How many fills from a cylinder

How many fills from a cylinder

Postby gaschef » Fri Jun 03, 2016 1:44 pm

As an RFD and a dive store owner I am often asked how many fills can I get out of my cylinder

Having a few hours of spare time I decided to calculate the amount of fills available from a range of commonly available cylinders

I have based the calculations on the following, these are approximate figures but are fairly near to the mark

400cc Buddy Bottle filled to a nominal 180 bar, this gives 72 litres of air in the cylinder
Once the pressure in the main cylinder drops below 180 bar I have used the pressure remaining to calculate buddy cylinder volume
I have stopped the list once the main cylinder pressure reached 130 bar as this is about the point most people have their cylinders refilled

A few other variables
I have rounded calculations up or down to the nearest Bar

No allowance has been made for temperature/pressure changes

Remember that many 300 bar cylinders are not fully filled to begin with. Many dive shop compressors struggle to reach 300 bar, cylinders loose a lot of pressure due to cooling. To get a true 300 bar quite often I will need to top the cylinder up about 3 times

No allowance has been made for ambient temperature changes when filling the buddy cylinder

I have also listed air fill prices, and estimated prices for new and used in test cylinders.

Always remember that it will cost about £35 for a cylinder test plus what ever else is required to test cylinder, so a cheap cylinder may not be cheap if it fails the test

AS I have said these figures are approximate, but there are so many variables I have tried to be as accurate as possible

If however someone was requiring a cylinder, the most cost effective route is a 232 bar 15lt or a 232 bar 12lt unit

Cylinder pressure remaining after each fill
No of fills 232 X 15lt 300 X 12lt 232 X 12lt 300 X 10lt 232 X 10lt 300 X 7lt 300 X 3lt 232 X 3lt
1 227 294 226 292 224 279 252 184
2 222 288 220 285 217 268 228 160
3 217 282 214 278 210 257 204 138
4 212 276 208 271 203 247 180 120
5 208 270 202 264 196 237 156 104
6 203 264 196 256 188 226 135
7 198 258 184 249 181 216 117
8 193 252 178 242 174 206 101
9 188 246 172 235 167 196
10 184 240 166 228 160 185
11 180 234 158 220 154 175
12 174 228 152 213 148 165
13 169 222 148 206 142 156
14 165 216 143 199 136 147
15 160 210 138 192 131 138
16 156 204 133 184 131
17 152 198 129 177 123
18 148 192 170
19 144 186 163
20 140 180 157
21 137 174 151
22 133 168 145
23 162 139
24 157 133
25 152 128
26 147
27 142
28 137
29 132
30 129

Fill Price £5.00 £6.50 £5.00 £6.50 £5.00 £6.50 £6.50 £5.00
New
Cyl Price £247.00 £290.00 £218.00 £279.00 £200 £262.00 £175.00 £128.00
Used
Cyl Price £125-140 £220 £120 £200 £75-£140 £180 £130 £60-£80

Hope this helps
gaschef
 
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Re: How many fills from a cylinder

Postby gaschef » Fri Jun 03, 2016 2:52 pm

eek help etc, for some reason the table has condensed itself together, any one know how to separate in columns, or how to post and excel spread sheet, as I cant work out how to do either.
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Re: How many fills from a cylinder

Postby TClancey » Tue Jul 18, 2017 10:04 am

My personal advice would be spend the bit extra and get a 300 bar bottle. Due to personal issues I bought a bottle pump, expensive but when you take time/travel and fill expense into consideration it's paid for itself in a couple of years. But I do shoot a lot.

The thing with the 300 bar bottle is, you get more warning when the bottle is running down, with the 232 you get a few full fills, then all of a sudden you don't. With the 300 you get loads of full fills and you start to notice when the high fill level slows down, there's your prompt to go and fill it. Also, if you want to fill your Galahad to 250, you can't with the 232.

Not sure if that's any help.
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Re: How many fills from a cylinder

Postby TenMetrePeter » Wed Aug 23, 2017 6:12 pm

Or you can apply some O level physics and work it out from first principles.

Assumptions for typical sirgun shooter...
1. Assume ar is an ideal gas at constant temperature throughout so using Boyles law to
convert all air to free atmospheric pressure volume
V2 = V1 * P1 / P2
2. Tanks start at 300 bar and finish up at 180 bar before refilling
3. Gun cylinders filled to 180 bar and are used down to 120bar
4. Gun cylinder is 2.8cm inside diameter and 30cm long (s400 MPR)
so gun volume is 2.8 * 2.8 * 30 * 3.142/4 = 185 cc = 0.185 litres
5. Tank is 3 litres at 300 bar.
6. No losses when connecting.
7. Atmosphere is 1 bar.
Using the formula...
Volume of free air in a full 3 litre 300 bar tank is 3 * 300 / 1 = 900 litres
Volume of free air remaining in tank at 180 bar is 3 * 180 / 1 = 540 litres
So air available to fill the gun is 900 - 540 = 360 litres.
Similarly,
Volume of free air needed to fill cylinder from 120 to 180 is
0.185 * (180 - 120) / 1
= 11.08 litres

Therefore fills for this scenario is
360 / 11.08

= 32 fills

Substitute your values into this! Remember the gun fill pressure (180) has to be the same as the tank minimum pressure.
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