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

[markfm]

Just for kicks, I checked my PADI OW book -- turns out it lists 20 - 40 bar (300 - 500 PSI) as the back-at-the-boat number (Dive Safety Practices Summary).
It doesn't go into any math, but right up in the first chapter it does explicitly talk about burn rate varying based on depth, that you are using 3x the air at 20 meters (The Effects of Increased Air Density).

 

[Kim]

but right up in the first chapter it does explicitly talk about burn rate varying based on depth, that you are using 3x the air at 20 meters (The Effects of Increased Air Density).

Which is of course completely useless (except for the most general of ideas) if you don't know what you use at the surface!

 

[Hooked-Again]

If you want to know why the agencies don't alter the standards , you should address your questions to them. I don't think Roakey's statement about most instructors being poor instructors is entirely accurate.

 

[Aquanaut4ata]

Whoa guys!! came up with this idea:

What if we installed a microchip in the tank that "tells" the SPG/computer the tank's volume and doesn't show you pressure, but volume (with the proper conversions, of course)??

Does anybody know a good, cheap patent lawyer??

Gio.

It's been thought of (or something close has)....on a Cochran Gemini computer you input tank size in liters. It doesn't show you remaining volume per say, but it's air integrated, so it monitors SAC and it shows remaining gas time in addition to remaining NDL time.

For the most part I agree with this thread. 500 PSI is most definitely not the same amount of gas in an AL80 as it is in a 100, or 120. I will say that I am surprised to hear so many people say that they are not taught any gas management in basic OW. Granted, the standards in my agency (and most I think) are that we teach divers to come up with 500 PSI, but there is more to it than that, and the standards are a minimum of what has to be taught, not a limit of what can be taught.

 

[WVMike]

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 did a google on Excalibur and can't find it. Do you have a url or source?

Mike

 

[djsmokyc]

Whoa guys!! came up with this idea:

What if we installed a microchip in the tank that "tells" the SPG/computer the tank's volume and doesn't show you pressure, but volume (with the proper conversions, of course)??

Does anybody know a good, cheap patent lawyer??

Gio.

The problem with measuring the volume of air in your tank is the fact that the volume in the tank never changes. You would need to know the tank volume and monitor the pressure in the tank to give a proper volume at use pressure calculation.

However, the information that is truly important is how many more minutes of air you have at the current depth and breathing rate. This information is something current air integrated computers can give you.

Teaching volume management during OW would be overwhelming for any student with lots of other things to worry about. You simplify the situation by standardizing tank size (Al 80) and teach the student to recognize and deal with the quantity their gauges are measuring directly - pressure. You wouldn't want an OW student, who can't even stay neutral, to be doing calculations on a table or slate underwater, trying to convert their current tank pressure into a breathable volume.

I just took my OW this summer and I definitely would have been overwhelmed by it, and I'm an engineer. I understand the idea that 500 psi on the surface is different than 500 psi at 100 fsw, so doing a volume conversion to prove or quantify that fact isn't going to help me understand that concept.

Gas management, or at least SAC rate calculation, is being taught to me for my Deep Diving class as part of my AOW that I'm currently taking through SSI. Deep Diving is where this concept most applies and the student should be advanced enough to be able to include this task along with other concerns.

 

[Ron Brandt]

Back on the boat at 500..... There are dive opps in Cozumel that either want you to end at 700 and one I dove with wanted us to end at 1000!

My take on this is that tey are allowing for air hogs on the safety stop.

I had one of our regular divers a couple of weekends back signal me he was low on air...he considered 1000 as low on air even for an open water lake dive to 37feet.

Ron

 

[Kim]

Back on the boat at 500..... There are dive opps in Cozumel that either want you to end at 700 and one I dove with wanted us to end at 1000!

700psi is roughly 50bar. Most places in Asia that I've been to want you back with around that. 1000psi sounds like a lot - would that be a thirds thing starting from 3000? Again though - 1000psi might not be too much if it's a small tank.

Dive ops obviously know the size of their own rental tanks. They probably set the limits from that. This removes the need for the diver to actually know it themselves as they probably don't expect the average diver to do gas planning in that way because they have never actually been taught how.

 

[Kim]

The problem with measuring the volume of air in your tank is the fact that the volume in the tank never changes.

I think you mean volume of your tank...not in your tank.

Using metric tanks you do know - my tanks are 14 liters. (fill them up with water and that's what would go in.)
Imperial measures tanks differently so it's a little harder to work out (IMO)

 

[PhinDiver]

What is the big deal for a RECREATIONAL diver? I have be diving for years within recreational limits and planning my dives so that I have enough pressure when I get back to the boat has always worked fine. For me it is 500 psi ON THE BOAT, not start ascending with 500 psi. Which means, I start ascending with more psi depending on the depth of my dive. Usually, i start going up on shallow dives at about 900 to 800 psi.