Are all Cu. Ft created equal?

[avw]

A common and simple rule for gas management is to return to the boat with 500psi in your tank. For the ubiquitous Al80, this is the same as saying return to the boat with 13cu. ft of air.

However, with another tank, HP130 for example, 13 cu. ft of air corresponds to ~340 psi, presumably because the HP130 has a larger interior volume.

My question: Is 13 cu. ft of air in a HP130 the same usable amount of air as 13 cu. ft in a Al80? Or more generally, when considering gas reserves is the amount (cu. ft) the only consideration? Does the difference in pressure matter at all?

I'm wondering if when you get to the bottom of a tank if there is any problem accessing that air, maybe b/c you are approaching the intermediate pressure of a regulator... I imagine depth might matter too.

I'm thinking of this b/c in another thread someone was strongly admonishing another poster for finishing a dive with ~400 psi. But said diver was diving a LP85, and 410 psi in a LP85 is equivalent in cubic feet to 500psi in an Al80 and if both are equally consumable, a diver would have the same remaining time left with either tank.

I DO NOT want to start an argument about the appropriateness of the 500 psi rule--I'm just curious if cu. ft at lows psi's are as usable as cu. ft at higher pressures.

Thanks,
Adam

 

[Thadmn]

My Regulator (poseidon) Makes A Strange Noise When I Get Down In The 200psi Range, But Still Delivers Air. Of Course, I Don't Do This Unless I'm On A Shore Dive And Just Playing Around In The Shallows Before Exiting. Normally You Would Use The Rule Of Thirds. Yes, I Would Have More Air In My Doubles Than In My Single Steel 66, But The Pressures Being The Same, My Regulator Would Make The Same Weird Noise, Just For A Longer Time Using The Doubles, Until I Ran Out And Drowned.

 

[captndale]

My question: Is 13 cu. ft of air in a HP130 the same usable amount of air as 13 cu. ft in a Al80? Or more generally, when considering gas reserves is the amount (cu. ft) the only consideration? Does the difference in pressure matter at all?

Yes the pressure does matter. With any regulator, when the tank pressure gets very low it starts to get hard to breathe. When tank pressure is at or below the intermediate pressure setting of the first stage the second stage will not perform well. In some regulators the second stage may not work at all or may break into a free-flow.

 

[redacted]

When your tank pressure drops below about 150 psi, most 2nds stages will start breathing a bit harder. How much harder depends on the specific design. With your HP130 example, you hit that pressure with about 5 ft^3 remaining. With that S13, it does not occur until the last ft^3. So, in that case, the difference may be rather large.

 

[captndale]

My Regulator (poseidon) Makes A Strange Noise When I Get Down In The 200psi Range, But Still Delivers Air. Of Course, I Don't Do This Unless I'm On A Shore Dive And Just Playing Around In The Shallows Before Exiting. Normally You Would Use The Rule Of Thirds. Yes, I Would Have More Air In My Doubles Than In My Single Steel 66, But The Pressures Being The Same, My Regulator Would Make The Same Weird Noise, Just For A Longer Time Using The Doubles, Until I Ran Out And Drowned.

Pilot operated Poseidon regulators require a minimum intermediate pressure to operate. If the i. p. drops too low there may not be enough pressure to close the demand valve. This can result in a freeflow and loss of any remaining gas. This is what is happening (in reverse) when you first turn on the tank valve and there is an immediate rush of air through the second stage until pressure builds up and the second stage demand valve closes off.

 

[Charlie99]

There are two things that make part of the reserve PSI-dependent, regardless of the size of tank.

The first is the IP and/or minimum operating pressure of the 1st stage (which increases 15psi per 33' of depth). Let's assume that you need about 140psi IP + another 50 or so for reasonbly easy to breathe operation. So an estimate of 200psi needed. (Yes, you CAN breathe off a reg below this, but it gets harder and harder).

The second is the inaccuracy of SPGs. You can eliminate most of this error simply by noting what the true zero point is on your SPG, but are you really sure that your SPG is reading 200psi rather than 100psi or 300psi?? This adds another 100 or 200 psi.

Put these two factors together and even with a monster sized tank you can't really count on getting to use of the last 300 or 400psi or so of the air in your tank.

 

[ClayJar]

All cubic feet of air are approximately equal with modern regs. I was draining a tank to do a viz on it, so I just hooked it up and dove a backyard pool until it was empty. It didn't feel difficult to breathe until the last few breaths. Just about any reg will let you breathe it down so low that you can unscrew the valve without opening it first.

The spot that gets you into trouble, however, isn't the reg. It's the SPG. Pressure gauges are best as measuring the middle of their range. You wouldn't want to use an IP gauge to measure tire pressure, as it's made to read best in the 100-150 psi range. It will not give as good results at 30 psi as a gauge designed for that range.

Near the bottom of your SPG's range, the measurements are less reliable. Do you *really* have 200 psi left, or might you have a mere 150 psi? Perhaps you have 250 psi remaining, but you can't be as sure. Maybe the gauge manufacturer put a "fudge factor" in, and anything less than 100 psi leaves the needle resting on the limit stop as "zero". Without a calibrated gauge, you can't be *completely* sure, and even with a calibrated gauge, the gauge is designed more for the middle than for the extremes (it's just a fact of physics and engineering).

If you have a gauge that might read 50 psi high at the bottom end, that would be 1/10 of your reserve in a 500-psi AL80. If you were diving double 130s with 3442 psi service pressures, the same 13.3 cubic feet of reserve would be about 177 psi. The same gauge error of 50 psi would be a full 28% of your reserve in the doubled 130s!

Of course, even if you had perfectly calibrated your gauge in a climate-controlled chamber with all sorts of high-dollar gear? How well can you read it? If you can't reliably read the difference between 120 psi and 140 psi, you'll have to adjust your reserve upward to compensate.

 

[ronbeau]

There are two things that make part of the reserve PSI-dependent, regardless of the size of tank.

The first is the IP and/or minimum operating pressure of the 1st stage (which increases 15psi per 33' of depth). Let's assume that you need about 140psi IP + another 50 or so for reasonably easy to breathe operation. So an estimate of 200psi needed. (Yes, you CAN breathe off a reg below this, but it gets harder and harder).

The second is the inaccuracy of SPGs. You can eliminate most of this error simply by noting what the true zero point is on your SPG, but are you really sure that your SPG is reading 200psi rather than 100psi or 300psi?? This adds another 100 or 200 psi.

Put these two factors together and even with a monster sized tank you can't really count on getting to use of the last 300 or 400psi or so of the air in your tank.

Are the electronic versions of the SPG such as those built into AI dive computers a lot more accurate at the low end than the mechanical SPG?

 

[Gombessa]

I think that depends on whether the actual mechanism inside the electronic SPG is the same. You've got a fancy digital display at the end, but if the gas-integrated part uses the same old bourdon tube mechanism, there's no reason why the accuracy would be appreciably different.

 

[RJP]

Double post - oops!