Checking my numbers for gas matching

[Ulfhedinn]

Do these numbers look correct? I fill the math is wrong somewhere but then math is not my strong suit.

Cylinder Size	AL80	LP85	LP95	LP104	HP120	HP130	LP72	HP100
Liquid capacity liters	11	13	15	17	15	17		13
Rated volume ft3	77.6	83	95	104	120	130	72	100
Working pressure (psi)	3000	2640	2640	2640	3442	3442	2640	3442
Liquid capacity ft3	0.38	0.46	0.53	0.58	0.51	0.56	0.40	0.43
Baseline (psi per ft3)	39	32	28	25	29	26	37	34
Current pressure (psi)	89	108	124	136	120	130	94	100

 

[tbone1004]

@Ulfhedinn psi/cf is complicated. LP104 and HP130 are the same, so are LP95/HP120 and LP85/HP100 because the water volumes are the same.
psi/cf is a bad unit because it's only applicable when everything is filled to the same pressure.

 

[Ulfhedinn]

I saw a post on cavediving.com and it prompted me to build out from their list. Seems the site is down so I thought I would build my own from scratch. @tbone1004 I was hoping I could print it out on waterproof and post it in my web notes for quick reference but I'm guessing that's not gonna happen.

 

[inquis]

You might consider using a different Baseline to give easier numbers to work with. A lot of people use cubic feet per 100 psi. For example, double LP85/HP100 would be a bit over 6. I find it easier to multiply/divide by 6 than by 32.

If the thought of doing math while in the water troubles you, another option is to put a table in your wet notes of pressures (rows) and tanks (columns) with gas volume in the cells. It's then trivial to see who has the smaller gas volume -- and is therefore the controlling diver -- and then convert to pressures for everyone else's tanks. These values can also consider the Z-factor (compressibility) for more accurate numbers.

 

[tbone1004]

in cave diving we are all assuming that steels are filled to 3600 so in that case with the LP tanks you would use their HP equivalents for cf/100psi. Unfortunately this is grossly misunderstood in most of the community which is annoying but it's the nature of indirectly measuring tank pressures vs. the rest of the world that measures capacity directly.

Just remember, a LP104 and a HP130 will ALWAYS hold the same volume when filled to the same pressure, no matter what any math says for cf/100psi or whatever other nonsense. Physically they are the same volume and as such will hold the same capacity when filled to the same pressure.

 

[EFX]

If the thought of doing math while in the water troubles you, another option is to put a table in your wet notes of pressures (rows) and tanks (columns) with gas volume in the cells.

Why would anyone do gas matching under the water? Shouldn't this be done on the surface among the team members? When I took my cavern course the gas matching was done on the surface. What am I missing?

 

[Lorenzoid]

Why would anyone do gas matching under the water? Shouldn't this be done on the surface among the team members? When I took my cavern course the gas matching was done on the surface. What am I missing?

On the surface, with the table written in wetnotes because they're waterproof. Alternatively, the table could be on a slate.

 

[tbone1004]

Why would anyone do gas matching under the water? Shouldn't this be done on the surface among the team members? When I took my cavern course the gas matching was done on the surface. What am I missing?

Usually done in the water but at the surface so tank pressures equalize.

The use of that math "under the water" is if you are recalculating thirds or if on CCR/DPV calculating bailout. Very useful once you get beyond tourist level cave diving.