What pressure to end a dive? A debate!

[ArcticDiver]

First of all I've never been taught the "500 PSI" rule. Sure, SOME dive operators use it. But, many I've dove with do not.

The actual measurement I use is a carry over from my aviation days. "Have enough fuel(breathing gas) on board for the expected flight(dive) plus enough to get to an acceptable alternate(handle unexpected underwater problems) plus enough for an emergency(a buddy OOA)." In the flying business there are regulations and company flight manuals that prescribe actual amounts. We don't have that in scuba so we must figure out our own.

BUT, it doesn't do any good to plan if you don't dive the plan. The other day a guy literally climbed over me on an ascent because he all of a sudden realized he was at 90' with 500 psi. So, he blew off his stops and went for the surface post-haste. When he climbed over me at safety stop depth I had a full AL80 he could have used. Pity.

I should add that this operator drops a hang bar at 15' with a couple regulators attached just in case someone needs extra air. He could have used one of them. So, he passed up at least three air sources because he got freaked by not diving his plan.

 

[fisherdvm]

I think the surface PSI requirement should be achieved by breaking the air need into 2 components:

1. SURFACE REQUIREMENT: If it is rough, with large waves, and the currents are bad. If it is a night dive, where your boat might not be able to find you in due time. If there is increased chance that your buddy will be in trouble, and you need to help - then you better have more than 500 PSI. If it is a shallow dive, no waves, and everyone is comfy and smug - probably 300 PSI would suffice.

2. DEPTH AND DECO REQUIREMENT: Then you can factor in the depth factor to determine the PSI needed to ascend. This would be added to your decompression need (slower ascent time, 3 or 5 minute 15 ft stop), etc... So that could be an extra 200 PSI, or as much as another 700 on top of the surface requirement.

Added together, the decision to ascend should be based on a PSI of 500 to 1200 depending your nitrogen status, depth, and surface environment.

Divers should be taught to breakdown these two componenents in their dive planning to calculate the needed PSI required to return to the surface.

 

[ianr33]

Please tell me that you did not really mean "Decompression" in the above post??

 

[jsmith89]

I was told by a tech one day that the real reason for the 500psi rule was simply because all tanks expand when filled. The greatest expansion in the tank is when fillin g the first 500psi so if the tank is always kept above 500psi then when filling the amount of expansion and risk of rupture is much less and thus safer for whoever is filling the tanks.

 

[String]

May not apply in america but in most places where metric is used its "back on the boat with 50 bar" which is about 730psi.

The reason for the red zone on the gauge isnt due to low gas - its the tolerance limit of the gauge. Below that figure the reading is not as accurate regarding gas pressure so shouldnt be relied upon.

 

[HowardE]

 

[SteveAD]

I was told by a tech one day that the real reason for the 500psi rule was simply because all tanks expand when filled. The greatest expansion in the tank is when fillin g the first 500psi so if the tank is always kept above 500psi then when filling the amount of expansion and risk of rupture is much less and thus safer for whoever is filling the tanks.

two words on that.....BEEE ESSS

 

[HowardE]

How about asking the question -

at what pressure will an average 1st stage STOP DELIVERING GAS to the user??

More than the error of margin with SPG's - think about that...

 

[String]

Not convinced at that.

Breathed from mine a few times with both gauges reading exactly zero.

 

[HowardE]

Not convinced at that.

Breathed from mine a few times with both gauges reading exactly zero.

i wasn't saying yours. i was saying average... in general... etc. For safety of the weekend diver