Gas Consumption

  • What Causes a Regulator to Free-flow?

    It’s very annoying, isn’t it? You jump into the water and your regulator starts to free-flow. It could even be life-threatening if that happened at depth. It’s as if someone has pushed the purge valve of the regulator in and held it there, losing you precious air. What causes it to happen?Dive2005B

    Modern day regulator manufacturers compete with each other to give the diver the most efficient and natural way of breathing. When you inhale from your regulator, the drop in pressure inside the body of the second-stage of the regulator drops and allows the second-stage valve to open, supplying you with air. Regulator designers try to make the valve as finely balanced as possible so that it takes the minimum amount of effort to pull it open against its closing spring, the spring that holds it shut and stops the gas escaping from your tank. Modern regulators can be so finely balanced in this way that it is often more effort to force the exhaust port open when you exhale than inhaling. (ANSTI breathing machines prove that!) So why do they sometimes free-flow?

    If you pull very gently on a regulator, the second-stage valve only opens a little to let air pass. The more you suck or the deeper you are the more it has to open to satisfy your needs.

    The depth the diver is at affects the pressure-sensing diaphragm. It operates a lever that pulls open the second-stage valve and also doubles as a ‘purge valve’. If for some reason the purge valve gets pushed in for a moment as it might when passing from air at the surface to water (a sudden increase in hydrostatic pressure) the valve opens and lets a whoosh of air past the back of the pressure sensing diaphragm. This fast moving air, just like the air moving fast over the top of an aircraft wing, causes a drop in pressure directly behind the diaphragm. This causes the valve to open even more and – viola! It’s exponential. The more the valve opens the greater the drop of pressure behind it and that leads to it opening the valve even more, resulting in that annoying rush of lost air.

    Luckily, that only normally happens at the cusp between air and water; that is to say at the surface. Putting your thumb over the mouthpiece is usually sufficient to cause a momentary increase in pressure inside the second-stage body to stop it. It’s annoying when it happens at the surface but it could be more than annoying if it happened at depth. Alas, in water that is colder than 10°C, it can happen at any depth if the mechanism of the valve is iced up or affected by ice. This is when it gets more serious than just annoying.

    Why is there ice? When air (or any other gas) is depressurised, it experiences a drop in temperature alongside the drop in pressure. The converse is also true. When a tank has been freshly pumped full, it feels hot to the touch.

    The water you are in may be at 10°C together with the air in your tank but that air in the tank might be as pressurised as much as 200bar. The first-stage drops it down to eight or ten bar more than the pressure of the water it is surrounded by. That’s a huge drop.

    It could easily cause a drop in temperature as much as 20°C and if you are in cold fresh water at 10°C you realise that it equates to minus 10°C for the air passing through the regulator’s first stage. This causes the water in its immediate proximity to freeze.

    Luckily, seawater rarely gets colder than 10°C around our temperate coasts but it’s a fresh water inland sites you might experience this problem – and it can be life threatening.

    You should have been taught how to breathe from a free-flowing regulator on your first diver-training course. The remaining air in your tank will give you time to get to the safety of the surface. Every diver should know how to do that.

    If you are in the habit of diving at inland sites, get a regulator designed for the job. They usually have first-stages that are environmentally sealed, with no working part coming into contact with the water, and they include extra metal to act as a heat sink to transfer what little warmth there might be in the water to the much colder air coming from the tank. Ask about that when you next buy a regulator. Ocean Leisure stocks cold water approved regulators by both Apeks and Scubapro.

  • Regulator Testing by Diving Magazines

    Everyone has had the experience of a broken domestic iron because it got knocked off the ironing board but would you think it a legitimate comparison test of domestic irons to see how well each fared subject to a drop test? No? I thought not, but that’s equivalent to what a British diving magazine has resorted to with its latest comparison test of regulators. They wanted to find out which regulator was toughest when subjected to the sort of disaster that rarely happens, by dropping a scuba tank on each or dropping a scuba tank in turn with each regulator fitted. Not very scientific nor easily kept consistent between each drop but they did it anyhow. It was like testing and comparing domestic irons by dropping them on the floor! Magazine editorials have been reduced to performing such stunts because since CE-certification for regulators was introduced and the ANSTI regulator-testing machine was developed to give an objective computerized result, quite frankly all regulators, certainly all those sold at Ocean Leisure, will give an easy breathe.

