What's the reason for 500psi/50 bar?

[DutchDown]

I don't know why I never gave it any real thought, but it just occured to me that I don't know WHY we are so often told to end our dives at 50 bar/500 psi. Following a fruitless search, I thought I would ask the question here. I fear that I have learned the reason somewhere, but do to advanced age, I've forgotten...

Phil

 

[OutdoorStud]

The main reason is to give oneself a fair reserve of air for unexpected circumstances.
(for example many cave and wreck divers use a thirds rule one third in, one third out, and one third as reserve)


Secondary to that is the fact that pressure in the tank keeps water out.

edit, this thread might five you some food for thought http://www.scubaboard.com/showthread.php?t=187953

 

[fisherdvm]

I think a scuba instructor, String, explained that, just like bathroom scales and gas tank gauges, the lower ranges are inaccurate, and psi's below 500's are not to be trusted. As I understand, the way how gauges work, is a metal tube that is slightly bent.

The air pressure straighten this tube and causes the needle to move. My understanding of spring operated scales is that the lower ranges do not register well, and the spring constant is more accurate in the mid ranges.

With this logic, readings below 500 might mean the number you read, more, or less... In anycase, it will not last very long, unless you are very near the surface.

 

[J.R.]

... didn't the ol' "J" valves generally also hold a 500 lb. 'emergency reserve'? Seems like this 'convention' has roots that go back a ways...

 

[amascuba]

Somebody determined that the average person would need 13ft3 of gas to do an emergency ascent from 100 ft. In a standard AL80 tank 13ft3 == 50bar/500psi.

In all reality though that is a liberal number and the proper reserve should be much higher for a dive to 100ft.

 

[merrken]

500 Psi Is Not The Only # To Remember The Condition Of The Dive May Call For More Psi Check With Dive Master Or Conditions For That Dive .current Or Depth .

 

[J.R.]

I was just reading through Lamont Grnquist's "Rock Bottom and Gas Managment" article at www.scriptkiddie.org/diving/rockbottom.html :

"No rock bottom pressure should be lower than 500 psi to take into account the possibility that an SPG doesn't read zero accurately. Even for a 30 fsw dive on dual LP-120s the rock bottom pressure should be 500 psi. I have retired an SPG which consistently read 300 psi over actual tank pressure when measured against a variety of other calibrated pressure guages at different shops. At 0 tank pressure it still read 300 psi. Vacation divers who rent all their gear should probably take note."

There's another thread going on [ http://www.scubaboard.com/showthread.php?t=187974 ] that addresses SAC rates and dive planning ... I think it may also have some relevance to the OP's question...

 

[vladimir]

I don't know WHY we are so often told to end our dives at 50 bar/500 psi.
Phil

I guess divers using imperial units need less air.

 

[Thalassamania]

Actually the old J-valves usually held back 300 psi, but that was with steel thanks that held, if I remember 2250 psi. When 3000 psi aluminums came out springs for 500 psi came available so that you could have about the same reserve. The j-valve was predicated on the idea that you pulled the rod and started you ascent (at 60 feet per min with no stops) to the surface, so you got there with basically 300 or 500 psi.

 

[fisherdvm]

I was reading the specification of a new analog SPG, and at least while new, they guaranteed accuracy to plus or minus 72 psi at ranges less than 580 psi, and plus or minus 145 psi at greater than 1450 psi.

As the metal age and fatigue, and without recalibration, my guess is that pressure gauges can be very inaccurate at ranges below 500 psi. I imagine, with a bathroom scale, at least you can re calibrate or re-tared. But with an spg, how would one do that??

My curiosity would be how reliable the digital wireless transmitters are? With these sensors being so small and dependent on digital technologies, would you really trust them to be accurate at the lower pressure ranges? It sure seems nice to get rid of a hose and gauge, but I think I would still like a simple pressure operated spg.