Why Surface with 500 PSI?

[johndiver999]

The "500PSI" rule is a rule by divemaster/boat captains for what you need to have on the surface when you end your dive. It isn't their job or responsibility to figure out a dive plan for you or what your tank pressure should be when you have to start your ascent from the dive to end up with 500PSI at the surface. They tell you what the "end result" must be and it is your responsibility to figure out how to meet the requirement. You don't expect the divemaster or boat captain to sit with every single passenger on their boat and bring out the calculator to work the math for the "end dive" pressure, do you? It is no more so than it is their responsibility to sit with each passenger to work out the dive tables for them or go over their computer.

I don't dive on charter boats, hardly ever. I wouldn't respond too well toward that kind of directive - for a variety of reasons. I might ask, "does it depend on what size tank I am using"? And I would expect the response to be "no, it pertains to all tanks regardless of capacity".

If I showed up "low" I might just explain to them that yes I have complied with the 500 psi rule because I still have 800 psi in my pony- "look see for yourself".

In all seriousness, if you do the calculation correctly, you will arrive with far more than 500 psi on the boat - under normal circumstances.

If there is a REAL emergency, then the 500 psi rule is explicitly disregarded and the "reserve" is to be used to facilitate a safe ascent - not keep some $8.50/hr DM happy. So in seemingly all situations, the 500 psi rule is nonsensical.

 

[tursiops]

an acceptable compromise by training agencies.

I've lost the bubble; which training agencies, and where, refer to "getting back on the boat with 500 psi?"

 

[Bert van den Berg]

Even if you are crippled for life and lost all income and the bank foreclosed on your house?

Accident Compensation (ACC) – No Lawsuits Allowed
In New Zealand you cannot sue someone for causing you injury.

• If you’re injured in New Zealand, regardless of cause or blame, the ACC scheme entitles you to:
  Free medical care.
• Payment of a proportion of your salary – up to 80 percent – while you recover.
• Payment of lump-sum compensation, if appropriate.

The ACC scheme replaces the right to sue for damages.

The ACC scheme is funded by a small tax paid by employees, employers and self-employed people on earnings.

 

[flyboy08]

Why? Because 501psi is too much!

 

[AfterDark]

Yet.. the 500 psi rule is nonsense in my opinion. Divers should be taught how much air to have in the tank when they leave the bottom, not getting on the boat. It is an ass backwards approach to the problem.

The whole premise is a ridiculous over simplification of the issue, but it somehow is simplistic enough to be thought an acceptable compromise by training agencies.

Using a tool like this spreadsheet, should be extremely useful and instructive to a curious and beginner diver.

We can argue about details like ascent rate variability/assumptions, appropriate SAC elevation during an incident and even time on the bottom to "solve issue" before an ascent is initiated, but refining our own personal assumptions is a necessary prerequisite, to being a reasonably competent diver.

A tool like this is VASTLY superior to a "be back on the boat with ..." type of protocol.

The end result of playing with this tool, should be the capability to generate a minimum gas pressure versus depth for each depth and type of tank. The end result is extremely simplistic and way more useful and reasonable than the "500 psi rule".

It fits well with the whole training model, simple as possible and still survive to dive again.

 

[rsingler]

My good friend @stepfen has just jumped in and translated things into a metric equivalent for you liter/bar folks!
Thank you to Chania, Crete!!!
Okay, team, check the new spreadsheet in Post #1 for errors.
Is the terminology right? SAC by convention instead of RMV, since you're already in liters/min and don't use "tank factor"?

Remember that if you download to your phone for use out at the dive site, most apps unlock the spreadsheet. Be careful not to inadvertently delete formulas.

 

[Doc]

“Why Surface with 500 PSI?”

Because you’re in Grand Cayman, you silly goose.

 

[CycleCat]

So far I have not seen anyone get yelled at for arriving at the surface with a bit under 500. The point is to leave a margin of safety and 500 is a nice round number. I have only been on a few dive boats in my time but I don't really think anyone really bothered to check. They yank off the regs and slap 'em on the next tank.

 

[Bob DBF]

Why 500 psi?

Because the 300 psi recommended in the 1980 PADI OW manual was not enough?
"Plan to surface from your dive so that you will have at least 300# of air in your tank." The predominant tanks were the old steel 72 and the Al 80, the rest were smaller.

The reasoning then was to prevent an OOA on ascent, SPGs can be off at the low end. And secondly, one may need air even though one is on the surface. In other words, don't cut it too close or you could get yourself into a jam where you need some air, and don't have it.

 

[Neokian]

Nice spreadsheets rsingler! Couple of small errors I found in the metric sheet:

- Cell H3 should be B3*D3, is now a constant value
- Cell C10 should be B6<18 in stead of B6<20
- Text in cell A10 should be "Ascent Calc #2 (18m to 5m):" but is now dependent on input depth. --> I see this error also in the imperial version. You could additionally change cell A9 to show the input max depth rather than just the text 'max depth'.