewitt
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Manifold closed - equalise afterwards..... results in more gas being transferred - try running a couple of calcs (or trust me)
If you think about it the first donate sets one cylinder of the twins to a higher pressure than the second donate.
The donor is left at that lower pressure.
Then you equalise the twins and they settle to a pressure part way between the two pressures.

But this must be more than the donor so it has given more gas than if they were all connected together and equalised so you must have donated more.
It's not much.
 

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Winding up the precious since 2009
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It does, do the maths and decide for yourself.
Oh FFS I'm bored,say 50bar in twins 220 in a 15.
OK Manifold open.
50 * 20 = 1000
220 * 15 = 3300
1000 + 3300 = 4300
4300/ (20 +15) = 123Bar

Manifold Closed
Side one
50 * 10 = 500
220 * 15 = 3300
...
3800 / 25 = 152 Bar Side one
Side Two
50 * 10 = 500
152 * 15 = 2280 (what is left in the 15)
2780 / 25 = 111 Bar Side two
Equalise
(152 + 111)/2 = 131 bar at the end.


So 8 bar additional,as I said about 10 bar.
 

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Discussion Starter · #7 ·
K....can i have a sanity check here pls...

Just run the numbers for decanting from a 15l 220 bar start into manifolded twin 10s with 100 bar in them

for closed manifold then equalise i get 162.4 bar and for decant into open manifold i get 155 bar does that sound right?


20L * 162.4bar = 3248L which is same as [email protected] 216.5bar ?
 

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Winding up the precious since 2009
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Thursday afternoon maths. Just take them to a filling station and get a 232 or 300 fill simples
No quite so simples when you are on a boat or even on land an hour+ from the nearest filling station!
 

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Nigel Hewitt
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No quite so simples when you are on a boat or even on land an hour+ from the nearest filling station!
Be careful though. Quite often the twinset after the decant ends up with less gas in it than the single had before hand.
 

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Winding up the precious since 2009
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Be careful though. Quite often the twinset after the decant ends up with less gas in it than the single had before hand.
Possibly but having swapped a twin for single wing and then converted twinset regs to a singles set on a rolling RIB bringing on seasickness (which I don't normally suffer from) then finding that my weighing was all wrong was not an experience I would want to repeat, especially as this is usually for a second dive and (reverse profiles thread not withstanding) is usualy shallower/not the main event.
 

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Quite often the twinset after the decant ends up with less gas in it than the single had before hand.
My buddy & I find that two 15s decanted to both our twinsets gives enough for a good second dive if we get our decant sequencing right. (First 15 to fullest twinset then other set, repeat with the second 15. First set always has manifold open, second set has cylinders filled separately.)
It's a bit of a faff but good when there's no refills within travelling range. Much better than 1:1 decanting.

RS
 

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Maths For Excel

Manifold Open
Twin Resulting Pressure: ((Twin_Size*Twin_Pressure)+(Decant_Size*Decant_Pressure))/(Twin_Size+Decant_Size)
Decant Cylinder Resulting Pressure: ((Twin_Size*Twin_Pressure)+(Decant_Size*Decant_Pressure))/(Twin_Size+Decant_Size)

Manifold Closed - filling each side separately
Twin Resulting Pressure: (((((Twin_Size/2)*Twin_Pressure)+(Decant_Size*(((Twin_Size/2)*Twin_Pressure)+(Decant_Size*Decant_Pressure))/((Twin_Size/2)+Decant_Size)))/((Twin_Size/2)+Decant_Size) + (((Twin_Size/2)*Twin_Pressure)+(Decant_Size*Decant_Pressure))/((Twin_Size/2)+Decant_Size))/2)
Decant Cylinder Resulting Pressure: (((((Twin_Size/2)*Twin_Pressure)+(Decant_Size*(((Twin_Size/2)*Twin_Pressure)+(Decant_Size*Decant_Pressure))/((Twin_Size/2)+Decant_Size)))/((Twin_Size/2)+Decant_Size)))

Twin's Size is based on total Size, i.e. Twin 12Ls = 24
 

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Or get a re breather and you will have more air endurance than you can shake a stick at.
 

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Nigel Hewitt
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Or get a re breather and you will have more air endurance than you can shake a stick at.
And you end up doing the sums to do 'top up' trimix fills on 2L tanks.

You can't win.
 

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K....can i have a sanity check here pls...

Just run the numbers for decanting from a 15l 220 bar start into manifolded twin 10s with 100 bar in them

for closed manifold then equalise i get 162.4 bar and for decant into open manifold i get 155 bar does that sound right?


20L * 162.4bar = 3248L which is same as [email protected] 216.5bar ?
Closed:
Decant 1: ((15*220)+(10*100))/(15+10)=172
Decant 2: ((15*172)+(10*100))/(15+10)=143
Equalise: (172+143)/2=157

Open:
Decant 1: ((15*220)+(20*100))/(15+20)=151

Matt.
 

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does any of that change if you use a dir cylinder , after all it is a friday
 
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