Minimal Acceptable Ending Tank PSI

[Dan G]

There's a lot of information here and I've learned a lot by reading it, so thanks OP for asking this question. Through much of this, perhaps the biggest practical take away needs to be that your psi gauge may not be accurate enough the further you get below 500 psi. You may be lower on gas than you think when it reads, for example, 250 psi. From what others are posting, I suppose if you are prepared for the possibility that you may run out of gas when you think you still have some, you should be OK.

I guess the question is: are you willing to run out of gas at the depths at which you run your cylinder down below 500?

 

[Flutter]

Thanks much for all your responses. On one of our dives on our last dive trip, my husband got a tank filled at 2300, which we noticed while gearing up at the site, a couple miles from the shop. (Yeah, I know we should have noticed it sooner). Rather than return to swap it out , I took the low tank & gave my husband my full one (I typically burn mine considerably slower). I kept a close eye on my consumption vs my husband's and we returned, completed our SS and headed in (I like to do a 5 min SS if air allows, and we stay within our NDL). The swim back could have been called & we could have surfaced at any point as we were in shallow, calm water. My SPG read 200 as we stood to walk our way to our truck. My reg was still breathing fine, & because I generally give my husband the fuller tank, we know that my spg reads 100 psi higher than his on the same tank. I was ok with the low reading, given the circumstances, but I did wonder if the lower pressure was going to give the shop tech grief.
On a previous, different trip, a shop gave me a nearly empty tank (by mistake) & I hooked my reg up just to see how a truly low tank would breathe (so I would know). At the 200 noted above, I did not experience the same beginning "difficulty to draw air" sensation as the nearly empty "test" tank (I believe that one was around 100 or a tad less).
Watching my air closely, & taking it easy, we did manage to get an hour dive out of the 2300 tank, but I did wonder if I was "risking" the tank's integrity, etc.
We do not run our air below 400 routinely, so thanks for your input. It's nice to know what shops consider acceptable vs unacceptable. (We don't want to irritate our shops. We tend to return to the same ones).

 

[fmerkel]

My personal experience on a balanced reg system; the couple times I was in very shallow water (less than 10'), and sucked the tank down, I got only a couple 'warning' breaths, then it seemed pretty much empty. I don't want to push that envelope as the tanks are steel and mine. Don't want salt water getting into those.
That's about the only way you are going to find out how accurate you gauge is at the low end.

 

[boulderjohn]

As has been indicated before, as long as there is positive pressure in the tank, there should be no problem; HOWEVER, any dive shop can have any policy it wants. Sometimes it is a scam, but sometimes they just don't know any better.

As for how far you can go in shallow water at the end of a dive both in regard to both safety and maintaining a minimum pressure in the tank, I was once present for a conversation that included a dive shop owner/instructor who was an avid photographer known for taking tons of pictures in the shallow water at the end of his dives. Someone asked him what his limit was--at what pressure does he finally surface?

He said, "CESA!"

 

[TMHeimer]

I shouldn't answer since I don't know and assume shops vary in requirements. I will say my shore dives usually end somewhat before I hit 500 PSI, depending on how cold it is. I have breathed my own tanks below that--to 200, even 100 on one occasion (the latter due to unexpected current), with no repercussions.

 

[Charles2]

When I was first certified, we did not have SPGs and since we were mostly just broke school kids, we breathed our tanks until we had to pull the J-valve. I think that was around 300 psi. We would continue diving shallow until we noticed an increase in breathing effort. The shop that filled our tanks only required that they be returned with positive pressure.

By the way, I calibrate my SPGs annually. If I ever found a 300 psi discrepancy, I would toss the gauge. I have never encountered that much error.

 

[HantsDiver]

We do not run our air below 400 routinely, so thanks for your input. It's nice to know what shops consider acceptable vs unacceptable. (We don't want to irritate our shops. We tend to return to the same ones).

That works both ways - the shouldn't want to irritate you. If they get funny and you really want to keep them sweet just get a decanting whip and top them up from another cylinder. It's what I do with my suit inflation bottle - it saves me having to get it tested.

In terms of spg accuracy - if you have or know anyone who had a transmitter then ask nicely to borrow it. You will be able to confirm if the gauge is accurate or not easily. You'd want to do with throughout the dive or at the very least at the start and the end. All the need to do is stick their first stage on and tell you the pressures.

 

[redacted]

If there is a hiss when the valve is cracked (to blow out water or contaminants) then no water entered your tank accidentally. If a hiss is audible and the shop objects, put your credit card away and leave.

 

[boulderjohn]

The last time I had a tank hydroed, the man who did the work asked me if I wanted him to put 500 PSI in it. I asked why. He said some local dive shops would refuse to fill it without a new visual inspection if I brought it in empty. Even with a brand new hydro stamp and a brand new VIS sticker? He assured me that it could be an issue in some places. I risked it and was fine, but he must have known of places where I would not have been fine.

 

[MXGratefulDiver]

I know of one dive op in British Columbia who has a blanket policy that if you end the dive with less than 500 psi you are done diving for the day.

His boat, his rules ... I don't agree with it, but since I usually end dives with way more than that (I like using large capacity cylinders) it's never been an issue for me ...

... Bob (Grateful Diver)