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

[diverdan214]

I guess divers using imperial units need less air.

Yeah I never figured out when diving abroad why if you had a Bar spg the redline was at the equivalent of ~735psi, I guess 35 bar doesn't have the same ring to it as 50 or someone just figured, eh, we'll drop a zero.

Also as many idiots I've end up consistently sucking a tank dry on the way back up, if they weren't told to be at the surface with 500 they'd definately be doing CESA or worse using their inflate button as an elevator button every time.

 

[String]

Yeah I never figured out when diving abroad why if you had a Bar spg the redline was at the equivalent of ~735psi, I guess 35 bar doesn't have the same ring to it as 50 or someone just figured, eh, we'll drop a zero.

You're assuming that imperial was there first. The opposite could be true in that it was always 50 bar and someone then decided to make it 500 psi to keep things even.

In reality reading 35 bar on an SPG is quite difficult, seeing 5 bar differences is hard on most gauges especially if not viewing from directly in front. 50 bar is also near 1/4 of a full tank when you back to the old days of aluminium and 206bar fills so it could be related to that as well. These days nearly all tanks are 232 bar and steel outside the US area so it isnt as valid but the convention started long before that.

And yes it was me that mentioned the red line traditionally indicated a minimum tolerance of a gauge below which it wasn't to be trusted as much.

Also as many idiots I've end up consistently sucking a tank dry on the way back up, if they weren't told to be at the surface with 500 they'd definately be doing CESA or worse using their inflate button as an elevator button every time.

Telling them to be there on 50 bar you get a range of people from 20 bar to 70 bar randomly. If you picked a lower figure, say 30 im fairly sure you'd see a LOT more people running out through not checking their gauge or their inability to manage their own gas.

FWIW 50 bar at 6m/20ft in a standard 12l tank will last most people 30 minutes or so so its a big safe reserve.

 

[diverdan214]

You're assuming that imperial was there first. The opposite could be true in that it was always 50 bar and someone then decided to make it 500 psi to keep things even.

You're right I did assume that, I appologize. I try to stay neutral in Metric vs. Imperial debates, but I guess having been raised under one system its hard to let go of some bias. I guess Cousteau would have been using bar but I don't know when the 'redline' was adopted. My point about dropping a zero was mainly fucitious. I didn't mean to suggest that 500psi was a superior arbitrary number than 735 (50bar) I think they're both convientient arbitrary numbers for whichever system.

Telling them to be there on 50 bar you get a range of people from 20 bar to 70 bar randomly. If you picked a lower figure, say 30 im fairly sure you'd see a LOT more people running out through not checking their gauge or their inability to manage their own gas.

That was my point...sorry if that was unclear, or maybe you were just reiterating that.

 

[String]

Chances are these days at least with better equipment etc that BOTH figures are just arbitrary numbers picked because they are easy to remember and both allow a sensible reserve for normal divers and those without not exactly good gas management skills.
Not too big, not too small and easy to remember. Gauge red line probably then adjusted to fit that.

 

[DutchDown]

Chances are these days at least with better equipment etc that BOTH figures are just arbitrary numbers picked because they are easy to remember and both allow a sensible reserve for normal divers and those without not exactly good gas management skills.
Not too big, not too small and easy to remember. Gauge red line probably then adjusted to fit that.

I actually suspected that it was a bit arbitrary to allow for those who can't manage their gas supplies. but I wanted to ask other's opinions. I understand the need to have sufficient gas for safety stops and emergencies, but to be told by dive operators that my dive should be finished with 25% remaining at the surface is a bit annoying. I am uncertain if I agree that spg's are innaccurate at lower pressures, unless that is a design specification. Consider that we drive our autos at speeds exceeding 120km/70 mph relying on 3 dollar tire gauges from Kmart to keep us safe. They operate at 30psi/2 bar. Personally (and this is not to encourage others to do the same), I generally dive until 20-30 bar is indicated, then surface. The argument could be made that I am pushing the envelope. That's okay...I like to argue.

Phil

 

[String]

Remember a tyre gauge operates over a very low range of pressures, typically 0 - 4 bar whereas an SPG has to cover from 0 bar to 300+.

For people who can actually manage gas common sense just prevails. 50 bar in a 12l tank at 6m is 30 mins or so of air, i see no problem in running it down to 20 bar or so provided its sane, shallow reef or whatever.

 

[Gilldiver]

Chances are these days at least with better equipment etc that BOTH figures are just arbitrary numbers picked because they are easy to remember and both allow a sensible reserve for normal divers and those without not exactly good gas management skills.
Not too big, not too small and easy to remember. Gauge red line probably then adjusted to fit that.

You are most likely very close to the target. The numbers are just easy to remember. They also:

Give a goodly reserve (1/6th of 3000 psi)
Allow for enough press to keep water out of the tanks
Allow for "cheap" regs to keep working, some of the lower cost regs just will not work well bellow 300 psi or so.
The numbers reflect the J-valve era and double hose regs.

Don't get scientific on some of these conventions. So are just because they are. They also are because time has shown that they do work.

 

[String]

Also the figures are a sensible reserve regardless of tank size.

50 bar in a 10l tank (500 litres) vs 50 bat in a 15l tank (750 litres) obviously are different amounts but even in a worst case scenario, the smallest tank its still a goodly volume of air. And using the same figure for all size tanks means that although it might be a bit too conservative for larger tanks its easy to remember and therefore apply to everything.
Simple is the key here.