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Thread: CF is CF correct?

  1. #1
    Barracuda
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    Arrow CF is CF correct?

    As I understand CF is the great equalizer?? Regardless of PSI etc CF is what matters. 19cf is aprox. 1/5th of 100cf?? Just want to paint a clear picture for new divers. Chime in techies.

  2. #2
    Barracuda
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    To save CompuDudes fingers from relentlessy typing all this info again I'll put it here for everyone.

    Al.80:
    2800psi=72cf
    2900psi=75cf
    3000psi=77.4cf
    3100psi=80cf (overfill, dangerous to Al)
    3200psi=83cf (overfill, dangerous to Al)
    3300psi=85cf (overfill, dangerous to Al)

    Al.80N
    2800psi=66cf
    2900psi=68cf
    3000psi=70cf
    3100psi=73cf
    3200psi=75cf
    3300psi=77.4cf
    3400psi=80cf (overfill, dangerous to Al)

    HP100:
    2800psi=81cf
    2900psi=84cf
    3000psi=87cf
    3100psi=90cf
    3200psi=93cf
    3300psi=96cf
    3400psi=99cf
    3442psi=100cf
    3500psi=102cf (slight overfill, not dangerous to steel)
    3600psi=105cf (slight overfill, not dangerous to steel)

    http://forum.scubatoys.com/tanks/233...html#post62528

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    Quote Originally Posted by WaScubaDude View Post
    As I understand CF is the great equalizer?? Regardless of PSI etc CF is what matters. 19cf is aprox. 1/5th of 100cf?? Just want to paint a clear picture for new divers. Chime in techies.
    I had this discussion a while back at my LDS. We were discussing using an LP 85 or an HP 120 when both were filled to 3000 psi. I couldn't get him to understand that they both had almost the same psi at that pressure. He kept saying sure, but the 120 is bigger. I said true, but the they have about the same amount of air, and the lp85 is much lighter. I tried the pound of feathers vrs a pound of lead thing, but I still don't think he got it.
    "Human beings are flawed individuals. The cosmic bakers took us out of the oven a little too early. And that's the reason we're as crazy as we are." - Desdemona

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    Quote Originally Posted by WaScubaDude View Post
    As I understand CF is the great equalizer?? Regardless of PSI etc CF is what matters. 19cf is aprox. 1/5th of 100cf?? Just want to paint a clear picture for new divers. Chime in techies.
    It's easier to understand the correlation if the tank volume is expressed in terms of water. Such as a 12lt tank, etc. That paints the image of a constant size, with varying pressures of gas pumped into it.

    FD

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    Quote Originally Posted by BSea View Post
    Quote Originally Posted by WaScubaDude View Post
    As I understand CF is the great equalizer?? Regardless of PSI etc CF is what matters. 19cf is aprox. 1/5th of 100cf?? Just want to paint a clear picture for new divers. Chime in techies.
    I had this discussion a while back at my LDS. We were discussing using an LP 85 or an HP 120 when both were filled to 3000 psi. I couldn't get him to understand that they both had almost the same psi at that pressure. He kept saying sure, but the 120 is bigger. I said true, but the they have about the same amount of air, and the lp85 is much lighter. I tried the pound of feathers vrs a pound of lead thing, but I still don't think he got it.
    This speaks to what I am getting at. does the lp hp matter at all! or is it just the 85cf vs 120cf. As I understand it the CF would represent the size of the phone booth that would hold Xcf at the surface. So 85 HP and 85 LP are equal???? Is this not so??

  6. #6
    Guppy
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    I may be wrong but with the LP80 vs the HP 120: the 120 will have more air due to the increased volume. It will take more air to reach 3000psi than the 80 will. They will have same psi, but volume is what keeps you alive.

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    Barracuda
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    And I thought I understood this...

    There are two things here

    1. Tank internal volume...

    2. How many cf the tank will hold.... Internal volume x number of atmospheres of pressure.

    Two tanks, with the same internal volume, will hold exactly the same at the same pressure.

    But, if one is rated at 3442, and the other at say 2400, then the 3442, when filled to it's spec pressure, will have more air.

    Does a 100 cf Faber XF tank have a larger volume than an AL 80? Yes it does, but not by as much as the 100 versus 80 would indicate.

    I like the reference to the actual tank volume, because that can be just as important as the pressure.

    The effect is that the 100 cf Faber, if only filled to 3,000, is still a 87 cubic ft tank, that weighs less and takes less weight over an AL80... that sort of comparison should be part of any dicussion, if one is going to buy tanks.

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    Quote Originally Posted by WaScubaDude View Post
    This speaks to what I am getting at. does the lp hp matter at all! or is it just the 85cf vs 120cf. As I understand it the CF would represent the size of the phone booth that would hold Xcf at the surface. So 85 HP and 85 LP are equal???? Is this not so??
    I Made a mistake in my earlier post (I really should proof read these). I said "they both had almost the same psi at that pressure." I should have said "the both had almost the same volume at that pressure.

