Tank overfill = mini hydro?

[SCUBA482]

I have been going to one LDS getting my HP100's tanks filled and they generously cap them off around 3800 - 3900 which cools down to about 3600 or so. I went to a chain store (Sport Chalet) to get some fills and they were really hesitant and told me they could only fill to the 3442 or 3500 mark. Of course when they cool down they are near the 3100 - 3200 mark. The clerk told me legally they couldn't go any higher than the rated amount from DOT on the tank. He conceded it was store policy for liability reasons, but he then went on to tell me I was "mini hydro'ing my tanks." He stated that one firehouse was bumping up there fills also on some new tanks and that they failed their first hydro. Oh yeah, he tried the scare tactic too telling me I'll blow the back of my head off because I'm using yoke. He then pulled an Aqualung 1st stage and showed me the 3300 psi rating. My Atomic is rated to 4350 so I asked for the higher fill. Seemed he was coming up with a bunch of reasons not to fill my tanks to 3600 cold.

The million dollar question - Would you consider the numbers above a "mini hydro" and if so is there damage occurring to the tanks being filled as such?

 

[noreastDIVER84]

I have had this discussion with the friendly people at my LDS because they now have a sign up about the DOT regulation and that they will not overfill a tank. But I have been told by them that it is not overfilling if you finish them off at say the 3800 you are talking about. The reason is that when the tanks are done filling, they are hot and that they cool off to the 3400 range. The burst disc will blow at a much higher range than the 3800 they are filling them to initially. I generally have plenty of time when I go into the LDS and tell them to take their time filling if there is no other fills. I have hung around there for over an hour before, and have even been out of there in 10 minutes. But when I am around for the hour, I generally have a full tank when I am ready to go diving, when I leave in 10 minutes, My tank will be good then, but almost 800 PSI less when I dive it.

 

[Bubbletrubble]

The Sport Chalet employee was correctly quoting company policy to fill only to the rated service pressure. I'm usually not a big hurry to get my tanks filled, so I'll just ask them to fill to service pressure (3442 psi) and then top them off once they've cooled. Simple solution. My local Sport Chalet location has no problems accommodating my request. If they're busy when I drop off the tanks, I make plans to pick up the tanks the next day. The inconvenience of a second trip (2 miles away) is worth getting a decent fill.

Theoretically, I guess it's possible that the steel tank is getting stretched out a little with each overfill...but the magnitude of overfill is quite small in my opinion. Your tank should still last several decades if properly cared for.

Do you know what the hydrostatic testing pressure of a HP steel tank is? It's 5/3 the rated service pressure. So, for your 3442 psi tank, a hydrostatic test would be conducted at 5737 psi.

Given this info, do you think that overfilling your tank to 3900 psi is a "mini hydro"?

 

[Scared Silly]

The recommended filled rate is 600 psi/min. That said it is legal to "overfill" if the finial fill pressure being at the working pressure at 70F.

 

[Chugwhump]

As I sit here gnashing my teeth....there...I just got over it....
I am once again so very happy to live in the Pompano Beach Florida area. Here this issue is just not much of a problem.
I am blessed, 97% of the LDS's around here have a clue about filling tanks (Finally).
Other Scubaboarder's who know much better than I, will drop in here possibly and define in proper terms what I cannot with specific and correct statements about how high you can fill a tank.
Chug

 

[jiveturkey]

is there damage occurring to the tanks being filled as such?

Not in a steel tank. No worries at all. These tanks are regularly filled to waaaaay above service pressure in cave country. 3900 is policy in the shop I worked in and we used to top that off after a 10 min cool down. We used to overfill Al tanks too but I was never as comfortable with those as I was with steels.

 

[imwright1985]

Good shops do cooler fills with soaking the tanks while filling them and filling slower than other places, this gives them the more accurate PSI and you more fill. Divers direct does hot fills and on an al80 i walked out with 2680 when i hit the water. if i use my usual shops when i hit the water the lowest they are is 3080 and usually run about 3200. beings that hydros are done at 5000psi on al80's im not too worried about the extra 200 on a fill, and the overfills strain harden the metal which causes it to loose elasticity so it is possible for overfills to cause a tank to eventually fail hydro

 

[RonFrank]

Sounds like these shops are filling tanks too fast.

I guess when busy a hot fill to a higher pressure can save time. I don't hurry the LDS on fills, nor do I really care around here if I get 2600 or 3000psi as I generally don't use more than 2000psi on a dive if that....

A couple of Boat Ops I dove with in the Keys did fast fills with partially submerged tanks. Seemed to work well for them.

 

[zaberman1]

1. tanks rarely fail hydro, and over-pressurized tanks (ie overfilled) fail even less. This is true because the metal is contracting and expanding, gaining a metal memory and able to acceptably expand within limits. If you leave an empty tank empty for a long time and take it to hydro without filling it and letting it regain its memory, it will probably fail. It is also true because water molecules are much larger than air molecules, allowing water to be retained in a tank with a very small hole that air can trap in. This is one of the reasons more tanks fail visual than a hydro...

2. If you have a nice slow fill (300-600 psi/min) at the dive shop, there should be very little, if at all gas change later on. At my fill station, we have cold/running water that we pour on the tank and break about half way in the fill to cool it down. Of course, this takes a long time, but we are very firm on regulations.

3. Fill station operators can do whatever they want...its their life on the line...

 

[Web Monkey]

I don't know why he was dancing around the issue. If he doesn't want to overfill a tank, he's not required to.

Terry