Don't breathe tanks to zero?

[captain]

http://www.osha.gov/SLTC/commercialdiving/more.html

 

[drdaddy]

One should let the air out slowly when draining a tank so the cold air won't condense into moisture. Inspection should take place as soon as possible and then the fill.

I had a shop where I went to retrieve my tanks from being viz'd, I found that they had been drained and not viz'd and therefore not refilled. I took them and immediately gave them some good air via a whip and am awaiting my viz and fill from my regular excellent shop (Eugene Skin Divers Supply). In our moist Oregon Coast climate, I would never want to leave my steel tanks unpressurized/open for any amount of time. I hope these don't need to be tumbled/cleaned due to the (never to recommend/go to again) shop's negligence.

 

[boulderjohn]

Depending on the due point of the air in the tank and to what temperature the tank dropped to it is possible for liquid water droplets to form on the interior tank walls but most would likely evaporate once the tank came back to ambient temperature for a period of time.

The air in your tank should be 100% dry, so there should be no dew point issue.

I guess what I'm wondering is what you're using for deco and why you don't (as it seems) breathe a deco mix (from your stage)

Since it appears your backgas and stage are pretty similar, what's your rationale for breathing the stage down?

You bring a stage with the same (or nearly the same) gas as on your back in order to add to the total amount of gas you are breathing on the bottom portion of your dive. You breathe the stage down and then switch to your back gas to finish that portion of the dive. You leave an adequate reserve in the back gas to deal with emergencies. Since there is no harm in breathing the stage down, it makes more sense to do that then to divide your reserve gas among three or more tanks when you can have it all in your doubles.

 

[Jon_R]

It depends. If the nitrox tank is O2 clean then no, you don't get "regular" air fills. Regular (aka grade E ) air fills will contaminate the tank. But if the LDS makes nitrox then it's a given that they don't pump grade E air. Their air is O2 compatible and therefore won't contaminate the tank.

At that point you get into the stupid "NITROX ONLY" stickers. Pointless bumper stickers don't belong on tanks. The tank contents marking is what matters. But yes, you can put EAN 21 in the tank and mark it as such and I would expect to pay $4 not $7 for that fill.

I checked with the shop and with a nitrox tank they will do an air fill no problem and will be the normal air price of $4 ($3 with the 10 fill card). They said no need to bleed it if you want air unless you want to. They will just top off with air so you might end up with 25% or so.

 

[swimmer_spe]

Plan the dive, and dive the plan. 1/3 down, 1/3 up, 1/3 emergency.

I like to come up with 1000psi in my tank. I have had it to where I do not have enough air on the surface to inflate my BCD. I have also ended dives with over 1500psi, with my buddy almost empty.

 

[OkByMe]

At dinner some divers were saying that you if a tank gets breathed down to near zero pressure, bad things can happen TO THE TANK. Something about humidity and rust, refilling the tank and then breathing head down.

What's the story here?

Bill

I did not read through all the responses, most of which seemed to be way off topic, but the answer is....

You can't breath the tank empty - period. There will always be pressure in the tank equal or greater than the intermediate pressure setting of the first stage regulator, usually 125PSI minimum and 150PSI maximum. So, as long as the pressure in the tank is greater than 14.7PSI at sea level, then no air can get into the tank if you open the valve. If you open the valve, air will only come from inside the tank. If you leave the valve open and the pressure inside is equal to the pressure outside, over time the moisture in the air will enter the tank. That could take several days if there is no influence for the air in and out to mix through the little teeny valve opening, such as the tank sitting outdoors where it will heat up in the sun and spill air, then cool down in intake air.

So, if you have redundant air supply while diving and you breath down the pressure until the regulators don't supply air effectively, then switch to backup, you will be fine. If you're not redundant - you're a fool.

When you exit the water, close the valve, rinse off tank and regulators, purge the regulators and remove them. As far as breathing head down... On the bottom of the valve, the intake, there is a tube that protrudes some 2" - 2 1'/2"- some call it a dip stick. If there is any liquid or particulate in the tank, it can't get by the the tube unless there is enough in there to slosh around.

 

[Tortuga68]

Plan the dive, and dive the plan. 1/3 down, 1/3 up, 1/3 emergency

That's a very conservative plan for rec. diving, it was originally for overhead environments although it's not always suitable for that either. Rock Bottom is much better for both

 

[billt4sf]

I did not read through all the responses, most of which seemed to be way off topic, but the answer is....

You can't breath the tank empty - period. There will always be pressure in the tank equal or greater than the intermediate pressure setting of the first stage regulator, usually 125PSI minimum and 150PSI maximum. So, as long as the pressure in the tank is greater than 14.7PSI at sea level, then no air can get into the tank if you open the valve. If you open the valve, air will only come from inside the tank. If you leave the valve open and the pressure inside is equal to the pressure inside, over time the moisture in the air will enter the tank. That could take several days if there is no influence for the air in and out to mix through the little teeny valve opening, such as the tank sitting outdoors where it will heat up in the sun and spill air, then cool down in intake air.

So, if you have redundant air supply while diving and you breath down the pressure until the regulators don't supply air effectively, then switch to backup, you will be fine. If you're not redundant - you're a fool.

When you exit the water, close the valve, rinse off tank and regulators, purge the regulators and remove them. As far as breathing head down... On the bottom of the valve, the intake, there is a tube that protrudes some 2" - 2 1'/2"- some call it a dip stick. If there is any liquid or particulate in the tank, it can't get by the the tube unless there is enough in there to slosh around.

13 pages, 127 responses. This answers the question very well, I think.

- Bill

 

[OkByMe]

You ask a question needing a specific answer, and all you get back is... should do, I do, should not do, need training, check with the LDS..... cracks me up.

 

[elan]

I feel that "period" actually has a question mark following...

Yes you can breath it down to ambient pressure.
Most of 1st stages are normally open so they will allow the air to pass through at any pressure. If you have a balanced second it will allow the air to pass through as well at any pressure. Unbalanced second through will not allow the gas to pass if the IP is low.

So if you have a balanced second stage you can breath the tank to near empty.

You can test it if you take a set , open the dust cap an breath on the reg. although a bit hard you will get air through.

I did not read through all the responses, most of which seemed to be way off topic, but the answer is....

You can't breath the tank empty - period. There will always be pressure in the tank equal or greater than the intermediate pressure setting of the first stage regulator, usually 125PSI minimum and 150PSI maximum.