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Scuba Professional: Column No. 4



“How Did That Get in There?”

Simon Pridmore looks at the common practice of filling scuba cylinders while they are standing in water and asks “Why?”

Anna is a new diver.

“I was on my ninth dive ever, about five minutes in and at a depth of around 12m when I realized that my air was not coming out smoothly. I couldn’t think why this should be. I had checked my pressure gauge on descent and it had shown 190 Bar. I switched to my octopus, but there was no difference. Soon the air became very thin.

I tried to stay calm. Everyone else was going deeper and swimming away from me. Then the air stopped completely. I held my breath. I looked around and there was my husband. He gave me his regulator and took me to the surface with him.”

Anna’s husband, an experienced instructor, takes up the story.

“I had checked my wife’s set up as usual before the dive. The pressure gauge was showing 190 Bar on the surface. A few minutes into the dive, I saw her hovering away from the reef in mid water, sensed something was wrong and swam towards her. She gave me a wide-eyed look and pointed at her regulator. I glanced at her gauge and saw it was at zero so I gave her my octopus and we went up together. “

“I was puzzled as to how a full tank could become empty so quickly. We were definitely no more than 10 minutes into the dive and there had been no O-ring blow out. After the dive, I removed the regulator and opened the tank valve. Nothing came out. Some instinct made me turn the tank upside down and, to everyone’s surprise, water started flowing out of the valve. It was fresh water!”

An Incredible Story?

Standard aluminium scuba cylindersIt seems incredible that a scuba cylinder should be so full of water that a diver would only get a few minutes of air. Some might interpret the story simply as a misunderstanding on the part of a new diver with a supportive husband.

However, when I heard the story, it brought to mind something that I came across when I had a dive store in Guam. We used to conduct annual visual inspections on cylinders owned by local residents and were surprised to often find a few centimetres of water in the bottom of cylinders. This was always fresh water, not seawater. We made some enquiries and found that this is fairly common and comes from the almost universal practice in dive stores around the world of filling scuba cylinders while they are standing in a trough of water.

The fill whips, hoses that are clamped onto the valves of scuba cylinders to fill them, sometimes drop into the water trough between cylinder fills. When the wet clamps are then attached to the valve of the next cylinder and the airflow is turned on, the pressure of the air in the fill whip is higher than the pressure of the air in the cylinder and water droplets are driven into the cylinder.

Fill the cylinder several times and the amount of water inside accumulates until you have a little pool in the bottom. The moisture inside causes corrosion to build inside the cylinder too.

Bear in mind that I was in Guam, part of the USA, where a stringent system of cylinder inspections applies and is enforced by dive operators. The maximum length of time between inspections is 12 months so that means the water inside the cylinders had accumulated in less than a year.

In many places in the world where people dive, including the island nation where Anna’s dive took place, there are no regulations governing cylinder inspection, so it is possible that a cylinder could go for many years without anyone examining the interior.

Therefore, Anna’s story is not as far-fetched as it might initially seem. With her cylinder containing a large amount of fresh water, the volume of air in the cylinder was quite small so once she started her dive she used it up very quickly. The reading on her pressure gauge must have been dropping fast during those first few minutes of her dive but she did not notice.

Why Do They Do It?

Dive centres fill cylinders while they are standing in water for three reasons:

  1. To keep them cooler during the filling process, which generates heat, so they can fill them faster and so the air pressure inside does not drop so much when they cool down afterwards and they don’t have spend time hooking them up again to top them off.
  2. As a convenient way to wash salt off them post dive, and
  3. To protect the filler from harm if the cylinder explodes during filling.

All three of these reasons are the product of fuzzy thinking.

  1. Filling the cylinder in water has very little impact on the cylinder temperature, partly because the heat from the filling process is transferred to the water, which then gets warmer. Fast filling still causes the cylinder to get hot, despite the water bath, so it still has to be filled to 20 Bar or so beyond its rated pressure. This makes sure that the pressure is 200 Bar or more when the cylinder is cool. Otherwise divers will complain. The only effective way to stop a cylinder getting too hot while it is being filled is to fill it slowly and from a bank of large cool high-pressure air cylinders rather than directly from the compressor.
  2. Using the fill bucket to wash the salt off the cylinder is ineffective as all that happens is that the water the cylinder stands in becomes a little salty. A quick fresh water blast with a hose before the cylinder is taken into the compressor room does a much better job.
  3. Aluminium cylinders have exploded during the filling process in the past and when this has happened the consequences have often been fatal. However, an exploding cylinder is a bomb and no concrete trough is going to protect the person filling it. Instead it will just contribute more shrapnel to the explosion.

Having said this, those aluminium cylinders that exploded were made from a 6351 alloy containing lead and it was the presence of lead in the alloy that created the problems that caused them to explode. Since 1988, no aluminium scuba cylinders have been made using this alloy or with any alloy containing lead. The vast majority of scuba cylinders in service today worldwide, including all cylinders made by Luxfer and Catalina, the top two manufacturers, are made from an alloy called 6061.

Although, during annual inspections, cracks have occasionally been found in 6061 cylinders, none has exploded: ever! And there are millions out there.

No Need for a Bath?

It is worth noting here that not all dive centres that fill cylinders in water are ignorant and negligent. There are many dive centres that fully recognise the potential issues and take elaborate precautions to make sure their whips and cylinder valves stay completely dry during the filling process so that water does not get into their cylinders.

But if,

  1. The likelihood that a scuba cylinder will explode during filling is so remote,
  2. A water trough will not protect a filler if it does happen,
  3. The water trough is not an effective way of removing salt or reducing the cylinder’s temperature sufficiently and,
  4. The practice of wet-filling can allow water to enter a cylinder and lead to an incident such as Anna’s,

why do it at all?

Ironic, isn’t it, that, from day one, divers are constantly told always to leave some air in their cylinder at the end of a dive so that no water can get in! And yet ……

Read more from Simon in his latest release Scuba Professional – Insights into Sport Diver Training & Operations and his bestselling book Scuba Confidential – An Insider’s Guide to Becoming a Better Diver, both available from Amazon in paperback and e-book versions.

Simon Pridmore is the author of scuba diving books, travel books and, as you might expect, scuba diving travel books. Originally from the UK, Simon has lived in Asia for over 30 years. As well as his books and guides, Simon writes regular columns for a number of magazines. He and his wife Sofie currently live in Bali, Indonesia but spend a lot of time exploring other places trying (but failing so far) to find a cure for their itchy feet. Simon's latest book - Scuba Exceptional – Become the Best Diver You Can Be - the follow-up to his best-selling Scuba Confidential, is available now in paperback and e-book versions from Amazon stores worldwide. Find out more about Simon and his books on

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Thanks for watching, Team!


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5 Ways To Use Less Gas When Scuba Diving



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Thanks for watching, as always! D.S.D.O James

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