Why the 50bar/700psi rule in the first place?

Kim · Sep 15, 2005

First off - I dive using the metric system so if 700psi isn't quite correct I apologise!

Anyway. I often think that this basic tenent taught in OW - 'back at the surface/on the boat with 50bar' - is fairly nonsensical. Because it takes no account of your tank size it takes no account of the real amount of air that you have left. For example, if I dive a 10ltr tank to 50bar then I have 500ltrs left - but if I dive a 15 ltr tank to 50bar then I have 750ltrs left. Wouldn't it be more sensible to teach new divers how to know exactly what volume of air they have, and need, to make safe ascents and keep safe reserves?

spectrum · Sep 15, 2005

You're right it''s just a nice easy palatable number that's thrown around.

A SPG may be off by + or- 200 psi down at low range so if you accept that you want to end the dive with a holding pressure to protect the gear from water entry then you get to 500 pretty fast.

It has no basis on really ending the dive safely and as you mention it does not reflect the true number of Cubic Feet of air you end the dive with. How many divers do you think will sit down and do a rock bottom calculation?

Pete

Kim · Sep 15, 2005

spectrum:
How many divers do you think will sit down and do a rock bottom calculation?

Pete

Maybe more would if they were taught to think in terms of volume rather than pressure right from the beginning?

markfm · Sep 15, 2005

I generally hear "On the boat at 500 psi" (34 bar) (Excalibur is a pretty good RPN calculator, for Windows, free, includes a good set of conversions)

I'd expect it's purely an OW thing, predicated on an average new person diving an AL80. 500 psi at the surface would be enough air to provide an assist in an OOA for someone diving at OW depths.

By the time you start diving 100s or larger tanks, the diver will hopefully know a bit more about air consumption, be able to adjust margins accordingly, be thinking in terms of his/her air consumption for different profiles while still leaving margin to help a buddy.

RIOceanographer · Sep 15, 2005

Well, first I agree, you are right about how it should be trained.

Second, just as a matter of speculation I wonder if that standard ending pressure thing isn't an artifact that has its origin in the old j-valve days when divers didn't have SPGs and used a j-valve to warn them when it was time to surface. The j-valves kicked in at a fixed pressure (I am not sure what the pressure actually was), so its about the same concept as setting a fixed pressure you are always supposed to return with.

roakey · Sep 15, 2005

"Back on the boat with 500psi" is not gas planning, it's gas guessing. This has been addressed dozens of times before on this board as just a plain bad idea. Search for the string "rock bottom" to understand what should be involved in gas planning for "simple" OW dives.

Roak

Kim · Sep 15, 2005

roakey:
"Back on the boat with 500psi" is not gas planning, it's gas guessing. This has been addressed dozens of times before on this board as just a plain bad idea. Search for the string "rock bottom" to understand what should be involved in gas planning for "simple" OW dives.

Roak

I agree - it's a really bad idea! My point was more along the lines of: being such a bad idea, why is it still being taught?

japan-diver · Sep 15, 2005

The 50bar/500psi rule is used because there is no gas management taught in OW and most instructors are to lazy to add it to there class. This has been talked about quite a bit on this forum though as mentioned earlier.

roakey · Sep 15, 2005

Kim:
I agree - it's a really bad idea! My point was more along the lines of: being such a bad idea, why is it still being taught?

You men besides the fact that most of the instructors are fast-track divers with little experience and are threfore just plain poor instructors to begin with?

"Back on the boat with 500psi" is what they've been taught, and by golly, that's what they'll teach others!

Roak

Kim · Sep 15, 2005

roakey:
You men besides the fact that most of the instructors are fast-track divers with little experience and are threfore just plain poor instructors to begin with?

"Back on the boat with 500psi" is what they've been taught, and by golly, that's what they'll teach others!

Roak

Partly that is what I mean. However it goes higher than that. Instructors don't set the standards normally - the agencies do. I actually wonder why the agencies themselves don't alter the standards to reflect a safer approach.

Why the 50bar/700psi rule in the first place?

Please register or login

Kim

Here for my friends.....

spectrum

Dive Bum Wannabe

Kim

Here for my friends.....

markfm

Contributor

RIOceanographer

Contributor

roakey

Old, not bold diver

Kim

Here for my friends.....

japan-diver

Contributor

roakey

Old, not bold diver

Kim

Here for my friends.....

Similar threads