"High Pressure Vs Low Pressure" How Much Air Am I Getting?

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“High Pressure Vs Low Pressure” How Much Air Am I Getting?

Trying to pick the right tank for you can be difficult if you over think it. Understanding exactly how much air you actually get during a fill is important to help make a good decision on what tank to purchase.

How Big Of Tank Do You Need

How Long Will My Tank Last

Calculating Your RMV Rate

Calculating Your SAC Rate

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4 Comments
  1. Bill Hazel says

    Thank you for the information, this particular video is especially welcome. I'm a relatively newer diver (2.5 years approx 250 dives) diving in warm waters of Guam , The Philippines and the like. I get the part about the 80's I have mostly aluminum but a couple of steel tanks as well. My question is I have a LP steel 80 stamped 2400. One dive shop regularly will only give me 24-2600 as I have no + stamped on the tank. The other regularly gives me 3000-3500. Am I damaging this tank ?

  2. Just so I can Fing comment says

    Would be nice if I did not have to stand over the fill attendants in SW Florida to get them to fill my tanks SLOWLY so they do not heat up and thus do not cool down and leave me with less air than when they finished and so they do not over fill my tanks to compensate for filling them too fast.

    Nice video, clarified some things for me.

  3. Thomas Nelson says

    why do you use 3000psi for a cave fill? In cave country, we fill 2400, 3442, and 3500psi bottles to 3600psi.

    + rating is only for 3aa tanks, the AL tanks are not eligible for that + rating, nor are the exemption tanks *3442 or 3500psi*. The + rating is what the tanks nominal rating is. I.e. a LP95 is 95cf when filled to 2640, not to 2400psi. That has been the case since the LP72's were released.

    The cave fill exists for a myriad of reasons, but it is anything but ignorance or lack of caring. It is a conscious decision to maximize the volume of gas that we are carrying while taking advantage of the buoyancy characteristics of the low pressure tanks vs the high pressure tanks. Historically we also have to remember that HP tanks didn't exist until the 90's and those were only 80's, 100's, and 120's. The E8 series from PST were the first high pressure 8" tanks and didn't come out until the early 2000's. Not really practical to replace all of the bottles at that point.

    Not sure which shops you are going to in NFL that are not filling over 3000 psi. Not sure of any that don't provide full fills that we go to in cave country, all of the "popular" cave diving shops fill to 3600psi or higher for steels.

    Also note that the exemption tanks, 3442 and 3500psi bottles are not hydro'd at 5/3's of their working pressure, but at 3/2's of their rated pressure. 3aa and 3al tanks are hydro'd at 5/3's working pressure.

    You talk about wearing the gear down. The tanks that are regularly sent to cave country have burst discs installed for the pressures we are filling, or are plugged. They are also supposed to be replaced every 5 years during the valve rebuild. We have been cave filling low pressure steel bottles since the 70's and have yet to have one fail hydro. Luxfer also rates their aluminum cylinders to 100k hydro cycles. If you are filling a bottle 2x/day, that is 135 years before that bottle is going to hit that rating. That is not a catastrophic failure, just a big number. LP72's are hydro'd at 3750psi, so you are well under their hydro pressure when filling to 3000psi. No permanent damage is going to be done to any of these bottles by filling them to 3000psi, or realistically even 3600psi

  4. Andrew Morency says

    Your terminology is very confusing. You are calling an overfill a "hot fill". A hot fill is typically when the tank is filled fast and it is hot when it is done filling and then cools down to a lower than rated pressure. An overfill is when you, well over fill a tank.

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