Breathing tanks down

[carbon]

I was wondering exactly how far down can you breath from a tank before it stops delivering air. I would assume that it would be roughly equivalent to the low pressure setting (120 to 150 psi) plus the water absolute pressure at your particular depth.

Has anyone had experience breathing a tank down? Do you remember what your pressure gauge read?

Do different regulator designs make a difference?

Thanks,
Carbon :11:

 

[DA Aquamaster]

It depends on the regulator and the SPG.

An unbalanced piston first stage will tend to breath harder as the pressure falls below about 500 psi. and they tend to stop breathing almost entirely if the tank pressure falls much below the intermediate pressure (usually about 110-120 psi at low tank pressures). With an unbalanced piston design, the intermediate pressure (IP) falls as tank pressure decreases which increases the inhalation effort required from the (normally) unbalanced second stage.

When the tank empties the piston does end up in an open position due to the spring forces involved, but the orifice is comparatively small and breathing off one at very low tank pressures is much like trying to suck air through a very long straw.

An unbalanced diaphragm first stage on the other hand will experience an increase in IP as tank pressure falls and the reg will actually breath easier and may even freeflow slightly if not properly adjusted.

A balanced first stage does not have the same noticeable increase in breathing effort at low tank pressures as an unbalance piston design and can be breathed at tank presures well below the IP with only a slight increase in breathing resistance particularly if they are combined with a balanced second stage. The orifice also tends to be larger whihc allows more airflow at very low pressures. In this situation, the last few to several breaths in the tank (depending on depth and tank size) will have a prgressively higher inhalation effort but not much warning will be given that you are almost out of air.

Mechanical SPG's tend to be most accurate in the middle of their range and accuracy at the full and empty extremes can be off a couple hundred psi. This can mean that you have a couple hundred psi left when the gauge reads zero or it can mean the tank will be virtually empty when it says it is empty. If you have a tank due for a VIP that needs to be emptied anyway, it is often worth the trouble to breathe the tank down in your living room to see just how accurate your SPG happens to be and to see how the reg breathes at low tank pressures.

I have had a digital pressure gauge and an air interated computer that registered 200-300 psi at the end of the dive and continued to insist this was the case when they were no longer even connected to the tank. They both went back to the manufacturer as an error in that direction is very unsafe.

 

[Viscya]

I'll go with DA on this. I've breathed a few dry on deco, and the guage said empty before the gas stopped flowing. It could easily be the other way.

Guages are mechanical and can be inaccurate. I've had my tanks filled at two different shops, both cool at 3000, when I got them home (still cool) one read 3000 the other 2800.

Depth also affects the preasure that the first stage will stop delivering gas.

 

[DandyDon]

I'll go with DA on this. I've breathed a few dry on deco, and the guage said empty before the gas stopped flowing. It could easily be the other way.

Guages are mechanical and can be inaccurate. I've had my tanks filled at two different shops, both cool at 3000, when I got them home (still cool) one read 3000 the other 2800.

Depth also affects the preasure that the first stage will stop delivering gas.

Carbon, are you new to scuba? You haven't posted any Dive Profile info?

DA Aquamaster has well answered your question (no surprise there!) but from my one Incident in which I ran dry, I can confirm what Viscya says about depth. I hit a dragging feeling on the last 2 breaths as I swam horizonally towards the deco line, knowing those were the last breaths I'd get at that 20 foot depth. I shared air from another, generous diver for my deco stop, then went back to my "empty" tank for the slow ascent, picking up a couple of additional breaths as I ascended.

I also had pulled my snorkel outa' my pocket and snapped it to my mask strap on the deco stop, in case I'd need it on the surface. That was the day that plans, A, B, C, D, and E all failed. I'm deveolping better equipment configuration and more humble & careful approaches to my deep diving :blush: next month.

 

[carbon]

Thanks guys,

I particularly hadn't considered that the pressure gauge reading at low pressures might be very inaccurate! As you suggest, I think I'll try my gauges to see how they read at very low pressures.

Carbon

P.S. DandyDon: I updated my profile

 

[Viscya]

Actuallly, this is one of the reasons they say to get on the boat with 500lb. It gives a margin of error for reg performance, guage acuracy, and sloppy surface conditions.

Dive safe,

V

 

[mech]

If you breath down a steel tank to zero it could very well rust up on you and require tumbling to clean it out.
Rust is not a good thing for a regulator.
It just isn't worth the risk.
Andy

 

[deadend]

It WILL breathe down to zero. Guage says zero, no air comes out, and when you disconnect first stage from tank, no air comes out. Do I do this intentionally or often? NO. Has it happened? YES.

 

[Don Burke]

If you breath down a steel tank to zero it could very well rust up on you and require tumbling to clean it out.
Rust is not a good thing for a regulator.
It just isn't worth the risk.
Andy

Going for a cleaning and visual as soon as you get the tank out of the water would preclude rust.

It takes a while to get a clean tank to the tumbling stage.

While I would not flatten a tank if there was another option, I sure would not cut deco short to surface with gas.

 

[mech]

DB,
I agree.