Pressure in tank

[Efka76]

All my dives were with AL80 tanks. During various dives I noticed that initial pressure of tank really varies from 180 bar to 210 bar and wanted to ask you what is maximum bar such tanks could hold and what psi I should expect when I receive freshly filled in tank. Somehow I thought that dive centers specifically fill in tanks not to full in order divers to have less time under water.

It would be interesting to get opinions from experienced divers on this matter. Also, it would be interesting to hear views about diving with steel tanks: what are pros and cons comparing to aluminum tanks.

 

[swimlikethefish]

I suppose you mean 180-210 bar. When a tank is filled there is generally a large amount of heat built up. So if the person filling the tank filled it to exactly 205 bar, the rated service pressure of the tank, and stopped. The tank would eventually cool off and when it did the pressure would probably read around the 180 bar mark. Effectively losing 2-400 psi in the process.

It is important to know that it really did not lose this air. Let's say this tank measured 180 bar after it was filled and cooled off. Then you placed it in the trunk of your car on a really hot day. Then you measure the pressure, it would read higher than the 180 bar from before. So the reason you get readings higher sometimes is the fill person either came back and topped off the tank after it cooled. Or they hot filled it to begin with, where they might pump in 220 bar and it cools to 205 bar.

As far as the pros and cons of steel vs aluminum, I will allow someone else to discuss in detail. But I will give you a few topics of discussion or research. Volume, weight, hp/lp, what is regionally available for sale.

 

[Steve_C]

If I were to pick up a tank on a normal day with normal ambient temp and the tank was at ambient pressure and they gave me 180 bar in an AL80 I would ask them to fill the tank to 200-205. For my local diving I dive my own tanks and the shops take their time and fill right.

 

[TMHeimer]

If I were to pick up a tank on a normal day with normal ambient temp and the tank was at ambient pressure and they gave me 180 bar in an AL80 I would ask them to fill the tank to 200-205. For my local diving I dive my own tanks and the shops take their time and fill right.

I believe "taking their time" has something to do with it. If I leave tanks and return a day or two later they are usually fuller than it I stay and wait for them. I'm not a physics expert, but I believe it has something to do with how quickly they fill them. A "hot fill" will result in less in the tank. Either way, they do "cool off". I'm sure someone will explain exactly how a hot fill differs from a normal fill. I'm happy with about 2800-3200 PSI starting a dive. Less than that and I may mention being short-filled.

 

[Scubis]

At my dive shop i fill aluminum tanks no more than 100 psi past their service pressure. Its a hot fill and usually looses 200-300 psi after cooling down. If someone is waiting for the tank i give it to them as is. Pressure check it in front of them and tell them "3100 psi warm." So whatever it is after cool down is what they get.
If it's a drop-off, i typically fill the same day and top it off the next day so that when i pressure check in front of the customer at pick up it has a true full pressure.

Steel tanks I'll usually pump about 250psi over on a hot fill so when they cool they are pretty close to their service pressure. Again, if a customer gives me more than 1 day to fill the tank, i have time to ensure it cools down and is topped off to full service pressure.

 

[Scubis]

Any dive shop should be willing to verify the tank pressure in front of you upon pick up. But just know that a good fill is a very slow process. So dont feel like you are cheated if your tank cools down and is 300 psi lower than your service pressure. Although many shops will over fill there is no obligation to do so. In fact it is a liability. So if you want a good fill, you need to give your fill station adequate time. I would suggest dropping of your tank, and getting some food. Come back amd feel your tank, hopefully its cooled down by the time you come back. Ask a staff member to check the pressure for you. Most shops will top you off if the pressure is low.

 

[Patoux01]

All my dives were with AL80 tanks. During various dives I noticed that initial pressure of tank really varies from 180 bar to 210 bar and wanted to ask you what is maximum bar such tanks could hold and what psi I should expect when I receive freshly filled in tank.

The maximum pressure a tank should be used at is written on the tank itself. Values I've seen are 196 (old steel tanks), 207 (aluminium), 232 (steel), 240 (alu), 300 (steel or carbon wraps). There are other values I guess.

On an al80, they will often be 11.1l tanks with 207b working pressure. Some people will fill them more taking into account that they cool down afterwards, others don't.

As the differences between alu and steel...
- Steel tanks are usually lighter out of the water
- Steel tanks are usually smaller in external dimensions
- Steel tanks tend to be negative throughout the dive, aluminium not
- Steel tanks need a protection layer of some sort to prevent rust
- Aluminium tends to be cheaper

These are the ones I see right now, I've likely forgotten some.

 

[TMHeimer]

Yes, thanks. Topping it off after it cools down explains it. I also have been to shops where they do significantly over fill it--causing me to think about that liability.

 

[Efka76]

Very big thanks for your explanations!

 

[pepperbelly]

When I started diving long, long ago fill stations put the tanks into a long tank of cold water while filling. There was usually room enought o fill 5-10 tanks at a time depending on the shop's size.
I don't remember ever having a "hot fill", and the pressures stayed the same after filling.
Why do fill stations not do this anymore?

BTW the coolest fill station I remember from when I was young in the early '70s was at Scuba Point on Possum Kingdom Lake- our best local diving. They had a pipe that ran from the bottom of the lake- about 70', up to their fill station. They pumped water continuously, and that water was cold!