    And you are right. an LP 85 will hold the same amount of air (volume) as an HP 85. It just that the HP will be smaller in size, and lighter.

    Quote Originally Posted by Puffer Fish View Post
    And I thought I understood this...

    Does a 100 cf Faber XF tank have a larger volume than an AL 80? Yes it does, but not by as much as the 100 versus 80 would indicate.
    I agree with you on everything except the above. Why wouldn't there be 20 Cubic feet of air difference as indicated by 100-80? Maybe I'm missing something. (it sure wouldn't be the 1st time)
    Last edited by BSea; 11-05-2007 at 11:07.
    "Human beings are flawed individuals. The cosmic bakers took us out of the oven a little too early. And that's the reason we're as crazy as we are." - Desdemona

    You don't know Desdemona? . . . "She runs this space station and bake shop down near Boomtown."
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    [quote=BSea;87026]
    Quote Originally Posted by WaScubaDude;86844This speaks to what I am getting at. does the lp hp matter at all! or is it just the 85cf vs 120cf. As I understand it the CF would represent the size of the phone booth that would hold Xcf at the surface. So 85 HP and 85 LP are equal???? Is this not so??[/quote
    I Made a mistake in my earlier post (I really should proof read these). I said "they both had almost the same psi at that pressure." I should have said "the both had almost the same volume at that pressure.

    And you are right. an LP 85 will hold the same amount of air (volume) as an HP 85. It just that the HP will be smaller in size.

    Quote Originally Posted by Puffer Fish View Post
    And I thought I understood this...

    Does a 100 cf Faber XF tank have a larger volume than an AL 80? Yes it does, but not by as much as the 100 versus 80 would indicate.
    I agree with you on everything except the above. Why wouldn't there be 20 Cubic feet of air difference as indicated by 100-80? Maybe I'm missing something. (it sure wouldn't be the 1st time)

    I should have been clearer.. volume in this case refers to the actual internal size of the the tank.

    When you fill a tank with say 77.4 cubic feet at 3,000 psi, you are putting 204 atm more gas in it (as we are starting at 0 =14.7. That means, the volume of the tank is actually....roughly .38 cubic feet inside (Watch me do the math wrong). With an identical volume tank, at 3442 psi in it, it would be 234 (approx) atm's...and around 88.8 cubic feet. The internal size of the tanks are just .427 versus .38 or around 12% larger.. the rest is due to the pressure differences.

    Note: every number here is actually off, as gas laws are not such easy math, but they should be close.

    If every tank you have can be filled to its rated pressure, you are luck.. as many places cannot do the 3,442 thing.

    So which high pressure tank has the closest internal volume and should you buy it, if you cannot get high pressures fills? Well a LP 85 worthington has an internal volume of .473 and a wothington X7 - 120 has an internal volume of .513.. so at the same pressure the 120 has more air (92 cubic ft). There is a weight difference of 4 lbs... A little more air... a little more weight.. would seem to be a draw..ok, there is a price difference.. a size difference, but just going on the two tanks, I would rather have the HP one... as some places i could get a much bigger fill... hope I made at least some sense there.

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    Quote Originally Posted by Puffer Fish View Post
    I should have been clearer.. volume in this case refers to the actual internal size of the the tank.

    When you fill a tank with say 77.4 cubic feet at 3,000 psi, you are putting 204 atm more gas in it (as we are starting at 0 =14.7. That means, the volume of the tank is actually....roughly .38 cubic feet inside (Watch me do the math wrong). With an identical volume tank, at 3442 psi in it, it would be 234 (approx) atm's...and around 88.8 cubic feet. The internal size of the tanks are just .427 versus .38 or around 12% larger.. the rest is due to the pressure differences.

    Note: every number here is actually off, as gas laws are not such easy math, but they should be close.

    If every tank you have can be filled to its rated pressure, you are luck.. as many places cannot do the 3,442 thing.

    So which high pressure tank has the closest internal volume and should you buy it, if you cannot get high pressures fills? Well a LP 85 worthington has an internal volume of .473 and a wothington X7 - 120 has an internal volume of .513.. so at the same pressure the 120 has more air (92 cubic ft). There is a weight difference of 4 lbs... A little more air... a little more weight.. would seem to be a draw..ok, there is a price difference.. a size difference, but just going on the two tanks, I would rather have the HP one... as some places i could get a much bigger fill... hope I made at least some sense there.
    Ok, that makes sense.
    "Human beings are flawed individuals. The cosmic bakers took us out of the oven a little too early. And that's the reason we're as crazy as we are." - Desdemona

    You don't know Desdemona? . . . "She runs this space station and bake shop down near Boomtown."
    - Jimmy Buffett

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