on the boat w/500 psi

[eponym]

@Bryan St.Germain: If you sum the percentages, it adds up to more than 100% because the survey allows people to vote more than once. In your attached image, it displays 15 tallied votes yet reports that 13 people answered. 2 people voted twice.

Well, as I said, sometimes I don't know math from a hole in the ground . . .

If it were me, I'd assign percentages based on number of votes.

But SB rocks and SB rules. I appreciate everyone who works therein.

-Bryan

 

[SoccerJeni]

In OW class I was told you need to be at the surface with 500psi. That was 4 years ago and I can't remember if he told us HOW TO PLAN AHEAD to make that happen. I did take a refresher course last November, and that instructor suggested we follow the rule of the thirds, using 1/3 of your tank going out, make the turn and head back on a 1/3 of your tank, then began your ascent. Obviously different depths and conditions are going to affect that rule of 1/3.

I'm hoping when I take AOW I'll learn much more about this planning. In the meantime I'll be reading NWGratefulDivers article.

 

[RTee]

When I started diving, only the rich instructor had a SPG. A few of us had wrist mounted depth gauge, were using 72 cft LP steel tank mounted on a backpack with a horse collar. You started to go up when you yank, I mean when you activated the reserve of your J valve set at 400 psi if I remember correctly...the 500 psi came out way after that when they added an octo, BC inflator hose and a SPG/depth gauge to the reg assembly.

 

[GrumpyOldGuy]

When I started diving, only the rich instructor had a SPG. A few of us had wrist mounted depth gauge, were using 72 cft LP steel tank mounted on a backpack with a horse collar. You started to go up when you yank, I mean when you activated the reserve of your J valve set at 400 psi if I remember correctly...the 500 psi came out way after that when they added an octo, BC inflator hose and a SPG/depth gauge to the reg assembly.

Back in the "day" I remember every single dive being planned for both gas and NDL, so even though we had J valve it was rarely needed.

Now it is all too easy to dive until either the computer beeps or the needle goes into the the red zone and react to the alarm rather than plan to avoid the situation.

 

[Buadhai]

When I started diving, only the rich instructor had a SPG. A few of us had wrist mounted depth gauge, were using 72 cft LP steel tank mounted on a backpack with a horse collar. You started to go up when you yank, I mean when you activated the reserve of your J valve set at 400 psi if I remember correctly...the 500 psi came out way after that when they added an octo, BC inflator hose and a SPG/depth gauge to the reg assembly.

Gee, that brings back memories. About ten years ago I was clearing out an old storage box and came across my 1970's era backpack and horse collar. As I lifted the horse collar all of the seems came undone and the thing became pieces on the floor. I'd have saved it otherwise.

I'd forgotten all about backpacks. Odd how they've come back into fashion...

 

[TMHeimer]

I'm not really sure how to answer. I learned in Deep Diver course to have 100 PSI for every 10 feet down when you begin the ascent. Charters vary a bit in their "requirements/recommendations"--the last one said 400 PSI. I haven't done any shallow charters to say 30-40 feet, but I imagine getting back with even a little less may do since a CESA is quite doable. But other that the Deep Diver recommendation I don't think anyone has ever mentioned it in any class, including DM class. Of course, Wreck course informs you of the "rule of thirds", but that's not exactly regarding a 500 PSI reserve.

 

[vondo]

I think the more interesting question is where did I unlearn to be back on the boat with 500 psi?

Don't remember where I "learned" if it was basic OW or from an operator.

 

[rob1967]

It doesn't really matter how much gas is in your tank when you are on the boat. What matters is planning when to start your ascent. Depending on depth and conditions, you should agree to start heading up at say around 1000 psi. You should be on your safety stop at around 700 psi, again depending on the dive and conditions. If there is cool marine life to see in the top 20 ft of water, I personally don't see a problem using up the rest of my gas in the top 20 ft. It is amazing how long your gas can last in the shallows. I would rather do a 20 min safety stop and end up with 200 psi, then do the bare minimum 3 min safety stop just to say you got 500 psi in your tank. To me, that gas is better used on prolonging your safety stop, again depending on conditions (if surface is rough, you may want to have more of a reserve for the surface).

 

[syntaxerrorsix]

Our OW classes put huge emphasis on calculating sac and approximating dive times based off of them. We have them calculate sac for a lot of their pool session. The 500psi is only really good for the 3000psi tanks because 500psi is a sixth of your total gas. good ish reserve for open water, but it's not enough with the HP tanks and likely too much for the LP tanks, all depends on your preference

That would be dependent on volume not pressure.

500 PSI isn't a relative number unless you assign it to a container of known volume.

 

[MXGratefulDiver]

That would be dependent on volume not pressure.

500 PSI isn't a relative number unless you assign it to a container of known volume.

Couple of things to think about ...

When considering how much reserve you need, size matters ... and so does working pressure. Consider four common cylinder sizes at 500 PSI ...

- an AL80 has about 13 cubic feet of air in it
- an LP95 has about 18 cubic feet
- an HP100 has about 14.3 cubic feet
- an HP130 has about 19 cubic feet

So you see from those figures that even though it's a much smaller cylinder, the low pressure 95 has almost as much reserve at 500 psi as a high pressure 130 ... and far more than the slightly "larger" high pressure 100.

It's important to understand how much usable gas you actually have.

Another consideration is that pressure gauges are notoriously inaccurate ... I can take any six of my regulators, put them on the same cylinder, and get readings that are as much as 300 psi different from each other. One way to find out how accurate your gauge is on the lower end ... which is where it counts for reserves ... is to take a tank you've already dived, sit in the comfort of a bench (or even in your living room) and breathe it down, watching what the gauge does as the cylinder approaches empty. Look at what the gauge is reading when it becomes difficult to breathe (or for high performance regs, just stops breathing). You might be surprised to learn that it still reads 200-300 psi. What does that tell you about the "reserves" in your cylinder when you're diving?

Both of those are good things to know about when you're planning how much reserves you really need ...

... Bob (Grateful Diver)