"Hot Fills"

[tddfleming]

Can some one explain a "hot fill?'" Are there an worries with them?

Thanks

 

[Paladin]

A hot fill is simply an air fill without water immersion. The tank gets warm from the compression of the air being put into it. It doesn't really get hot, but it does get noticeably warm. When the tank cools, the pressure inside decreases somewhat. For that reason, most shops will slightly overfill a tank so that when it cools, it has the proper full pressure. I have been to a couple of places that won't give that slight overfill and I ended up with a short fill once the tank cooled.

Hot filling an AL80 to about 3200 will give about 3000 when it cools. Hot filling a steel 72 to about 2500 gives about 2250 when it cools.

This is an old practice and as long as the tank is in good shape, there is nothing to worry about. If a tank is in poor enough condition that hot filling to slight over pressure is hazardous, then the tank needs to be condemned anyway.

 

[vinegarbiscuit]

It's a 'fast' fill, or a fill that leaves the tank in a heated state. Maybe the filler (you or the shop staff) are in a hurry and wanted to get out of the shop to be some place else, so you opened the banks very wide to cut down the fill time; or the fill was done without immersing the tank in water for a variety of reasons (again, 'saving time' is a frequently cited reason!). The biggest bugaboo for the recipient is that a 'hot' tank when cooled will show a lower pressure, generally about 10%. You might walk out of the dive shop thinking you have 3,000 psi in your HP tank, only to find there's only 2,700 psi in the tank once it's had time to cool down. I've also heard that repeated and frequent hot fills compromise the tank's integrity.

 

[on_two_wheels]

I've also heard that repeated and frequent hot fills compromise the tank's integrity.

As have I. It was explained that every time you do the tank expands/flexes. Over a period of time this is what causes the deterioration in integrity. On a steel this is obviously less of an issue and the tank will still last a very long time. Not sure about aluminum tanks.

Feel your tank as soon as it's unhooked on a boat. It's almost always warm. That's why it's very important to check pressure at the beginning of a dive IN the water rather than on the boat. Once you jump in you lose a few (or even a few hundred) psi in less than a minute. So you check just before descending and discuss turn pressures based on the cold tank psi.

 

[DA Aquamaster]

I have noted that SCB tanks used by fire departments that have been slam filled are more likely to fail a visual inspection due to neck cracks. Obviously filling an aluminum tank in 30-45 seconds is probably not good for it.

But the average "hot" fill you get when filling a tank at a normal 600 psi per minute fill rate is not really a problem.

Per the DOT regs, it is also legal to fill the tank over the service pressure, as long as the pressure decreases to the service pressure once the tank cools to room temperature (70 degrees). This means that a shop filling a tank to 200-300 psi over the service pressure is in fact ot overfilling the tank, and with a little practice feeling the shoulder of the tank, the tank monkey can get very good at estimating exactly how much of an "overfill" is needed to have a "full" tank when it cools to room temperature.

Unfortunately many shops don't understand this and will stop at the service pressure to avoid overfilling the tank since they don't understand the term or the regs. When that occurs the pressure in a 3000 psi AL 80 filled at 600 psi per minute will fall to about 2800 psi or so when the tank cools to room temp.

 

[horsediver1]

If you look on your tank after the DOT number, PSI number and probably just below the numbers near manufacturer date you may or may not see a plus sign. This is what most dive shops look for. The plus means during filling it is within DOT regs to fill 10 percent over operating psi. Meaning a 3000 psi aluminum with plus sign can be filled to 3300 a steel 2400 = 2640.

hope this helps

Enjoy

 

[Mike Boswell]

Tddfleming - This is a great topic, and something we should all know more about. There is a lot of info on Scubaboard about this, and some real good info at various websites as well.

Generally, the recommendations I have read are as follows:

Fill tanks very SLOWLY, and only to their working pressure. Allow them to cool, and then "top them off" to the desired fill pressure.

Do not overfill (exceed the working pressure of) tanks. This causes increased stress and reduces the life of the tank.

Do not use water baths. A water bath does not result in significant cooling during filling, it does not "contain the blast" in the case of a tank failure, it increases the odds of getting water into the tank during a fill, and most water baths, unless changed religiously, accumulate salts which are not good for scuba tanks.

 

[aquaregia]

The plus means during filling it is within DOT regs to fill 10 percent over operating psi. Meaning a 3000 psi aluminum with plus sign can be filled to 3300 a steel 2400 = 2640.

I believe only 3AA tanks are eligible for the '+', not any aluminiums or the 14157 exemption tanks. I could be wrong.

 

[cool_hardware52]

If you look on your tank after the DOT number, PSI number and probably just below the numbers near manufacturer date you may or may not see a plus sign. This is what most dive shops look for. The plus means during filling it is within DOT regs to fill 10 percent over operating psi. Meaning a 3000 psi aluminum with plus sign can be filled to 3300 a steel 2400 = 2640.

hope this helps

Enjoy

There are no "+" ratings for aluminum cylinders.

Tobin

 

[cool_hardware52]

Tddfleming - This is a great topic, and something we should all know more about. There is a lot of info on Scubaboard about this, and some real good info at various websites as well.

Generally, the recommendations I have read are as follows:

Fill tanks very SLOWLY, and only to their working pressure. Allow them to cool, and then "top them off" to the desired fill pressure.

Do not overfill (exceed the working pressure of) tanks. This causes increased stress and reduces the life of the tank.

Humm, Well no not really.

What is actually being tested when a cylinder is being retested or "hydro-ed"?

The elasticity of the cylinder. In simple terms the hydro pressure stretches the tank, and it either returns to it's original size (passes) or it remains deformed (fails)

The elastic limit for any solid material is the maximum stress *before* permanent deformation.

elastic limit (mechanics) -- Britannica Online Encyclopedia

Hydro's (with certain exceptions) are conducted at 5/3's of the working pressure. If 5/3's of the working pressure is expected to *not* permanently deform a cylinder in good condition please explain how "hot filling" 200-400 psi over the working pressure will reduce the life of the cylinder.

Tobin