I had some bottles re-hydro'd last week. While I was there, I figured I'd ask a certified technician about overfilling since it's been popping up on here recently.
Long story short - DON'T!
As I asked the question, I could tell he was anxious to answer before I finished asking. A resounding NO was his answer. And, he continued... If you overfill above the rated psi it immediately condemns the bottle. There is no further discussion to this. Overfill = condemn. I told him I understood but asked him to explain a little further as to why this is so that I could share it with you guys and possibly avoid an injury, or worse, for someone enjoying our sport.
The bottles expand and contract under pressure. When a bottle gets over filled, it stresses the sidewall of the bottle beyond what it was designed for, on a microscopic level. An example of this that I really like is the coat hanger example. Take a coat hanger and flex it back and forth enough so that it retains just a very slight bend. You'll be able to do this many many times without failure. This would be considered normal use. If you were to put two harder bends in the hanger during this time, you would weaken it a bit but you'd still be able to go back and forth without failure. This would simulate hydro testing which is required 2 times in 15 years. Bend a coat hangar back and forth as much as you can, 180deg. It won't last long before you feel it get easier to bend and ultimately break apart. That's an example of what you're doing to the bottle on a microscopic level when you overfill.
I've heard of putting a piece of masking tape on an empty supply bottle. When you fill the bottle, the tape will break. I've never tried this. But, I did have an opportunity about 2 years ago to talk with a different certified technician about what happens during a hydro test. Basically, they fill the bottle with water and then pressurize it to ~7500psi. While he was working, I asked how long he keeps the bottle at that pressure thinking the test might be, if it blows before XXX amount of time then, it failed. He said, not long at all!!! At that pressure, the bottle WILL explode, probably in less than 20 seconds. So, what was he measuring? He was measuring the amount of expansion taking place over a short period of time only ~3-5 seconds. In other words, He's putting one very hard bend in the coat hanger and measuring how much weaker it got while he was bending it. One hard bend isn't going to break your coat hanger.
But, overfilling repeatedly will damage your bottle and it's not just a little bit at a time. It's works on a compounding scale. The first time you damage it (and condemn it for use). The next time you damage it more and more severely. The next time, more severely and more quickly. And, it goes on getting worse and worse every time you do it.
Now, I'll tell you that I've gone past 4500 on my supply bottles. But, it's not by much and, I don't do it on a routine basis. Certified SCUBA shops will fill bottles over 4500 psi. They do that so that when the bottle cools, and the pressure decreases, it ends up at or near 4500. Filling a hint over, only for a short amount of time, isn't going to over stress your bottle. But taking a 250b bottle and bringing it up to 300b adds ~700 psi and will damage your bottle.
Be safe out there guys and stay "happy shooting!!"
Tom
Long story short - DON'T!
As I asked the question, I could tell he was anxious to answer before I finished asking. A resounding NO was his answer. And, he continued... If you overfill above the rated psi it immediately condemns the bottle. There is no further discussion to this. Overfill = condemn. I told him I understood but asked him to explain a little further as to why this is so that I could share it with you guys and possibly avoid an injury, or worse, for someone enjoying our sport.
The bottles expand and contract under pressure. When a bottle gets over filled, it stresses the sidewall of the bottle beyond what it was designed for, on a microscopic level. An example of this that I really like is the coat hanger example. Take a coat hanger and flex it back and forth enough so that it retains just a very slight bend. You'll be able to do this many many times without failure. This would be considered normal use. If you were to put two harder bends in the hanger during this time, you would weaken it a bit but you'd still be able to go back and forth without failure. This would simulate hydro testing which is required 2 times in 15 years. Bend a coat hangar back and forth as much as you can, 180deg. It won't last long before you feel it get easier to bend and ultimately break apart. That's an example of what you're doing to the bottle on a microscopic level when you overfill.
I've heard of putting a piece of masking tape on an empty supply bottle. When you fill the bottle, the tape will break. I've never tried this. But, I did have an opportunity about 2 years ago to talk with a different certified technician about what happens during a hydro test. Basically, they fill the bottle with water and then pressurize it to ~7500psi. While he was working, I asked how long he keeps the bottle at that pressure thinking the test might be, if it blows before XXX amount of time then, it failed. He said, not long at all!!! At that pressure, the bottle WILL explode, probably in less than 20 seconds. So, what was he measuring? He was measuring the amount of expansion taking place over a short period of time only ~3-5 seconds. In other words, He's putting one very hard bend in the coat hanger and measuring how much weaker it got while he was bending it. One hard bend isn't going to break your coat hanger.
But, overfilling repeatedly will damage your bottle and it's not just a little bit at a time. It's works on a compounding scale. The first time you damage it (and condemn it for use). The next time you damage it more and more severely. The next time, more severely and more quickly. And, it goes on getting worse and worse every time you do it.
Now, I'll tell you that I've gone past 4500 on my supply bottles. But, it's not by much and, I don't do it on a routine basis. Certified SCUBA shops will fill bottles over 4500 psi. They do that so that when the bottle cools, and the pressure decreases, it ends up at or near 4500. Filling a hint over, only for a short amount of time, isn't going to over stress your bottle. But taking a 250b bottle and bringing it up to 300b adds ~700 psi and will damage your bottle.
Be safe out there guys and stay "happy shooting!!"
Tom
