Question Calculate gas cubic feet

[marc.collin]

hi

i have a 85 cubic feet low pressure so 2400+, 2640.

this bootle had 1376 psi

I need to feed it to 1920psi

I search to calculate number of cubic feed to add

My try

1387 / 2640 * 85=44.6
1920 / 2640 * 85=61.81

61.81-44.6=17.21

17.21cubic feet of gas added?

thanks

 

[Manatee Diver]

17.21cubic feet of gas added?

Pretty much.

 

[rx7diver]

hi

i have a 85 cubic feet low pressure so 2400+, 2640. ...

If your 85 is a Faber LP 85, it holds only 81.1 cu ft at 2,640 psig. (See Faber LP85 Cylinder w/Boot, Hot Dip Galvanized, for example. Scroll down.)

rx7diver

 

[gbirch]

When considering any cylinder, dividing the fill capacity by the working pressure tells you how many cubic feet are added per psi of fill pressure (sometimes referred to as the "index" of the cylinder):

Cylinder Capacity / working pressure.

It is not clear from your question if your cylinder design spec is for 85cuft at 2400, or 85cuft when plussed (2640). Assuming the former, adding 544 psi (1920-1376):
adds 544*85/2400 = 19.2cuft

If it is the latter case (85cuft @ 2640psi) then 544 psi (1920-1376):
adds 544*85/2640 = 17.5 cuft

 

[marc.collin]

it's Faber LP 85 who has been used

 

[inquis]

If your 85 is a Faber LP 85, it holds only 81.1 cu ft at 2,640 psig.

Yeah, it's weird how an LP85 has nothing to do with 85 cubic ft. OP, you're looking at 16.4 cf added (= (1920-1387 psi) / 2640 psi * 81.1 cuft). However, many (most?) shops don't know this and will multiply by 85 cuft instead when figuring the price (if they charge by cuft).

 

[rx7diver]

Yeah, it's weird how an LP85 has nothing to do with 85 cubic ft. ...

Yes, similar issue with the Luxfer Al 80, which holds only 77.4 cu ft nominal at 3,000 psig.

rx7diver

 

[justinthedeeps]

Are you actually adding gas in measured amounts of volume? Do you have some kind of volumetric gas gauge? I am guessing that you don't.

The only way I've ever added gas to a cylinder is in measured changes of pressure.

The Faber "LP85" is actually a 13 litre internal volume cylinder, manufactured in a metric country.

I suggest ditching cubic feet entirely, and working in metres, bar, litres.

Re-measure the pressure in bar.
Multiply this number by the internal volume (13 L).
That is how many bar-litres you have in the cylinder.
Every bar of pressure added will be 13 more litres.
Why is it so easy? Because it was designed to be simple.

Canada should switch over entirely to metric for SCUBA. Why has it not?

There really isn't any good reason to keep using feet or psi in diving anymore--unless your gauges can only show feet or psi. That is rarely true now. Sell or retire the feet-only and psi-only gauges--keep them around as cherished relics, like olde muskets form the 1700's.

 

[Boarderguy]

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Plug in the numbers and do the math based on true volume of your tank.

 

[inquis]

Yes, similar issue with the Luxfer Al 80, which holds only 77.4 cu ft nominal at 3,000 psig.

Actually, that one makes sense: it's 80.1 cf ideal, so within rounding. (It's 77.4 cf including compressibility effects.)