Why I Want To Fill My Own Tanks

Image: Max-Air.com

I was not expecting it until next Tuesday, but my new portable high pressure breathing air compressor arrived on island today. Instead of waiting for the trucking company to deliver it on Monday, I went down to their warehouse and picked it up myself.

I thought I was going to have to race to the dive shop tonight after teaching classes all day to get my tanks refilled from my dives yesterday before they closed, but instead I was able to get all 4 of them filled with the new compressor. It took me just over an hour to cold fill them, but all 4 of the tanks are filled and ready to go again now.

There were always two things that I did not like about how the local dive shop fills tanks, and they are both caused by the same thing – Hot Fills.

Almost all of the local dive shops hot fill tanks. That is why even though their gauge will show that there was 3,000 in the tank when they filled it, once it cools down that pressure drops by as much as a couple hundred pounds. I have often gotten tanks at 2600 or 2700 and the dive shop thinks that is fine. No thats not fine, thats lazy!

Image: biobug.org

A properly filled tank should be filled to 3,000psi, yet by taking shortcuts and hot filling tanks a perfect 3,000psi tank is hard to achieve. Basically they have to guestimate how much the pressure is going to drop once the tank cools down. Meaning that they have to intentionally overfill the tank by 500psi or more, causing additional stress on the tank so that the cooled tank will be closer to the 3,000 mark.

When I was renting tanks from the dive shop I didn’t worry too much about the additional stress that hot fills cause on a tank. Now that I own my own tanks thats a different story entirely.

Every time a tank is overfilled, or dropped, or banged around it gets closer to not being able to pass inspection and needing to be replaced. That is if your not so unfortunate to have a tank fail on you like the one in this photo. Just imaging if that were to happen while it was strapped to your back.

Over time every tiny bit of stress on the tank adds up causing metal fatigue until it eventually fails. Tanks receive enough stress on them being used without “tank monkeys” at dive shops intentionally overfilling them simply because they are too lazy to do it the right way and properly fill the tanks while keeping them cool, immersed in water.

Complete immersion in a water bath draws this heat away at a significant rate (25 times faster than air, as you may remember from your Open Water Diver class). The colder the water is, the more efficient the heat transfer. This not only reduces the time necessary to obtain a full pressure fill, it also means that you will have a greater volume of air or gas in your tank.

Heat that is generated during the filling process – will cause a significant pressure drop once the cylinder cools to room temperature, and again to diving water temperature. This means you will have a smaller volume of available gas in the event of an emergency. By having a cold, full pressure gas fill, you will have a greater volume of gas for longer dive times, and greater safety.

In recent years many dive stores have gone to dry hot filling, in fact here on Oahu I only know of one dive shop that does not hot fill tanks, Aaron’s Dive Shop in Kailua ($5 for an air fill). This change to hot filling is incorrectly based on published information which says it is not necessary to cool the cylinder. In reality though, to obtain a cool, full pressure gas fill, the cylinder must be filled extremely slow to avoid heating – or it must be allowed to cool to room temperature from the heat that is generated and then topped off to full pressure. Either way, this will take an inordinate amount of time. Other countries have issued warnings in recent years advising dive shops about the dangers of hot filling tanks, but here in the US it is now common practice.

The principles of physics cannot be altered. Any cylinder that is filled in an air environment is going to heat excessively. This heating will result in a substantial loss of pressure as the cylinder cools to ambient temperature. When you have a cylinder filled, it should be at rated pressure and at the very least cool, if not cold to the touch. Anything less is unacceptable. The practice of dry, hot filling is actually a decreased level of customer service, and pure laziness.

The way I see it, to not ensure a cold, fully rated pressure fill is simply a lack of commitment to customer service, and that is why I will be filling my own tanks from now on. If they want to be lazy and not give me good customer service and protect my investment, I can do it myself and save the money that I would have given to them for my tank fills.

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