Overfilling Scuba Cylinders
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Paladin
Contributor
I do not overfill my tanks. Reason: metal fatigue. Metal will only flex or expand/contract so many times before its crystalline structures becomes weakened to the point of failure; even when used within its design parameters.
A Boeing 737 is designed to fly at about 40,000 feet, give or take. In order to do that and keep the passengers comfortable (and alive) it must be pressurized. Pressurizing the plane puts stress on the fuselage skin, just like filling a SCUBA tank stresses the tank wall. When the plane returns to lower altitude, the pressure is decreased, releasing the stress. This repeated cycling slowly weakens the metal until it fails. Remember the 737 that lost part of its fuselage over Hawaii some years back? The metal failed even though it was being used as designed.
Take a paper clip and straighten it out. Then, bend it slightly and straighten it repeatedly. If the amount of bend is slight, the number of cycles it goes through will be much higher than if the bend is pronounced. How soon the metal fatigues and fails is directly dependent upon the degree of stress involved in each cycle.
Overfilling a tank will shorten its life, of that I have no doubt. But how short? Personally, I have no desire to find out. But, as in all things diving, it's a personal choice. If you want to dive with a potential bomb strapped to your (vulnerable) body, go for it. Not me.
A Boeing 737 is designed to fly at about 40,000 feet, give or take. In order to do that and keep the passengers comfortable (and alive) it must be pressurized. Pressurizing the plane puts stress on the fuselage skin, just like filling a SCUBA tank stresses the tank wall. When the plane returns to lower altitude, the pressure is decreased, releasing the stress. This repeated cycling slowly weakens the metal until it fails. Remember the 737 that lost part of its fuselage over Hawaii some years back? The metal failed even though it was being used as designed.
Take a paper clip and straighten it out. Then, bend it slightly and straighten it repeatedly. If the amount of bend is slight, the number of cycles it goes through will be much higher than if the bend is pronounced. How soon the metal fatigues and fails is directly dependent upon the degree of stress involved in each cycle.
Overfilling a tank will shorten its life, of that I have no doubt. But how short? Personally, I have no desire to find out. But, as in all things diving, it's a personal choice. If you want to dive with a potential bomb strapped to your (vulnerable) body, go for it. Not me.
Paladin954,
Your example of an airliner is appropriate. Airplane service life is designated by the number of 'pressure cycles' the plane sees. They are, however, rated for something like 10,000 pressure cycles. Scuba tanks don't see that kind of use.
Your example of the paper clip, however, does not apply. In bending the paper clip you are exceeding the elastic limit of the material. In overpressure filling steel scuba tanks we are not.
Fruits and veggies again...
Paladin
Contributor
Cave Bum,
The paper clip example was for illustration. I am fully aware of the differences. I also said it is a personal choice whether or not to take the chances involved with routine overfilling. I, myself, am by no means a Politically Correct, Always by the Rules diver. However, I dive within parameters that I believe are reasonable (for me) within the current set of conditions and my own abilities and limitations. Yes, I sometimes take calculated risks if I am confident of my success and the outcome is worth it. But the amount of risk one is willing to take is largely dependent on a number of subject criteria. For me, I can easily imagine the stresses going on within the walls of my tanks every time they are filled and I am not inclined to increase those stresses for what is, to me, questionable gain for the sort of diving I do. But, as in all other areas of life, to each his own. If you feel confident that you can routinely overfill your tanks and get way with it, this is your choice and I will not judge you. It is your right to choose what risks you are willing to take.
I understand that cave diving entails risks that are absent from other types of diving and that certain, unusual measures and practices have been developed to deal with those risks. As I said in another post, I have much respect and admiration for the guys who willingly go sliding into the dark places. I tried it once an nearly bought the farm. This is one of those areas where I feel it is beyond my personal limits and confidence/comfort level. Diving in the dark is bad enough, but strapping an over-pressurized tank to my back would increase my apprehension and stress levels. It just isn't for me.
I once knew a diver (my father's cousin's boy) down in Virgina who routinely overfilled an old Voit 72. It was one of those that had the 1/2 inch J valve that was sealed with teflon pipe tape instead of an O-ring. He was diving solo when it blew. When he didn't come home that evening, his wife called the Sheriff and they started searching for him in the area where he usually went diving. The family asked me to drive down and help look for him. It took two days to find him.
Such things tend to stay with a man.
The paper clip example was for illustration. I am fully aware of the differences. I also said it is a personal choice whether or not to take the chances involved with routine overfilling. I, myself, am by no means a Politically Correct, Always by the Rules diver. However, I dive within parameters that I believe are reasonable (for me) within the current set of conditions and my own abilities and limitations. Yes, I sometimes take calculated risks if I am confident of my success and the outcome is worth it. But the amount of risk one is willing to take is largely dependent on a number of subject criteria. For me, I can easily imagine the stresses going on within the walls of my tanks every time they are filled and I am not inclined to increase those stresses for what is, to me, questionable gain for the sort of diving I do. But, as in all other areas of life, to each his own. If you feel confident that you can routinely overfill your tanks and get way with it, this is your choice and I will not judge you. It is your right to choose what risks you are willing to take.
I understand that cave diving entails risks that are absent from other types of diving and that certain, unusual measures and practices have been developed to deal with those risks. As I said in another post, I have much respect and admiration for the guys who willingly go sliding into the dark places. I tried it once an nearly bought the farm. This is one of those areas where I feel it is beyond my personal limits and confidence/comfort level. Diving in the dark is bad enough, but strapping an over-pressurized tank to my back would increase my apprehension and stress levels. It just isn't for me.
I once knew a diver (my father's cousin's boy) down in Virgina who routinely overfilled an old Voit 72. It was one of those that had the 1/2 inch J valve that was sealed with teflon pipe tape instead of an O-ring. He was diving solo when it blew. When he didn't come home that evening, his wife called the Sheriff and they started searching for him in the area where he usually went diving. The family asked me to drive down and help look for him. It took two days to find him.
Such things tend to stay with a man.
OldNSalty
Contributor
This is probably bad timing given what Paladin954 just posted but does anyone know how much you can overfill a 72? I've got 2 (found one and picked up another the other day for $30). I am thinking about dropping the Al 80's for the 72's...until I can aford some HP 100's
I do not have any data that suggests that filling a cylinder to 160% takes it beyond its elastic limit. In fact, I don't know where that magic line lies, between the elastic and inelastic boundaries. I'll bet the manufacturer knows, which is why they chose the rated pressure that they chose. Clearly, though, pressures used during hydro do purposefully exceed elastic limits of the cylinders.
But I would like to see your data that proves that filling a cylinder to 160% does not take it beyond its elastic limit. If you have access to a hydro station you could prove that very easily. Pump it up to 160%, release the pressure, and then see if its permanent expansion ratio is 0%. If the permanent expansion ratio is > 0%, then clearly a 160% fill does exceed elastic limits. Can you do that for us? It would be very interesting. I am not sure that we could get this kind of data directly from the manufacturer. Thank you.
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