How much is really left in tank?

[treidm]

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?

 

[deepstops]

Not the dumbest thought by far but it's insignificant.

Maybe a breath and half and that's with a 7' primary hose.

 

[ptyx]

It isn't a stupid question - at which point isn't there enough pressure left in the tank for your regulator to work correctly?

I don't remember the answer, but it has been asked before and it probably depends a lot on the regulator. That said, it is of limited practical use as:

• it is below the standard 500 PSI minimal reserve
• in that range, your SPG is not accurate enough to be trusted to that level of precision

 

[scubadiver888]

This is an excellent question because it shows you are thinking.

The truth of the matter is that the amount of air isn't significant. The margin of error on your gauges is probably greater than the amount of air in your lines. You might be able to find how much air is consumed by the system but it would be difficult to measure the error in your gauge. Additionally, the error in your gauge will change over time.

So rather than worry about how much air is used by the lines, regulators, etc. you want to leave a safety margin.

You have probably heard that you should end your dive with 500 PSI or that you should never breath a tank down to 0 PSI. This is not only so the tank remains pressurized and keeps moisture out but because it is hard to say when the tank will actually be empty. Your gauge could read that you have 50 PSI left but the system really has 37 PSI left.

 

[DevonDiver]

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A ScubaBoard Staff Message...

Moved the post into Basic Scuba.

It's a well-informed debate, but nothing that fits specifically within the scope of the Advanced Scuba forum.

Advanced Scuba Discussions - A forum dedicated to discussing the transitional process of migrating from the traditional recreational divng envelope into more advanced areas such as advanced nitrox and trimix diving, decompression diving, cave diving and wreck penetration diving, blackwater diving, etc. This forum isn't a place to discuss technical diving per se - that's what our technical diving forums are for - but rather to discuss those baby steps to becoming a true advanced diver who's ready to step into tech.

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[knotical]

Interesting question.

You can get a rough idea of how much gas is "lost" by considering how much the volume of your high pressure cylinder is effectively increased when you pressurize your regulator. In a typical setup, start with the amount of space in the high-pressure side of your regulator and the space in the HP hose and SPG. That's not much. Those spaces, and the inside diameter of the HP hose are all typically small. Add to that a fraction of the space in the low-pressure side of your regulator, your LP hoses and your second stages. A fraction because a small amount of high pressure gas expands to fill a much larger volume in the low-pressure part of your system. Say 1/20 if your cylinder is pressurized to 3000 psi and your IP is 150 psi.

And not all of that gas is "lost. You'll end up breathing some of it.
As others have said, the total amount is pretty small.

Sometime you might want to try the following. Measure the pressure in a cylinder with a stand-alone pressure gauge. Then put a regulator set on, pressurize it, depressurize it and remove. Then re-measure the pressure. I doubt you'll see a difference.

 

[LeadTurn_SD]

Very little volume is lost.... but some is. If am understanding the original question... if it referred to the volume of the hoses.

Example: When I service my regs, I am using a full scuba tank to provide air for testing and tuning the regulators.

Before I start to service the regs, I pressurize them, check the Intermediate Pressure (IP), and inspect all fittings and hoses for subtle leaks. Then I turn off the tank, purge the regs and service them. After the regs are reassembled, they get pressurized / depressurized several times (first to check 1st stage IP, then to individually "tune" the 2nd stages).

At the end of this process, I will notice that a couple hundred pounds of tank pressure on an Al80 have been used. Maybe more if I had to "fiddle" with the 2nd stages a bit to get them tuned properly.

So while the hoses do not hold very much air.... they do hold some. Depending on the hose length and regulator, enough air for maybe 2 normal breaths.

Best wishes.

 

[Garrobo]

Just get out of the water at 500psi and you won't have to worry yourself about mundane issues such as this.

 

[TSandM]

I'm not a physicist, but this is how I understand the issue: The first stage brings the pressure in the tank down to 140 psi (may vary with regulator) above ambient pressure. Therefore, it would seem that, when the pressure in the tank is less than 140 psi above ambient, the first stage will fail to open. At 4 ATA, that's approximately 200 psi. So yes, there is gas in the tank when the reg stops delivering it, which is why the agencies teach that, if you are doing a CESA, you should retain the regulator in your mouth -- as the ambient pressure falls, the first stage will regain the ability to deliver gas, and you may get another breath on the way up.

 

[DevonDiver]

It's also one of the reasons why agencies teach divers to maintain a reserve of 500psi/50bar.