Shop drains tank before fill

[edwants2dive]

I recently moved and am using a new shop to get fills. The partial pressure blend for Nitrox. Somehow in the discussion about the mix I wanted they revealed that they drain the tank before filling. They start with the O2 then add the air to get the mix. They reasoned that the O2 mixes better that way. Most shops I know that partial pressure blend use a calculator to find the amount of O2 and air to add to achieve the desired mix then add the O2 and top with air. Just wondering if that could be allowing humid air into my tank and cause problems at vis time.

 

[Wibble]

Probably end up with a hot tank that cools to a much lower pressure. Then have to moan at them to top it off.

Bit surprising they do this though as they will need to run their compressor for a lot longer. Sure, sometimes it's unavoidable if their oxygen pressure is low. Mostly they won't do this -- or certainly not in my experience.

 

[johndiver999]

It is not going to let humid air in unless they are idiots. It CAN however cause condensation and rusting, if they do it way too fast causing rapid cooling.

It is much easier for the shop monkeys to always drain the tank down and start near zero, so they have no calculations to mess up. In reality, if they leave a few psi in "the bottom" it won't have any meaningful effect on the final mix, but it is hard to talk sense to the guys with strong backs and weak ....

 

[Wibble]

It is not going to let humid air in unless they are idiots. It CAN however cause condensation and rusting, if they do it way too fast causing rapid cooling.

It is much easier for the shop monkeys to always drain the tank down and start near zero, so they have no calculations to mess up. In reality, if they leave a few psi in "the bottom" it won't have any meaningful effect on the final mix, but it is hard to talk sense to the guys with strong backs and weak ....

Sad isn't it!

If only there were some simple gas mixing apps available... Oh, there are.

Maybe it's them giving enough of a damn to analyse the contents, write it down, an' type it into the app for the mix solution.

Hey, it's so easy to do 32%. Drain it and fill with 30bar of oxygen, then top off to 230bar. (... letting it cool to 200 if you're lucky...)

 

[DiveTucson]

Most likely they are doing this as they don’t have a gas booster and are just trans filling across from the O2 supply bottle. By bleeding it all the way down they can blend Nitrox for a while without needing a booster.

 

[Whitrzac]

Most likely they are doing this as they don’t have a gas booster and are just trans filling across from the O2 supply bottle. By bleeding it all the way down they can blend Nitrox for a while without needing a booster.

This. There are a few dive shops I know that have a o2 cascade but no booster.

 

[scrane]

It is not going to let humid air in unless they are idiots. It CAN however cause condensation and rusting, if they do it way too fast causing rapid cooling.

It is much easier for the shop monkeys to always drain the tank down and start near zero, so they have no calculations to mess up. In reality, if they leave a few psi in "the bottom" it won't have any meaningful effect on the final mix, but it is hard to talk sense to the guys with strong backs and weak ....

"monkeys" ?

 

[edwants2dive]

Most likely they are doing this as they don’t have a gas booster and are just trans filling across from the O2 supply bottle. By bleeding it all the way down they can blend Nitrox for a while without needing a booster.

Probably end up with a hot tank that cools to a much lower pressure. Then have to moan at them to top it off.

Bit surprising they do this though as they will need to run their compressor for a lot longer. Sure, sometimes it's unavoidable if their oxygen pressure is low. Mostly they won't do this -- or certainly in my experience.

That reminded me they mentioned something about the O2 pressure. I think they use a single large O2 bottle or small bank they don't have a pressure booster on their O2 supply. I would guess the cost of using extra O2 from a cylinder is less than the cost of a pressure booster system.

 

[rjack321]

I recently moved and am using a new shop to get fills. The partial pressure blend for Nitrox. Somehow in the discussion about the mix I wanted they revealed that they drain the tank before filling. They start with the O2 then add the air to get the mix. They reasoned that the O2 mixes better that way. Most shops I know that partial pressure blend use a calculator to find the amount of O2 and air to add to achieve the desired mix then add the O2 and top with air. Just wondering if that could be allowing humid air into my tank and cause problems at vis time.

Any shop that doesnt have a booster will do this to maximize their ability to use up their O2 supply. Otherwise if you bring back a tank with 700psi in it their O2 supply needs be higher than that to even start transfilling.

It's fine.

 

[Pavao]

It is not going to let humid air in unless they are idiots. It CAN however cause condensation and rusting, if they do it way too fast causing rapid cooling.

I have never been able to observe this, not ever!
I‘vê worked for shops that perform over 1500 visuals a year, tanks are put outside (because of the noise) to drain basically as fast as possible, tons of condensation on the OUTSIDE, tanks are fully wet, dripping in “sweat” from condensation, but INSIDE is always bone dry, this observation was performed from immediately after draining to hours after draining, not ever have I seen it cause condensation INSIDE a cylinder, steel or aluminum.

Short of an explosion, burst disk failure will drain tanks as fast as it can be drained, I’ve been around a few of those, generally you’d take the opportunity to perform a visual in those cylinders given they‘ve been drained, again, never seen condensation inside.