What if you need to use some of that 500 psi contigency reserve?

[Doc]

How did this 500 psi thing all get started anyway.
Moisture will only enter a pressurized tank while filling it (forcing it in) or if the tank has less than surrounding ambiant pressure in it (vaccuum or haul the tank under water with 0 psi in it) or leave the valve open. Those dive shops with corroded tanks should be looking into some new water seperators.

Looks like you read my explanation http://www.scubaboard.com/forums/3364320-post11.html but somehow didn't believe it could be possible.

Again- put yourself in the position of the owners of the tanks. I am not asking you to act on science or probabilities, just think like they do. I saw this Commonly Accepted Truth first breathe life, just like the 15 foot safety stop.

 

[SparticleBrane]

Honestly, I'm not quite sure how to take your posts ... but I'm quite sure I should not take them seriously ...

They're worth a good laugh! He has a zillion dives and is very experienced! He must be right! Everything he says must be an exclamation!

 

[peterbj7]

In the UK the norm for back on the surface is 50 bar, which is around 750 psi.

I always tell new guests that they should try to be on the surface with 500 psi, leaving the bottom with around 700. The reasons are simple- (1) I don't know these divers, so I don't know how negligent or reckless they may be. Believe me, we see all sorts; (2) if they're using their own gear I don't know how accurate their gauges are, and if they're using my gear they don't know how accurate they are. Either way, some conservatism is a good idea; (3) we don't know what surface conditions will be like, as they can change quite quickly. I want people to have ample air to inflate BCs and breathe from if necessary, as otherwise their lack of awareness becomes my emergency.

It is possible to get water into a tank even if it isn't drained to "empty". It's happened to me too many times and tank cleaning is a nuisance. If I don't spot what happened and that tank is just refilled, the moisture/salt will still be there and it'll have a much shorter life.

You don't actually want to take a tank down too close to the inter-stage pressure of your regulator, as with some regulators that can make them very tough to breathe from. That pressure varies with different designs of regulator. Others may produce unpredictable results - have you ever seen a Poseidon freeflow simply because the tank has been run too low?

 

[The Horn]

Looks like you read my explanation What if you need to use some of that 500 psi contigency reserve? but somehow didn't believe it could be possible.

Huh? I guess you didn't read what I wrote. I believe it is possible if you suck the tank right down or vent it off and leave it open: but then mark the tank for later, pull the valve rinse, dry, vis and put back in service. Should be generally accepted for operating a bait boat that stuff like that will happen from time to time, one of those service industry pearks. Don't you expect tips anyway?

 

[Teamcasa]

...It is possible to get water into a tank even if it isn't drained to "empty". It's happened to me too many times and tank cleaning is a nuisance. If I don't spot what happened and that tank is just refilled, the moisture/salt will still be there and it'll have a much shorter life.

I doubt it. It's pretty hard to put water into a tank that has even 50psi (3bar) unless the water is under a higher pressure. More than likely the water is introduced by the fill system.

 

[Doc]

It is possible to get water into a tank even if it isn't drained to "empty". It's happened to me too many times and tank cleaning is a nuisance. If I don't spot what happened and that tank is just refilled, the moisture/salt will still be there and it'll have a much shorter life.

I doubt it. It's pretty hard to put water into a tank that has even 50psi (3bar) unless the water is under a higher pressure. More than likely the water is introduced by the fill system.

Experience 2 , Doubt 1.

 

[diver 85]

The real reason is dive ops. don't want their tanks drained & want a little air left in them.......

 

[captain]

Experience 2 , Doubt 1.

How about explaining the law of physics that lets a low pressure move into a high pressure area.

 

[redacted]

How about explaining the law of physics that lets a low pressure move into a high pressure area.

Come on captain, the water had to either come from the LDS's compressor or the user got water into the tank. And we all know that no shop would ever let water get into a tank thru the compressor and fill operation.

 

[Doc]

How about explaining the law of physics that lets a low pressure move into a high pressure area.

I don't have to. Re-read the posts with greater care.

I say "when they're empty", Pete from Belize talks of the issue with extreme low pressure. Low pressure results in "empty". Here's why....

Here's the fact: Tank valves that are in resort-use, they all leak. It may be barely noticeable, you might need bat's ears or soapy water to notice- but they all leak. Tanks that are filled 3200 psi on Friday Night and racked as the dive staff goes home (for the resort changeover days) are often down to 2200 psi on Sunday morning for the first divers of the new week.

Do a little math. If you rack a tank at, say, 100 psi, the air will leak out within minutes allowing the ambient salty humid air to insidiously enter and affect the threads and further mutz-up the valve. Mutzing is a technical term.

It's easy to have an empty tank begin to assume ambient air, even if you know you racked it with 500 psi. They all leak.

This factor also gives rise to another Diver Myth. That 2200 psi tank that I mentioned... the one that new divers often find all racked up for the first dive of the new dive week? You will see this comment often in trip reports and resort/op comment cards: "The tanks were low on the first day so we complained and they kept them filled after that". No, you just pulled tanks that were very recently rotated and filled. They hadn't been sitting, leaking slowly for a weekend.

I've seen my share of dive op and resort operations in tropical environments. If I had a nickel for every tank valve I pulled and worked on, I'd have gotten a real job on an island. In a large-op, quite often at least one guy pulls and changes out valves, one day a week. All day, for 8 hours.