Minimal Acceptable Ending Tank PSI

[Flutter]

When shore diving, my husband & I like to stretch our dives as long as possible breathing our tanks fairly low after completing our SS (sometimes seeing amazing critters in the 5' shallows). We never drain our tanks completely dry, but we do run them down lower than the 500 psi required on many boat dives. From a dive shop perspective, how low is too low - and therefore a returned tank becomes an inspection issue? 300 psi, or 400 or ?? Is there an minimal acceptable amount, as long as it is still breathing air? Thanks. (For those who might be concerned about safety matters, our SS is completed by 500 & we are just generally burning air in the shallows afterward, seeing what we might see, in good viz, low current, ideal conditions).

 

[HantsDiver]

Depends on the size of the cylinder. And its personal really. You are interested in the amount of gas you have to breath - so 500psi in a ali80 is different to 500psi in a 18litre twinset in terms of usable gas.

First thing to work out is will you have enough to surface. Then work out if you will have enough to surface with your buddy diving off your alternate air source. If you have then all good. From 5 foot I wouldn't really care personally - if it goes to ratshit it's a very very short cesa.

All of that goes out of the window if you start to need mandatory decompression. But I'm assuming you don't.

I don't think I've ever seen a shop check the pressure for air. As long as it's got some pressure then nothing has got in so they won't care. If they do say something tell them to mind their own business.

 

[CptTightPants21]

From a dive shop perspective, as long as there is still actual tank pressure there shouldn't be any issues as water couldn't have entered the tank.

 

[Got2Go]

From a dive shop perspective, how low is too low - and therefore a returned tank becomes an inspection issue?

If you always fill at the same few dive shops, ask them.

If you want to be sure you get fills at unfamiliar shops, it's a crap shoot.
I've seen some that expect 300psi minimum, some that don't check (just connect whip and fill), and some that just check to make sure there is enough pressure to move the gauge needle a bit, and yet others that just crack the valve open and listen for gas coming out.

In my opinion, if you want to be on the safe side, stick to 300psi.

 

[g1138]

500psi is acceptable if you really get into the risk assessment and gear abilities at lower pressure.

PSI (Professional Scuba Inspectors) is the US nationally recognized & scuba industry wide experts on tank inspections, safety, and the like.
They're guidelines suggest as long as the tank contains positive pressure, it should not have water intrusion from taking it below minimum pressure.
So for tanks anything above 100psi and you'll hear a hiss and can assume tank integrity is still sound.

However, industry standard also allows SPG's to have a deviation of up to 300psi from the master calibrated gauge.
So your SPG may be 300psi above actual, or can lose drastic accuracy as you go lower in tank pressure. Sometimes I see gauges that hold great at 3000psi, but by 1000psi and lower it register above the actual tank and master gauge pressure. Essentially 800psi on the gauge was actual 500psi and that is still industry "legal".
Digital or Air Integrated gauges have a benefit over analog in these cases. They tend to be more accurate throughout the entire tank.

So for SPG's I wouldn't trust below 300psi.

Most 1st stages have an Intermediate Pressure of 125-150psi. Many shooting for 145psi. So at 145psi tank pressure, you will not get air through your hose. So 300psi is realistically 135psi of usable tank pressure before you suck dry. And that's assuming again that your SPG is accurate at that pressure.
You can also wrench loose the 1st stage by hand or impact at 200psi and below if you try hard enough. Heck, I had a deckhand take off my first stage at 500psi when he didn't know better.


This is why I plan to surface by 500psi.
That remaining air is for my absolute emergency or a surf exit assist.

 

[herman]

500psi is acceptable if you really get into the risk assessment and gear abilities at lower pressure.


Most 1st stages have an Intermediate Pressure of 125-150psi. Many shooting for 145psi. So at 145psi tank pressure, you will not get air through your hose. So 300psi is realistically 135psi of usable tank pressure before you suck dry. And that's assuming again that your SPG is accurate at that pressure.
.

This is inaccurate. The 125-145 IP is correct but you will get plenty of air from the reg at pressures will below that. The HP stage will stop regulating below it's set IP and simply stay fully open, delivering full (all be it low) tank pressure to the second stage. At this point the second stage will start breathing harder and harder until the tank pressure is very low, it is possible to drain a tank down to almost zero pressure but the breathing resistance will be quite high at that point. This is assuming an unbalanced second stage, balanced ones tend to breath much easier down to considerably lower IPs but they too will begin the breath hard at some point. Ambient pressure does have to be taken into account so it's more accurate to say you can still get air from the regulator as long as the tank pressure is above ambient but the OP was asking about very shallow depths so it's not much of a difference there.

From an equipment (tank)standpoint as long as the pressure does not drop to zero, ie any pressure at all in the tank is OK, as long as the internal pressure is above ambient, water or outside air for that matter can not enter the tank. The nothing less that 500psi or something might get into the tank is pure BS. ........unless ambient pressure is above 500 psi...which is over 1000 ft deep. Dive shops might not like it but there is not reason at all to suspect anything has "gotten into the tank" is pressure is only a few psi.

From a diver safety standpoint, it not a big deal either AS LONG AS the diver understands what they are doing. As I said before, the breathing resistance will get harder and harder with each breath after the tank reaches the regs IP so the diver has a lot of warning that the tank is very low. It's not one or 2 breaths like is often reported but quite a few, how many depends on the tank, the diver and their activity. It is a fairly long progression of harder and harder breaths before there is nothing remaining. I often drain tanks down to basically zero psi when they are going in for inspection (vip/hydro) after the dive so this is not speculation but actual experience backed up with a good working knowledge of regulator operation. That said, I will only do it when pool diving or in very shallow water when shore diving. While I am not advocating diving tanks down to very low pressures, lets face it, if you are diving in a few feet of water where you can simply stand up if the reg quits, how unsafe is that ,esp if the diver understands what they are doing and is expecting how the regulator will react.

 

[Neilwood]

The 500PSI realistically is a guideline that makes for easy counting. For the type of diving you are doing, there shouldn't be a need for a reserve once you have completed your stop (so long as you are within NDL, you can blow off a safety stop if required) so I wouldn't worry about going below 500PSI.

Saying that though, if your LDS won't fill a tank below 500PSI for whatever reason, then you need to comply with that (no point getting into an argument about it).

 

[tbone1004]

0 issues as long as you have positive pressure in the tank as said above.
As also said above, keeping it above about 200psi will make sure the regulator breathes normally, but I have no issues from a safety point taking it to 200psi in the shallows where you can make a direct ascent to the surface.

 

[RyanT]

What Herman and tbone said. If I'm doing a shallow reef dive on a single AL 80, I have no qualms about breathing the tank down to 200 psi by the time I finish my safety stop.

 

[kelemvor]

If you don't own the tank then it's got to be just a matter of the shop's opinion. The only way to know for sure is to ask the shop.