Pressure, pressure, can you feel the pressure??
Some straight talk on cylinder pressures, fills and other myths.
LP vs. SP vs. HP – divers shopping for tanks are often confused with the selection of the right tank pressure to purchase when buying scuba tanks. And ask most folks in the industry and you’ll get more answers and rumors and whispered-down-the-valley distorted facts than you can shake a stick at. Here’s a short factual summary that we hope will help you decide what is best for you and your diving needs..
Myth #1 – Buy as big a tank as you can!
Cylinder size and pressure selection ultimately often boils down to personal preference, coupled with whatever influence your diving mentors and buddies have had on you. The bottom line when selecting a scuba cylinder is that you want as much gas with you as you need for the diving you intend to do, including whatever safety factors you wish to apply (no need for endless pontification on this topic, just making a point. I know we all have our own guidelines for safety). Different size divers, improved breathing techniques, more efficient finning, better dive planning – all these factors add up to help you determine how much gas you really need or want to carry. And don’t overlook reserve gas needs for your buddy, especially if you are planning penetration or deco dives! So determine the gas capacity that you require, then see what cylinder selection works best for you. Buy only the amount of gas capacity you actually need – anything larger or with greater capacity than that is simply more weight you need to hump around. And isn’t diving supposed to be fun?
Myth #2 – You can’t get a High Pressure tank filled everywhere
Twenty years ago, divers primarily depended on the standard low pressure 72 cubic foot tanks that were the industry standard since WWII. However, as the scuba industry evolved, and aluminum tanks became popular, many dive shops were challenged to be able to offer a full 3,000 psi fill. Well, times change, and today nearly every breathing air compressor built and sold in the last ten or fifteen years has the capacity to pump to 4,500 psi or greater which clearly provides enough pressure to fill any standard scuba cylinder, be it low pressure, standard pressure, or high pressure. The supplier’s storage bank or cascade systems generally do not affect this; as long as the dive shop or gas supplier maintains a system pressure higher than what you want in your tank, then you can get that pressure into your tank. Cylinders that are considered HP (typically 3,400 or 3,442 psi) are so common today that almost any dive shop can fill them fully. If in doubt, call ahead!
Myth #3 – Buy a smaller tank and overfill for more capacity
No cylinder should be ‘overfilled’ (the exception being a + stamped steel cylinder). Period! No more needs to be said on this. No need for mathematical conversions here. Buy the tank in the capacity you need at the design pressure and don’t play games.
Myth #4 – You can never get a full fill if your shop doesn’t fill tanks in a water bath
All cylinders should be filled slowly so heat should never pose a problem. Slow filling also extends the life of the cylinder by “working” the cylinder walls less from fill to fill. For an optimal fill, they can be left connected to a regulated fill system and allowed to top themselves off as any gas expansion caused by a warm fill dissipates. If for some reason a cylinder is filled quickly, or other cylinders are in the queue, then simply letting a filled cylinder sit and cool and then topping it off later will yield a more accurate fill. The whole water bath thing is another leftover from the “that’s the way we’ve always done it” mentality – from an engineering point of view there is no significant circulation of gas within the cylinder during the filling process, and air itself has a very poor thermal conductivity coefficient, so the thermal transfer of heat from the gas to the cylinder wall and then to the surrounding room air is very slow and inefficient. Water baths at room temperature, which many shops swear by, do not have any magical abilities to reach into your cylinder and draw the heat out of the gas within, making water baths simply an unnecessary mess at any fill station. In fact, pre-chilling cylinders as some do in a water bath actually can actually cause condensation to form on the interior cylinder walls during the filling process, which obviously is something we all wish to avoid in our tanks. An additional drawback to water baths for tank fills is the number of times a fill whip gets accidently dropped into the water, then placed onto a tank valve for filling – blowing the water that is on the fitting end right into the tank!
Myth #5 – Water baths make tank fills safer
Get real – does anyone really believe that partially submerging a pressurized metal container in a tub of water and then exploding that cylinder will cause any less injuries (or worse) to bystanders than would occur to someone who experienced the same event during a dry fill? Not a chance. The good news is that with proper filling procedures, including using a regulated air supply and verifying cylinder fill pressures prior to filling, should reduce the chance of a cylinder failure to near zero. Ask your shop if they use a regulated air supply for safety, or if they just count on the fill operator to open and close valves at all the right times? You might be amazed to find how few shops actually use a properly designed regulated supply system. Everyone likes to play the lottery, but this is one game of chance you should pass on.
So, let’s summarize – Give your dive planning some thought, calculate your gas needs and your safety or reserve needs, and then choose the tank that best fits your body and budget. Look at the cylinder specification chart (insert link to the spec page here) and buy what you need! In the end, buy the tanks that have the capacity you need, and make sure you can get the fills you need in the area(s) you plan to dive. Simple as that!
Dave's Dive (b)Log 
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