Does an spg measure vs atmospheric or ambient pressure?

[rjack321]

just a little trivial nit, if that's the case then that's not really gauge pressure in the context of SCUBA diving.

That's more like a differential pressure measurement with 1ATM as the reference (or whatever the reference pressure is on the outside of the tube/inside of the sealed case)

Gauge pressure is a differential pressure measurement referenced to ambient.

Shrugs, it's not referenced to anything other than ambient. They are not designed to be brought to outer space, the face oring will fail and the face will pop off.

 

[bradlw]

ambient just means surrounding.... It doesn't mean 1atm, or any other particular value or property.

The ambient temperature you are experiencing is the temperature of the room you are sitting in, My ambient temperature is the temperature of the room I'm sitting in.
Standard atmospheric pressure at sea level is 29.92 inches of mercury, or approx 14.7 psi absolute, but actual ambient pressure at sea level is rarely exactly that. An hour ago, where I am, it was 30.00 inches of mercury...

but yes...shrugs. It's negligible for our purposes. Even deep, it's an approx 44psi difference at 99 feet of seawater

 

[NothingClever]

@bradlw

Bro, just go dive already.

 

[Rogerdd]

@inquis one thing I was thinking, due to the non-ideal gas behaviour it’s indeed not really necessary to adjust the amount of gas available by the depth at which it will be breathed when using the rule of thirds directly (to account for the reg’s min working pressure @ambient + 10 bar), as once we hit the first third (in pressure terms), the 2 remaining thirds will be a larger amount of gas than 2 times the first third. Conservatism is already embedded as you said (now whether this natural conservatism is enough depends on the depth and other factors but let’s assume it is)

But if we run a dive plan and calculate the estimated DCR for each part of the dive, and want to make sure that total estimated consumption is less than total available gas minus a reserve, say 1/3 reserve, shouldn’t we in this case take into account both the fact that the free liters capacity is lower (reduced by the z-factor), and the fact that the final few bars won’t be available to be breathed at depth?

For example twinset 11.1 L tanks, pressure 210 bar, z-factor at that pressure and temperature = 1.03, atm pressure = 1.013 bar

Free liters capacity = 2*11.1*210/(1.03*1.013) = 4468 L

Then assuming some SCR/SAC, bottom time at depth, etc.
For example estimated consumption = 2900 L

If we assume that we will be mostly using this back gas at 40m:
-Should we check that 2900 is less than 2/3 of 4468
- or less than 2/3 of (4468 - 2*11.1*14/(1*1.013))?

 

[Jai Bar]

You should be more concerned with the temperature difference between, say, the cylinder in boat/sunny beach/car trunk and once you enter the cool water. The pressure drop will be greater than whatever intrinsic accuracy the spg has.

 

[inquis]

But if we run a dive plan

Yes, absolutely use the real gas capacity (z*P*V). It's clear you like to address all the details, so sure, you can add in a reserve for the IP. (Your use of z=1 was correct for that.)

FWIW, Subsurface is one planner that understands non-ideal gases.

Also, consider picking your reserves based on consumption (i.e., get 2 stressed divers to the next source of gas) rather than some fraction of starting volume. A fraction is great for penetration since you're retracing your steps, but otherwise there are better approaches.