Reserve limit 500 psi always? What about doubles?

[mddolson]

I recently had to deal with an annoying charter captain.
This was an advanced o/w wrech charter.
No technical dives, no penetration, all within sport diving rules.
However a few pairs in the group were technical divers, (myself included) diving with double steel 100's, isolation manifolds reduntant regs & gauges) .
At the safety brieifing the reserve limit was set at 500 psi. In that all divers would surface with at least 500 psi min gauge pressure.
I questioned the captain about the 500 being valid for doubles, & he responded with a hard line " thats the rule" dive by it or don't dive.
I was annoyed with his attitude though, when I pointed out he was requiring the doubles divers to have twice the reserve as singles, he wouldn't listen, didn't care, no discussion.

With an isolation manifold this is not a real problem. Close the crossover. Dive like you have two singles. No biggy or dive the twins down to 500-600 psi, then close the cross over and use the remaining from one tank down to 200 or so, and still show 500 psi reserve. In fact one would finish with the equivalant of 700 psi in a single 100 cu ft tank.

I'm not one who pushes the limits & I ended my 3 rd dive of the day with 600 psi in both tanks, I didn't need to close the crossover to meet the Captain's rule.

However, I'm interested in hearing others opinions.
regards

Mike Dolson

[TSandM]

Pressure gauges, whether digital or analog, are most accurate in the middle of their calibrated range. They can be quite inaccurate at the far ends. That doesn't matter very much at the top, but can be quite important at the bottom. This is the real reason for the 500 psi limit, not the volume, which as you point out, varies from one configuration to another.

[Bubbletrubble]

The primary purpose for adhering to a safety reserve is to ensure that the diver doesn't go out-of-air.
A secondary side benefit of this is that boats renting out tanks to customers won't have to worry so much about water infiltration into the tank.
Water infiltration would probably necessitate whipping or tumbling and that would add to the cost of tank maintenance.

If the only purpose of the captain instituting a safety reserve is the safety issue, then I would think that cubic foot capacity would be a better measure than tank psi, gauge inaccuracies at the lower end notwithstanding.

Since you were diving doubles, you almost certainly were diving your own tanks, so the captain should not have been concerned at all with the tank maintenance issue.

On some level, I would hope that the captain knows enough about diving to understand the difference between 500 psi in an AL80 and 500 psi in independently isolated double HP100s.

During the safety briefing, I could also see the captain not wanting to be bothered with having a drawn out conversation on tank factors and gas management issues. It's just easier to say: "Those are the rules of the boat. Adhere to them or you aren't diving with us."

If you don't like how the captain handled it, choose to dive off of other boats, I guess.

[saxplayer1004]

the pressure gauge is the main one. Even our nice brass and glass SPG's aren't entirely accurate way down in the low range, but it's an industry standard I guess and better to not fight it. 500psi is still entirely practical to surface with in doubles, much better than the 1200 that I usually "have" to surface with. It also makes sure people aren't being stupid and saying "oh, woops I'm at 500, i should probably go up now, then end up with wicked deco obligations. See that on the coast here with the breather divers taking for bloody ever on their first dive

[PfcAJ]

Less than 500psi doesn't really give you much gas to deal with, in event of an issue. You still have to drive the regs.

[Bob DBF]

Less than 500psi doesn't really give you much gas to deal with, in event of an issue. You still have to drive the regs.

And fill the BC, if necessary. And as TSandM mentioned, it only says you have 500, my gauges are pretty accurate but if I were on a rental...


Bob
--------------------------------
I may be old, but I’m not dead yet.

[NetDoc]

I almost always end a doubles dive with a third or more pressure left in the tank. For a 3,000 psi tank, that would be 1,000. As a tech diver, I dive thirds. A third to get there and a third to get back. A deco at 10-20 ft deco/safety stop doesn't use much. I really can't remember a dive with doubles where I came up with less than a third. Of course, EMERGENCIES, which the third was designed to handle, could cause me to go below that line. I have been very successful at avoiding those. Go figure.

[JahJahwarrior]

I don't think "technical diver" and "500 psi reserve" belong in the same sentence, unless it's a recreational dive at very shallow depths.

500 psi in lp120 steel tanks is 22 cubic feet. Below the IP of the reg, I'd imagine performance will begin to suffer, so lets go with 350 usable psi which is 15.9 cubic feet of gas.

At 500 psi, even the lp120 is reduced to being a "spare air" and we all know how useful those are. In an aluminum 80, that's 9 cubic feet, or 4.5 each.

And, on a recreational boat with a recreational time limit, how many people are breathing off your doubles to get you down to 500 psi?

If your boat captain is the one determining your safety gas requirements, you should probably start taking technical classes from a different instructor. I think they start talking about planning your own gas needs at the OW level. 500psi is a completely arbitrary and pointless rule made because people are too foolish to make rational decisions about their own life, and to prevent them from requiring the tank owner to open up the tank every fill because someone drained it to 0 and did a CESA, and possibly let water into the tank.

</rant> Some of that was meant to be tongue in cheek humor, the rest was just a sad commentary on the abilities of most certified divers, or lack thereof.

[Mr Carcharodon]

My spgs seem to work fine down to a couple hundred psi. that is something to findout in shallow water and when you do not need the gas. Insisting on 500 psig is more hand holding than is needed from a boat captain.

[Jaydubya]

So who the hell is checking your pressure gauage when you surface? Tell them to go screw themselves. I wouldn't dive a boat that wanted to know anything more than my run time. On the other hand, I don't think I've ever gotten back on the boat with less than about 900-1000psi in doubles even on 3 tank days.