Overfill Differences for LDS - Question

[UB]

I recently purchased a Faber 120 HP and have a question about the different fills I get from different LDS:

Shop 1 - They say they overfill it to ~3700psi hot and I get ~3500 psi cold. (GREAT!)

Shop 2 - They say they overfill it to ~3700psi hot and I get ~3100 psi cold. (Not so Good)

FYI - I wait at both places to get the fills. They both take around 10-15 minute to fill the tanks. I know if I would wait longer they could do a better job but I get fills between beach dives.


Answer 1 - I figure either one of the shops has a bad psi gauge or is not reading the gauge correctly or

Answer 2 - It has something to do with their compressor (ie one is more efficient and provide a "cooler" hot fill so when the tank cools down the pressure drop is not as significant).

One side note - Shop 1 (better cold fill) does use a water bath for the tank while Shop 2 doesn't. I always understood this does not make a difference though

Does this make sense and has anyone else run into this problem?

 

[captndale]

I always fill to 10% over my target pressure. I don't use a water bath. I find that I usually get very close to the pressure that I am looking for.

 

[captain]

The water bath definatey does make a difference. I have filled my own tanks for 40+ years both with and without a water bath. If I use the water bath I almost never have to go back and top off tanks, if I don't fill in water I have to let them cool and top them off by at least 200 psi.

 

[pescador775]

Make sure you use a real pressure gauge, not an SPG.Submersible pressure gauges read 100-200 psi low. I checked several SPG's against a 300 dollar calibrated gauge and two random shop gauges. This error is done for liability reasons and has been common practice since the 1970's. Recently, I compared a new Genesis gauge and it was off by 200 psi. I repeat, there is nothing mechanically wrong with any of the SPG's which I have examined, they are set to read low.

 

[SparticleBrane]

The water is the main factor, but there's also the waiting time.
When I worked at a dive shop, if the customer could wait a few hours or perhaps overnight, I'd fill the tanks slowly and set them aside for a while. Several hours later I'd top them off to be sure everything was perfect. If you can wait, that's the best thing to do.

 

[JahJahwarrior]

Water takes the heat away from the tank alot faster than the air. That's why you can get hypothermic in relatively warm water given enough time, even though you'd be fine in the same temperature air for allof your life. Water baths don't necessarliy make a safety difference, when your tank blows up it'll hurt things water tank or not.

 

[Thalassamania]

Many years ago when we were designing a filling station we instrumented a water bath to see what the heat transfer was to that water while filling. I hate to burst bubbles and crush life long beliefs, but there is very little heat transfer to the water during a 5 to 10 minute fill (at least with steel 72s).

 

[scubawally]

the use of a water tank will provide cooling to the tank much more quickely than in air (scientific fact) you may not have noticed much heat going in the water but i guarentee you that it did happen. In addition you should find out if both shop fill your tank off of the compresor of if they fill them off of banked air. with banked air you can fill a tank in less time this will result in greater heat production....when the tank then cools you will have 'less' air. However you did say that you had an hp 120 and that you got it topped up between shore dives right....where do you dive that you need 120cuft of gas for a shore dive multiple times a day?

 

[SparticleBrane]

Many years ago when we were designing a filling station we instrumented a water bath to see what the heat transfer was to that water while filling. I hate to burst bubbles and crush life long beliefs, but there is very little heat transfer to the water during a 5 to 10 minute fill (at least with steel 72s).

That's why you fill them extra, extra slowly.

 

[Thalassamania]

That's why you fill them extra, extra slowly.

Roger that.