Reserve limit 500 psi always? What about doubles?

[weitodive]

I am not a tech diver. I am sure I don't have the amount of dives that some of you have. I am an instructor and I do believe in the rule of thirds and also a minimum tank pressure at the end of the dive. I can understand the attitude that the captain might have given you probably set off the issue more than really asking about the amount of air after the dive, but if you are going to go on a tourist boat then why make a big deal about the rules. It frustrates me when I get on a boat and have people arguing with the crew about stuff. They have a job. They try to do it to the best of their ability. If it means checking for air pressure at the end then so be it. I guarantee if something happens to someone on the boat, they wouldn't hesitate to sue them. They are covering their own and hopefully the idiot diver's ass.

 

[DevonDiver]

Here's what I teach and what I usually adhere to: Gas Management Workshop/Rock Bottom

 

[franny]

The only way it would be annoying to me, is if they told you as you were about to step in the water, that that was the limit and they would check your tank when you get back on board. Even then, it would only be annoying because you can't easily do the calculations to get the right values for when to turn and when to ascend.

If you are doing a dive that requires that much air, chances are you could need more than 500PSI to safely bring yourself and your buddy to the surface. You should be calculating the need for the dive, and working out how much gas you need as a rock bottom. Cutting the dive a couple of minutes short is better than finding yourself short in an emergency.

As long as you know a bit before the dive, just plan the dive around the more conservative of the two options (500psi or rock bottom), if you know the operator will check the gas on the way back. If not... well the choice is up to you and on your head.

 

[Rhone Man]

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.

As usual, Lynne beats me to it. If you try going down to 250 in doubles, you may discover your 250 is more like 50, or even zero.

I also think more gas = more potential to get into trickier situations. All the most reason to keep a bigger reserve. If it was a "no deco" charter, I am sure you had plenty of gas without needing to tap the bottom of the tank.

 

[JahJahwarrior]

I've seen two other comments in here that I think are interesting to comment on

First, "my SPG works fine at low pressures." How do you prove this? It's a fact that bordon tube gauges are most accurate in the middle of their range. The SPG will continue to read all throughout the range, but at the ends, it is not as accurate as it is in the middle. That's a fact we live with, because the accuracy difference isn't generally enough to make much of a difference on a dive, but it gets important when you are talking life or death.

Second, "I'm a tech diver so I dive thirds." That's a good rule of thumb, but a more proper statement would be "I'm a tech diver so I evaluate my gas needs and dive appropriately to ensure enough gas for myself and my buddy given an emergency at our maximum depth or penetration and run time, to reach the surface safely" For many dives, that is less than thirds. I was surprised I had to explain the idea of diving with more than one third reserved for safety to a cave diver recently.

 

[redacted]

I don't tip anybody who inspects my SPG.

 

[GrumpyOldGuy]

I used to get worked up about DM/Captains demanding stupid limits like "back on the boat with 500PSI" because its merely a simple limit catering to the feeble minded and is not even close to proper gas planning.

Then I realized I almost never returned to the boat with less than 1000PSI and simply decided it was not worth my time arguing with them. I would rather just enjoy my dives rather than try to publicly confront a captain about his policy, its like pissing upwind...

 

[vladimir]

The captain is just setting a minimum standard, one that's easy to adhere to, easy to enforce, and easy to explain. I don't see the problem, and I hate being told what to do.

 

[Mr Carcharodon]

First, "my SPG works fine at low pressures." How do you prove this?

y.

By comparing observed gas consumption against expectations. For example in a 130 I have 4 cu ft/100 psi, or 20 cu ft for 500 psi. So if I'm crusing around at 15 fsw I should have about 20 minutes of gas left and sure enough that is how it works. So what does that say about all bourbon tubes being very nonlinear?

Also with 20 minutes of gas left using a goodly fraction of it seems reasonable as long as you're not holding up a boat.

 

[vladimir]

So what does that say about all bourbon tubes being very nonlinear?

Bourbon tubes can be cylindrical or somewhat cuboid, even. They are typically labeled "Maker's Mark," "Jim Beam," "Wild Turkey," etc., and are often referred to simply as "bottles," or "bourbon bottles," or, sometimes, "Old No. 7." They are nonlinear in the sense that you are progressively less able to walk a straight line as you empty them.

A bourdon tube, named after Eugene Bourdon, is a different matter.