    Test divers with multiple tanks head out to deep water. Test divers with multiple tanks head out to deep water.
    86-87-1 Two divers breathe simultaneously from one regulator.
    I’m partly to blame for this state of affairs because around thirty years ago I started doing comparison tests of regulators, not only with a breathing machine but with groups of divers who actually breathed off each tested model at depth and compared the qualities of each. When we started doing this we discovered some horrors. Some regulators were not safe to take deeper than eighteen-metres while others were excellent breathers. The test made good copy for the magazine I was technical editor of and we managed to get the overall quality of regulators available up to a high standard. I went with a group of divers and multiple tanks each time down to fifty-metres-plus where they were able to experience the different way in which these regulators delivered air (the densest gas likely to be put through such a device) and make notes. In fact copious notes were made and two divers would breathe off one regulator to check each was good for an out-of-air emergency.
    Copious notes were made at depth. Copious notes were made at depth.
    It was uncanny in the way the experiences of the different divers coincided. I was always careful to choose experienced divers who were competent to work at depth and checked by springing upon them a written test at depth for nitrogen narcosis before we started the regulator comparisons in earnest. Then CE-certification came in and manufacturers had to make sure their products met the standard or go out of business. In the most recent tests I orchestrated, we found that there was little to choose between them unless any had a positive manufacturing fault. Most recently we were reduced to noting cosmetic differences and by-and-large these tests became pointless. Hence the ‘toughest regulator test’ we have recently witnessed. Of course there are some design differences. Piston-type regulators deliver the most air and for this reason they prove popular with those that dive in warm water conditions, but they are less suitable for use in cold fresh water than diaphragm-type regulators. Many of these have heat-sinks incorporated to take what little warmth there is in the water and transfer it to the very cold gas that is passing depressurized from the diver’s tank. If you are going to use a regulator in water polluted with muck or fine sand, one that is environmentally sealed might be more appropriate._DSC0159 Then there’s the question of servicing. Some makes are less well supplied with spare parts in remote parts of the world than others. Some enjoy very long servicing intervals indeed whereas others should be serviced annually, The staff at Ocean Leisure are exceedingly knowledgeable and if you can tell them your particular needs and requirements will be able to advise you which regulator is best for you. Whichever you choose to buy, hopefully, you won’t experience a car driving over your regulator anytime soon and they all breathe well!

  • How Long Will Your Air Last?

    The big question that every new diver wants answered is about how long the air in the tank will last. It all depends on how much there is to begin with, how deep it is going to be breathed at, and how much the diver is going to breathe it.

    How long your air will last depends on several factors including your own physiology. How long your air will last depends on several factors including your own physiology.
    The first two parts to this are easily identified. You can read on the shoulder of the tank its fixed volume and you can read from the pressure gauge how much it has been filled. Multiply one by the other. A 10-litre tank filled to 200 bar has 2000 litres of air in it. A 15-litre cylinder filled to 230 bar has 3450 litres of air. It is best to set aside a reserve of air and conventional thinking suggests that quarter of the initial supply is kept aside. This may be over cautious with a large tank but you have to make a judgement based on the circumstances you expect to encounter. Let’s assume that our 10-litre tank has only 1500 litres of air at our disposal with the rest (50 bar) is held in reserve. The next thing to identify is the depth the air is going to be breathed at. At 30m deep, the regulator delivers air at four times the pressure as it would at the surface. Thus if we are diving at this depth, we have only 375 litres of air compressed to four bar to breathe. The final part of the equation has a big question mark hanging over it. How much air do you need? A woman with small lungs will probably breathe a lot less than a male heavy-weight boxer. A man with large lungs will have the ability to pump a huge amount of air through his lungs. A relaxed man might breathe only eight litre every minute but increase his heart-rate by increasing his work load or stress him in some way, and this can easily leap to 30 litres each minute. It has not too much to do with fitness either. An old diver who has smoked all his life may not be very fit at all but if he is relaxed, and often that comes with experience, he will use less gas than a young trained athlete who is working harder than he should underwater. Even thinking hard uses a lot of energy. If you have to swim hard you will consume more than if you are merely hovering in the water. So what figure should we use for a breathing-rate in this calculation? Training agency manuals usually use a figure of 25 litres per minute in their examples of how to calculate air consumption. At 30m, our 10-litre tank (with 50 bar held in reserve) would last only 15 minutes. So what does it feel like to breathe from a regulator underwater? You can easily try this for yourself because it feels exactly the same as it does if you try it on land in a dive shop. You suck, there is a faint resistance and the valve pulls open. The mouthpiece floods with air that you inhale. It stops when you stop. When you exhale, there is also a slight resistance as the exhaust valve opens and allows that air to escape. It bubbles away past your head. Remember, the amount of air in your tank is divided by your respiratory mean volume or breathing-rate multiplied by the absolute pressure in bars at the depth at which it is breathed. If you can get your head around that you are well on your way to understanding your air consumption.

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