What pressure to end a dive? A debate!

[D_B]

my limited experience ... try coming back with less than 500psi and your going to get a talking to to find out why

Edit: not for me, but I've heard the warning

 

[Rick Murchison]

First stages typically deliver gas at 10bar above ambient. However that doesnt mean they stop giving gas when the pressure is lower than that, merely that the IP is lowered before eventually stopping.
I can quite easily imagine 2-3 bar being the point the valve will no longer open and thats well inside the gauge and human read error on most SPGs.

Once tank pressure is below target IP, the first stage will be - and remain - wide open. It will "deliver" air until the tank is at ambient pressure. What you will experience is a steadily increasing resistance to breathing until tank pressure is exhausted as IP drops. If you'd like to see just how much inherent breathing resistance is in your regulator, just remove the dust cap and inhale through it without it being hooked to a tank. You'll notice that resistance is substantial, but that you will get air, even if you only apply a small suction. You'll also notice that this is not a condition you'd ever like to experience under water.
The point is, though, that there is no valve opening to do. The first stage is wide open at zero pressure, and the second stage opens when you suck on it, regardless of any pressure in the system. (That's why you want to be careful not to purge during rinsing, or you could get water into your hose and first stage.)
Rick

 

[Codyjp]

Has anybody just ran a tank empty while sitting in the shallow end of a pool just to see what would happen? It seems it may be a good learning device if done under safe, controlled conditions.

 

[MikeFerrara]

Once tank pressure is below target IP, the first stage will be - and remain - wide open. It will "deliver" air until the tank is at ambient pressure.
Rick

Hey, is there an echo in here?

 

[MikeFerrara]

The "how to complete your dive" (the part they usually make after the site briefing) section of the dive briefing is usually part of the reminder to vacation divers to come back with air in your tank. Not to teach a lesson - I don't think its bad to say to one of these vacation divers "be back with 500 psi" - it gives them something to think about.

A couple of things. First, what is a "vacation diver". Is it a special cert or what?

Second, if a diver is able to plan his/her gas, the recommendations is completely unneeded. However, if the diver doesn't know how, it does nothing to tell them how. Better to say something that imparts real wisdom like "Two donuts in the air is worth one elephant in a bush". You know, something useful?

In all honesty, I think the DM is just repeating what they remember hearing from their own OW course.

 

[Web Monkey]

A couple of things. First, what is a "vacation diver". Is it a special cert or what?

Those are the people I dive with on cruises.

They're easy to identify, since they're generally bleeding, terrified or saying "My ears really hurt. How can you stand the pain?"

Terry

 

[HowardE]

A couple of things. First, what is a "vacation diver". Is it a special cert or what?

Second, if a diver is able to plan his/her gas, the recommendations is completely unneeded. However, if the diver doesn't know how, it does nothing to tell them how. Better to say something that imparts real wisdom like "Two donuts in the air is worth one elephant in a bush". You know, something useful?

In all honesty, I think the DM is just repeating what they remember hearing from their own OW course.

I usually dive in the warm waters of South Florida... I don't know about how it is in caves, quarries, or lakes, but we frequently see many vacation divers on a typical dive boat in South Florida.

Vacation Divers are typically divers who dive whilst on vacation in Florida or someplace else warm; and dive to have fun. Maybe they do 6 - 10 dives a year if they're dedicated... They're typically non-scuba board members too.

Some boats I've been on recently, when they know everyone on board (as being good divers, and maybe they're all regular customers too) the dive briefing is, "ok are you ready to go diving?" and that's it. When there's new people on the boat, it's a comprehensive site briefing, and they often tell the divers to return with 500 psi. Merely for the simplicity of it (i've always thought). It gives these "vacation divers" a target - these divers don't care about rock bottoms and rules of thirds. They are going to suck their tank to about 1100 psi, and start their ascent to the boat - to return with 500psi, as instructed by the DM. I would also say that these types of divers are usually diving air, and sucking down their tank long before NDL's.

Experienced divers generally know their SAC, and how to plan dives according to their gas consumption and situation they're diving in. Experienced divers can figure out how much gas they'll need for reserve, and any other stipulation that may be required.

So if you're listening to the DM's briefing on how to "be back at the rope at 1200psi and surface with no less than 500 psi" and you think it's B - S - then it's probably not directed at you anyway

 

[PaulChristenson]

They're easy to identify, since they're generally bleeding, terrified or saying "My ears really hurt. How can you stand the pain?"

Terry

I thought those were the people in the front of a heavy metal concert....

 

[Rick Murchison]

It seems that all will agree on the falacy of the '500psi rule'.

I wouldn't call it a "fallacy."
Note: I'm stealing my own post from another board here...
The 500 psi isn't entirely random.. it's about 13 CF in an AL80, which turns out to be just about the gas the average recreational diver needs to make a 30fpm ascent and safety stop from the bottom of the "recreational envelope" of 100 FSW.
So... by planning to arrive at the boat with 500 psi you're also (sort of) planning to reserve enough gas to make an air sharing ascent with a buddy who needs it.
Whether this was actually calculated or just a happy coincidence is really irrelevant... it's a pretty good rule for typical recreational dives with AL80's.
For the second part of the question, for two 100CF tanks, one HP and one LP, 500 psi is more gas in the LP than it is in the HP, so if you use a 500 psi (rather than calculate an actual required reserve based on the dive) reserve then yes, you can dive longer on the HP than the LP because you've specified a lower reserve (in CF) for the HP.
But you ain't being very smart

Rick

 

[String]

What you will experience is a steadily increasing resistance to breathing until tank pressure is exhausted as IP drops.

Thats still something ive not experienced with any of my reg sets. During deliberate depletion and testing i get little or more likely no warning at all before air just stops flowing.