How much is really left in tank?

[Akimbo]

We account for “usable gas” in commercial diving all the time, especially when looking at the bail-out bottle — the cylinder carried in case gas supply is lost from the hose\umbilical.

I use a spreadsheet with precision as high as 10 decimal places for many conversions. Rounding errors can drive you nuts and isn’t any more effort.

0.035314667 Ft³/Liter
14.69594878 PSI/Standard Atmosphere
33.01429999 Feet of Seawater\Standard Atmosphere (based on a density of 64.1 Lbs/Ft³ average)
0.445138889 PSI/FSW​

This calculation centers on subtracting bottom pressure AND the lowest breathable intermediate pressure your first stage delivers. Most balanced first stages can deliver reasonably low inhalation resistance down to the +/- 135 PSI intermediate pressure where an unbalanced first stage is closer to 300 PSI. Also remember that calculations are in Gauge Pressure (Sea Level Pressure), not absolute, since you can’t suck the cylinder down to a perfect vacuum.

So, at 100’ you lose something like 179.5 PSI plus the floodable volume in the tank. This is just a rounding error on Bourdon Tube SPGs that are +/- 2½% accurate or worse. However, this becomes usable information with a typically +/-¼% accurate integrated air computer display. This may or may not matter to recreational divers, but when that number is 580 PSI at 1000’ and time back to the bell is measured in seconds rather than minuets, it matters a lot. It is up to you and your dive profile to figure out when it becomes important to you.

 

[aquaregia]

How hard can a person suck to reduce this? I'm guessing not a lot. I've used a 12" straw to drink water, so that'd be like 0.5psi or something?

 

[Akimbo]

How hard can a person suck to reduce this? I'm guessing not a lot. I've used a 12" straw to drink water, so that'd be like 0.5psi or something?

The actual pressure is way less important than what it feels like to you. I think it is prudent for everyone to know what pressure their regulator(s) performs before it becomes unacceptably difficult to inhale. Test it for yourself anytime between completing a dive and filling your tank.

Hook up your regulator and start breathing the tank down. Note the pressure you first notice any increased breathing resistance and again when you can’t suck any more out without turning blue. It is good to know what it feels like and the pressure it happens at. Just remember that you are at sea level and must add the depth/pressure at the depth you are using it.

You can get the most out of the exercise by doing the same thing in a few feet of water in a swimming pool. Swim at a moderate pace and note the time between noticing and turning blue. Be sure to divide this “drop dead” time by the absolute pressure of your dive — yet another reason to dive an SPG or computer in Bar rather than PSI.

This drill will also help “calibrate” you to your SPG/computer — is really empty when your gauge reads 50 or 200 PSI?

Edit: Don’t forget that the time factor is not only tied to depth, but the size cylinders you are testing. You can get the feel and pressures from a pony bottle, but time changes with the capacity.

 

[knowone]

Another example of omitted necessary practical usefulness, for the start of ones foray where solid instructional emphasis should focus, rather than at
the end.

 

[Diver0001]

Could not really see a forum that truly fit this question, but will post it here.

For some reason this thought came in my mind and I can't shake it, so have to ask.

Anyone thought about how much pressure could be lost driving air to and through regulator or other devices?
In other words to be conservative, does anyone deduct a set amount, psi or bar, to account for this?
Is it so small it is not relevant?
Is this the dumbest thought anyone has come up with on the forum?

It should be about 2 lungs-full of air. Maybe 4-6 litres max, which in terms of pressure is about 1/2 bar in an average sized tank. That's not worth accounting for.

@ knowone... mate I can't understand what you're saying in your last post. It looks like your post got mangled in an attempt to edit..... can you try re-wording it? Thx.

R..

 

[treidm]

Trained in the 500 psi school of thought, but through planning dives (Don't use dive computer), have thought there must be a more accurate way than a set amount (too many variables).

Can't afford more classes, so been thinking on my own.

I am trying to plan as accurately as I can to curb the possibility of OOA occurrence in worst case scenario and buddy breathing is needed "Rock Bottom". Will not ever plan less than 500 psi, but plan for more if it's needed.

I thought of this one night and if it is a variable note worthy of adding to my planning......

I misstated the question, sorry, wasn't really referring to volume of gas (Although it would contribute a small amount), actually in hoses/reg cavity etc., was trying to get more at what post # 9 (TSandM) started to touch on and post #11 (Akimbo), truly hit on.

More accurately:
At what point does regulator stop delivering gas, to be able to know true gas available for use in dive?

