How much air to surface with?

[stuartv]

I was doing a training dive where we had a person acting as a dive safety officer. His briefing included telling everyone to arrive back on the surface with at least 500psi in their tank. I asked him: For people who are diving bigger tanks, do they still need to be back with 500 psi? For example, another diver had double 120s. If he got back to the surface with 250psi, he would still have more air in reserve than the guy with an AL80 at 500psi. The DSO said "no, everyone needs to be back on top with 500."

500psi in an AL80 seems to be pretty standard as the "get back with" figure. If you do a safety stop at 10m, that means 2 atm (i.e. just under 30psi) of air inside the tank will have you OOA. In other words, at 10m, 500psi left means ~470psi usable, which means, actually, 12 cu-ft of usable air at the end of your final stop.

To get to the end of a safety stop at 10m with 12 cu-ft of usable air in an HP120 would mean having (if I've done the arithmetic correctly) 380 psi (again where ~30psi of that is "unusable at 10m depth", right?).

And double HP120s would mean 205 psi to have 12 cu-ft left.

Is it pretty universal and standard to expect everyone to "get back on the boat with 500psi" even if they are diving bigger tanks or doubles? Or would it actually be considered correct to plan for a specific volume of usable gas on exit, rather than a specific psi that is used without regard to the capacity of the tank being used?

 

[tbone1004]

fairly common. You also have to consider the IP requirements for the regulators to function properly, so you need to keep tank pressure well above IP, so bare minimum 200psi, and ideally significantly higher than that. That said, if you plan your rock bottom calculations properly, you will never surface with less than that in any tank unless you are actually sharing air due to an emergency, which of course rules out a minimum surface rule. With a normal balanced regulator, this isn't much of an issue, but with Poseidons, it can cause them to freeflow if the IP drops. Also with less than a few hundred PSI in the bottles, the valves are no longer seated securely and if hit the right way they can start turning, which is bad....

In general, you are correct, you need to calculate your reserves based on CF instead of PSI, but have to keep the IP scenario in mind. Run rock bottom and you'll never have to worry.

 

[Patoux01]

(again where ~30psi of that is "unusable at 10m depth", right?).

Why? AFAIK, most if not all regs are depth compensated?

 

[scubadada]

Correct, it's more reasonable to consider surfacing with some acceptable amount of gas in reserve rather than an arbitrary psi, something around the 12 cf with an AL80 at 500 psi in not unreasonable. It depends on the dive, I often drift dive at about 60 feet and have free access to the surface at any time. I often choose to surface with less than 12 cf of gas.

 

[stuartv]

Why? AFAIK, most if not all regs are depth compensated?

Unless I'm mistaken, if you're at 10m, the ambient pressure is 2 ATA. If the tank pressure is also 2 ATA then you're not going to be able to pull any air out of it. The whole scuba system relies on tank pressure being higher than ambient pressure. 1 ATA is 14.7psi, so 2 ATA is 29.4 psi (rounded to 30 in my previous post for simplicity).

---------- Post added September 9th, 2015 at 03:35 PM ----------

fairly common. You also have to consider the IP requirements for the regulators to function properly, so you need to keep tank pressure well above IP, so bare minimum 200psi, and ideally significantly higher than that. That said, if you plan your rock bottom calculations properly, you will never surface with less than that in any tank unless you are actually sharing air due to an emergency, which of course rules out a minimum surface rule. With a normal balanced regulator, this isn't much of an issue, but with Poseidons, it can cause them to freeflow if the IP drops. Also with less than a few hundred PSI in the bottles, the valves are no longer seated securely and if hit the right way they can start turning, which is bad....

In general, you are correct, you need to calculate your reserves based on CF instead of PSI, but have to keep the IP scenario in mind. Run rock bottom and you'll never have to worry.

If I'm interpreting all that correctly, you are saying that, instead of calculating usable gas left based on 30 psi, I should calculate it based on 200 psi.

So, an AL80 with 500 psi means 300 psi "usable", or 8 cu-ft.

A single HP120 would be 433 psi (for 200 psi + 8 cu-ft).

Double HP120s would be 317 psi.

Is that better, as a "how much do I get out with" calculation?

