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1 Introduction

1.1 In a thread on Yorkshire Divers there was discussion on decanting, and some questions were asked about the maths involved in working out how much gas pressure you would expect in the receiving cylinder. The examples below ignore such factors as tank heating and cooling and must not be used to work out mixed gas fills.

2 Filling of similar sized tanks.

2.1 This is the easiest of the calculations to understand, as the pressure differential between the donor and receiver tanks is the only thing to consider. For example:

Donor tank 10L 200 bar

Receiver tank 10L 100 bar.

The pressure difference of 100 bar [200-100] will be halved and both tanks will end up with 150 bar.

2.2 To start looking at the maths involved in decanting filling, the above example can be used. The donor tank has 2 000 litres of available gas [200 bar x 10 litres] and the receiver tank contains 1 000 litres of gas [100 bar x 10 litres]. All you need to do to determine the final fill pressure is add the tank volumes together [in this case 10L+10L=20L], then the amount of gas in litres that is available [2 000L + 1 000L=3 000L]. Finally you divide the number of litres by the total tank volume to find the new pressure. In this example it is 3 000 litres of gas in 20 litres of tanks, or 150 bar in each.

3 Filling of dissimilar sized tanks.

3.1 One of the most common decanting procedures is to decant from a large donor tank into a small receiver tank. If we look at the example of a 15L cylinder being used to top off a 3L pony, we can use the example above and just change some of the figures:

Donor: 15L, 230 bar = 3 450 litres of gas.

Receiver 3L, 100 bar = 300 Litres of gas.

Total tank volume = 18L

Total gas volume = 3 750L

3750 divided by 18 = 208 bar [rounded down]

3.2 Another way of decanting is into a large tank[or tanks] from a smaller donor, sometimes of a higher working pressure than the receivers:

Donor 12L 300 bar = 3 600 litres

Receiver: twin 12L at 90 bar = 24L at 90 bar =2 160 litres

Total tank volume = 36L

Total gas volume = 5 760 L

5 760 divided by 36 = 160 bar.

4 Sequential Decanting.

4.1 There is a more efficient way of decanting into a twinset with an isolator valve from a single higher pressure source, by closing the isolator and decanting into each of the receiver tanks in turn.

Donor 12L 300 bar = 3 600 litres

Receiver: twin 12L at 90 bar = 24L at 90 bar =2 160 litres

4.2 Closing the centre isolator, it is now possible to decant into the tanks in turn, and the twinset can be thought of as a pair of single 12L tanks:

Donor 12L 300 bar = 3 600 litres

Receiver A 12L 90 bar = 1 080 litres

Total tank volume = 24 litres

Total gas volume = 4 680 litres

Final pressure [gas vol / tank vol] = 195 bar.

Now we fill the second cylinder of the twinset:

Donor 12L 195 bar = 2 340L

Receiver 12L 90 bar = 1 080 litres

Total tank volume = 24L

Total gas volume = 3 420L

Final pressure = 142 bar [rounded down]

4.3 By opening the centre isolator the pressures will equalise, and a tank of 195 bar and 142 bar will give a final pressure of 168 bar, 8 bar and 192 litres more efficient than filling the tanks with the isolator open.

5 Filling from a series of cylinders into a single donor.

5.1 Before air compressors of a high enough quality for breathing gas were available, dive clubs used to buy in a set of ‘J’ cylinders [O2 cylinders in dive shops are normally J size] to decant gas into diving cylinders. Modern divers don’t normally need to do this, but it is possible to decant from several donor cylinders into a single receiver. The important thing to remember is to go from the cylinder with the least pressure [but still higher than the receiver] to that of the most pressure to ensure that the gas is used in the most appropriate manner, and to ensure the highest final fill pressure.

If we look at filling a 3L pony from four 12L cylinders we can see how this would work:

Donor 1 12L 230 bar = 2 760

Donor 2 12L 210 bar = 2 520

Donor 3 12L 200 bar = 2 400

Donor 4 12L 190 bar = 2 280

Receiver 3L 50 bar = 150 bar.

After donor 4 the pony has 162 bar in it
After donor 3 the pony has 192 bar in it

After donor 2 the pony has 206 bar in it

After donor 1 the pony has 225 bar in it

If, however, the filling is done in the reverse order the pony will have less gas in it:

After donor 1 the pony has 194 bar in it

After donor 2 the pony has 206 bar in it

…and connecting it to tanks 3 and 4 would drop the pressure in the pony, so you would lose out on nearly 20 bar of pressure.

"Kite is right"
1,899 Posts

very useful summary, thanks for that

have some green


One half of team dip sh!t
1,638 Posts
Dr T said:
very useful summary, thanks for that

have some green

"o no he's going to blow himself up "

if you are going to start that caper, stay on the other side of the boat.:teeth:
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