Main/Primary Regulator = SP MK25/S600
Mainly 80cft AL, 3000 working (77.4 cu ft)

So if regulator can't deliver gas below x psi, I don't wish to count on this gas (that can't be used)

Also considering doubling RMV's for both divers in emergency situation into my figuring, is this too high?

Thanks to all for contributing thoughts & sorry for posting in the incorrect forum.

And sorry, if this turns out to be an insignificant post for most, just trying to figure things out as I progress.

Akimbo, I sure would like to have a working copy of your spread sheet. hint, hint

 

[1_T_Submariner]

I have breathed a tank on purpose until it quit (Not far under water). When I took my regulator off I could open the Tank valve and stop the pressure with my finger (I'm guessing a few PSI) it registered nothing on my SPG.

 

[Bubbletrubble]

Also considering doubling RMV's for both divers in emergency situation into my figuring, is this too high?

@treidm:
As written, "doubling RMV's for both divers" would result in quadrupling the gas supply to get one diver to the surface safely. That will probably get you in the ballpark of a good enough gas reserve. Bear in mind that calculating the number in this way assumes that the dive team can ascend directly to the surface. Don't forget to account for ascent time at a controlled rate and stops in your calculations.

Generally, a person calculates a "rock bottom" gas supply for a given depth by allowing for a stressed RMV (2x the average RMV for one diver?) for a minute or two at depth (to calm down, problem solve, organize dive team), figuring out gas usage for ascent time, adding gas usage during any stops, and then doubling that entire sum to account for the dive buddy. The scenario being planned for here is complete loss of a gas supply for one diver at depth.

FWIW, from my own experience, I've found that it's entirely possible for a "stressed" RMV to be actually greater than 2x an "average" RMV. Just something to think about...

 

[treidm]

@treidm: I'm not sure I understand your question.
As written, "doubling RMV's for both divers" would result in quadrupling the gas supply to get one diver to the surface safely. That will probably get you in the ballpark of a good enough gas reserve. Bear in mind that calculating the number in this way assumes that the dive team can ascend directly to the surface. Don't forget to account for ascent time at a controlled rate and stops in your calculations.

Generally, a person calculates a "rock bottom" gas supply for a given depth by determining a stressed RMV (2x the average RMV for one diver?) and then doubling that number to account for the dive buddy. The scenario being planned for here is the complete loss of a gas supply for one diver in a two-man buddy team and getting both divers to the surface safely.

FWIW, from my own experience, I've found that it's entirely possible for a "stressed" RMV to be actually greater than 2x an "average" RMV. Just something to think about...

"Also considering doubling RMV's for both divers in emergency situation", was again misstated, as I didn't add enough information, sorry..

But you did get at what I wanted, for an emergency ascent with buddy OOA situation. Yes other factors, safety stops etc. will be figured in, I was only asking about a few aspects, I've been thinking about, so didn't include all possible parameters.

your method = ex. RMV is 1cfm, double that for stressed is 2cfm, then you say double that for other diver, so total of 4cfm.

I said doubling for both divers. ex. regular RMV diver1 is 1cfm, double that is 2cfm, RMV diver2 is 1cfm, double that is 2cfm, add the two is 4cfm.
Same answer.

The reason I wanted to separate the divers is in case I had 1 RMV and diver2 had 1.5 or 2cfm RMV (gas hog). Knowing just my RMV and not my buddies doesn't seem the way to do it, but I will consider and think this all through. Thanks for responding....Reid

And again, sorry, I don't articulate very well in typing/written form.

 

[Bubbletrubble]

The reason I wanted to separate the divers is in case I had 1 RMV and diver2 had 1.5 or 2cfm RMV (gas hog). Knowing just my RMV and not my buddies doesn't seem the way to do it, but I will consider and think this all through.

@treidm: Yeah. If you are buddied up with an air hog, that needs to be taken into consideration when you're working out the gas plan. If you know your buddy's RMV, then by all means use that number for your calculations.

A RMV of 1.5-2 cfm is...ummm...very much an "air hog," but a person's RMV is what it is. With more dive experience that number moves down into a more normal range. Experienced divers have "average" RMVs that are in a range between 0.30 cfm and 0.70 cfm. For a given person, the RMV can vary quite broadly based on a multitude of factors: current, cold, low vis, stress, fitness level, sickness, just finning around more than usual, being uncomfortable, not being used to gear, etc.

Also, don't forget to allow for increased gas density of each breath at depth. RMV is calculated at 1 ata (the surface). A regular RMV of 1 cfm means that, at a depth of 99 fsw, for example, the diver will be consuming 4 cfm. After running all of these numbers, you begin to see how conservative one should be with gas reserves.