1/3 Rule?

[windapp]

Also, mechanical pressure gauges can be wildly inaccurate, especially with use. I've seen old SPGs that were off by 150-psi. A guy ran out of air when his SPG showed 150-psi and I had to feed him my air for a bit. Luckily we were at 30-ft.

Pressure gauges like all other equipment should be checked regularly. I would not dive with an SPG I knew was off by 150 PSI.

 

[uncfnp]

Yes, BUT... How much pressure do you actually need in your tank for nothing to be able to get into it at say 20 meters?
3 bar should do, lets say 5 just for good measures.. In other words, less than you want in the tank to be able to surface..

So, as long as I surface with roughly 100 psi, my steel tanks are safe? Theoretical speaking of course. :cool2:

 

[oldschoolto]

Pressure gauges like all other equipment should be checked regularly. I would not dive with an SPG I knew was off by 150 PSI.

I guess you would not had dived in the early 70's then.... First time I had a SPG was like 1979-1980.... J valves were your SPG....

Jim...

 

[Tigerman]

So, as long as I surface with roughly 100 psi, my steel tanks are safe? Theoretical speaking of course. :cool2:

The tank should be. If you are is a whole other matter

 

[Steve_C]

I have a sensus ultra in my BCD pocket. Give the software the start and finish PSI and it will compute the SAC in PSI. You then need to adjust for the tank size.

 

[fnfalman]

Pressure gauges like all other equipment should be checked regularly. I would not dive with an SPG I knew was off by 150 PSI.

When was the last time you sent out your SPG and mechanical depth gauges for calibration?

When was any shop?

 

[thirdcoastdiver]

Steve Lewis (SB Member "Doppler") wrote a book (The six Skills and Other Discussions) .the book has some excellent information on dive planning and how to determine air requirements. I wish that I would have read the book when I started diving.

 

[chrpai]

When was the last time you sent out your SPG and mechanical depth gauges for calibration?

Not "calibration" but I do have 2 SPGs and 2 Computers that I will use to cross check each over.

 

[Colliam7]

I would not dive with an SPG I knew was off by 150 PSI.

I am curious about this. Why not? If you know that it is off by 150 psi, why would you not dive with it? I presume by this that you mean a SPG that shows 150 psi when the cylinder is off - i.e. it is at ambient pressure.

 

[lamont]

In terms of gas available, PSI is a relative measure. 500 PSI on an HP 130 is very different from 500 PSI on an AL80 (much less 500 PSI on an AL19 pony)

Depth (in feet) * 10 + 300 works for Al80s, HP100s,LP85s and similar sized tanks
Depth (in feet) * 10 works for 130s/119s/LP104s/etc

Gauges can vary a bit. Do you really have 200 PSI when your gauge gets down to 200 PSI?

i've tossed a few gauges over the past 10 years that read 200-300 psi when not hooked up to a tank.

---------- Post added April 29th, 2013 at 08:15 PM ----------

I would like to thank you all for the quick and precise response.

That "rock bottom" has opened my eyes, and shockingly so! Clearly one very important factor was not taken into consideration in everything I was told. And that was, 2 of us might be breathing that air. An just how much air we will both be consuming for a safe ascent.

Your answers at the least might of saved a very dangerous situation for me or a buddy, and at worst (case) might of saved a life in the future.

That link is now saved, and will be reread many times (I will become comfortable with the equation).

Thank you.

John

Also, you're probably just not very good at solving problems at depth and moving shallower quickly and aborting your dive, and you may not have considered issues like the need to swim back to an upline.

Consider the case where you encounter something unexpected at 100 feet on a wreck dive and you may be significantly delayed at depth dealing with the problem, and then may need to be sharing gas during some or all of the exit.

500 psi will last for a long time when you're hanging at 10-20 feet. 500 psi will get sucked out of your tanks real fast at depth when you've got two excited divers breathing off of it. 2 cu ft / min for 2 divers at 100 feet on an Al80 is 310 psi per minute. So if you're at 500 psi and your buddy goes OOA on you, and things get exiting, you run a good chance of being OOA again in another few minutes. 1300 psi in an AL80 at that rate will last 4 minutes at depth. If you assume maybe a more reasonable 0.8 cu ft / min breathing rate (two fairly typical male divers kind of excited), then you get a little over 5 mins. If you are both in reasonable shape and can stay calm and breathe at a combined 1.0 cu ft / min then the Al80 wil last 8 minutes.