This is based on your statement that regulators won't function properly without tank pressure being above IP. I thought that normal balanced regulators would still work right down to the point where tank pressure equals ambient - though probably getting harder to breathe. What makes a normal balanced 1st and 2nd stage set not work when the tank pressure gets down below the normal IP? Say, you're at 10m, normal IP is 140psi, and the tank pressure has dropped to 100psi. You're saying at that point you would not be able to pull a breath of air? Something would start freeflowing?

As far as running rock bottom, yes, I've been taught that for Tech dives. But, I have never been aware of anyone I've been around doing that for a Recreational dive. It's really Rec dives that is motivating this question. I dive HP120s and what I was really wondering about was what would happen if I go out on a boatful of people diving 80s and, during the drive briefing, they say to be back on the boat with 500 psi. And then I really maximize my time underwater and get back on with 400 or 450. If someone on the boat crew notices that, are they going to give me a ration of crap? Are they going to bench me? Or are they going to understand when I say "hey, it's a 120" and have no problem with it.

 

[KevinNM]

No, if the tank can't supply the IP required by the second stage you are OOA. That is typically 130-150 psi. Plus at under 500 psi/40 bar the SPG isn't particularly accurate, and the lower you go the bigger the error.

 

[scubadada]

fairly common. You also have to consider the IP requirements for the regulators to function properly, so you need to keep tank pressure well above IP, so bare minimum 200psi, and ideally significantly higher than that. That said, if you plan your rock bottom calculations properly, you will never surface with less than that in any tank unless you are actually sharing air due to an emergency, which of course rules out a minimum surface rule. With a normal balanced regulator, this isn't much of an issue, but with Poseidons, it can cause them to freeflow if the IP drops. Also with less than a few hundred PSI in the bottles, the valves are no longer seated securely and if hit the right way they can start turning, which is bad....

In general, you are correct, you need to calculate your reserves based on CF instead of PSI, but have to keep the IP scenario in mind. Run rock bottom and you'll never have to worry.

Interesting post, I went back to my downloaded dives to check my lowest surfacing PSIs. I have 2 dives below 200 PSI, 130 and 140. Of course, the breathing was at the safety stop, I recall no unusual characteristics, and I was paying close attention.

 

[stuartv]

No, if the tank can't supply the IP required by the second stage you are OOA. That is typically 130-150 psi. Plus at under 500 psi/40 bar the SPG isn't particularly accurate, and the lower you go the bigger the error.

I would have expected that with an unbalanced reg. I would not have expected that with a balanced 2nd stage. Interesting. Why is that? I thought that was the whole point of a balanced 2nd stage. Just like the point of a balanced first stage is to keep the IP (its output pressure) the same even when the tank pressure (its input pressure) drops. Similarly, I thought a balanced 2nd stage would keep its output pressure steady even as the input pressure drops - obviously, until the input pressure drops below what the output pressure is.

---------- Post added September 9th, 2015 at 03:41 PM ----------

... and I was paying close attention.

I bet you were! LOL

 

[BRT]

No, if the tank can't supply the IP required by the second stage you are OOA. That is typically 130-150 psi. Plus at under 500 psi/40 bar the SPG isn't particularly accurate, and the lower you go the bigger the error.

I dove a bunch with a surface supplied rig I build that only produced 50 psi. At 50 feet you had to breathe slow and steady but the regs worked fine.

 

[Russoft]

...so bare minimum 200psi, and ideally significantly higher than that. That said, if you plan your rock bottom calculations properly, you will never surface with less than that in any tank unless you are actually sharing air due to an emergency, which of course rules out a minimum surface rule. With a normal balanced regulator, this isn't much of an issue, but with Poseidons, it can cause them to freeflow if the IP drops.

I was breathing on a Poseidon Cyklon at 10bar/150psi during a safety stop and didn't notice any ill effects. I shortly thereafter switched to buddy's octo to preserve pressure on the tank/first stage. Maybe I would've had breathing resistance soon afterwards had I continued breathing below that pressure. Wasn't aware that this could be a problem.

As a side note, when I took OW, the "ascend at 50 bar" rule was hammered into my head.

When my wife took OW 13 years later, it was "surface with 500 psi".

I'm beginning to believe the first rule of thumb is only useful for shallow dives, while the latter is more useful if you can plan your gas for ascent. We never learned gas planning in OW.