Minimal Acceptable Ending Tank PSI

[g1138]

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 ~~~~~.

When you get to about 200psi, how many breaths or minutes do you get on the tank?
And what size & what depth?

 

[redacted]

When you get to about 200psi, how many breaths or minutes do you get on the tank?
And what size & what depth?

At the depth the OP said they were at, they could easily be looking at 10 minutes with an Al80. 5 minutes should be quite easy before they need to stand up as inhaling may start to get noticably difficult.

 

[HantsDiver]

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)

Do they really run around the boat demanding to see your spg after a dive? What if you want to do a weight check at the surface so run them down? I can't believe people put up with this and still hand over cash to him. If any captain tried that in the UK they'd be out of business quickly.

 

[BRT]

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.

The part about not getting air through your hose at or below intermediate pressure is false. To prove this you just have to turn off your tank valve and breathe your tank down...to zero.

 

[EricTheDood]

Do they really run around the boat demanding to see your spg after a dive? What if you want to do a weight check at the surface so run them down? I can't believe people put up with this and still hand over cash to him. If any captain tried that in the UK they'd be out of business quickly.

A freeflowing reg during the surface swim could drain a tank pretty quickly to somewhere below 500psi.

Said event could also be used as an excuse or a cover up. Not that it should be. Just sayin'...

 

[g1138]

200psi in an Al 80 is roughly 5cf.
135psi (before IP is reached) = 3.4cf
Ok, so after doing the math, yeah a couple minutes of breathing at a passive SAC rate. Lazy generalizing got to me again.
But you're working for that remaining 2.6cf remaining cf once it gets below IP. Again then assuming your SPG is accurate.
I still don't see the risk/reward in going below 500psi to get a few more jollys from the tail end of the dive.
From a tank inspector issue though as long as it puffs, it gets refilled without an internal inspection.

The part about not getting air through your hose at or below intermediate pressure is false. To prove this you just have to turn off your tank valve and breathe your tank down...to zero.

I'll fight that one. Cubic footage is different in the hose VS in the tank.
You can't empty a 3000psi Al 80 in 4 breaths like you can on a 3000psi filled reg set.
200psi tank open is different from 200psi tank closed.

 

[BRT]

200psi in an Al 80 is roughly 5cf.
135psi (before IP is reached) = 3.4cf
Ok, so after doing the math, yeah a couple minutes of breathing at a passive SAC rate. Lazy generalizing got to me again.
But you're working for that remaining 2.6cf remaining cf once it gets below IP. Again then assuming your SPG is accurate.
I still don't see the risk/reward in going below 500psi to get a few more jollys from the tail end of the dive.
From a tank inspector issue though as long as it puffs, it gets refilled without an internal inspection.


I'll fight that one. Cubic footage is different in the hose VS in the tank.
You can't empty a 3000psi Al 80 in 4 breaths like you can on a 3000psi filled reg set.
200psi tank open is different from 200psi tank closed.

Cubic footage is different but you can breathe it to zero, tank or reg hoses. If you think there is some difference between the air in a hose and the air in a tank blow the tank down below IP and see if you can still breathe on the reg. You can. Right to zero. Which is why you can push your purge and clear the air out of your hoses and remove your tank valve. The first stage spring drives the first stage HP seat wide open when IP drops below the setpoint and the IP pressure becomes whatever the tank pressure is. It isn't magic, try it.

 

[redacted]

200psi in an Al 80 is roughly 5cf.
135psi (before IP is reached) = 3.4cf
Ok, so after doing the math, yeah a couple minutes of breathing at a passive SAC rate. Lazy generalizing got to me again.
But you're working for that remaining 2.6cf remaining cf once it gets below IP. Again then assuming your SPG is accurate.
I still don't see the risk/reward in going below 500psi to get a few more jollys from the tail end of the dive.
From a tank inspector issue though as long as it puffs, it gets refilled without an internal inspection.


I'll fight that one. Cubic footage is different in the hose VS in the tank.
You can't empty a 3000psi Al 80 in 4 breaths like you can on a 3000psi filled reg set.
200psi tank open is different from 200psi tank closed.

i breath all my regs down to empty just so I know how they will behave. This also allows me to see how each SPG is working in the low range. Unless you happen to bhe diving a Scubapro Mk7, which will stop delivering usable gas from the primary at about IP, you will be able to breath with reasonable comfort well below IP as long as your RMV stays low.

 

[Dan G]

Do they really run around the boat demanding to see your spg after a dive? What if you want to do a weight check at the surface so run them down? I can't believe people put up with this and still hand over cash to him. If any captain tried that in the UK they'd be out of business quickly.

The OP is shore diving when doing this and was wondering if dive/fill shops would want the tanks to come back with some gas remaining.

My understanding is that boat Captains make the rules. If he/she says, "Be back on the boat with x amount of gas," that is their right to enforce that rule. It is also a diver's right to take their business where they wish or buy their own boat.

 

[Moor3061]

My target is always to be at SS with 500psi. Which according to my gauge is plenty to complete obligation plus allows much extra time on shore dives, I normally breath down to 200 and slow ascend. Boat dive with boat captain rules is different story, I follow the rules when asked.