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CUP's vs PSI's

Discussion in 'Handloading and Reloading' started by Blakenzy, Dec 28, 2006.

  1. Blakenzy

    Blakenzy Active Member

    Jun 12, 2004
    How do you go about converting pressure information in CUP to PSI, and viceversa? I tried to look this info up but have had no luck finding a converting formula. I am asking this because not all load data is expressed in the same unit, and I would like to be able to compare them side by side.
  2. ClarkEMyers

    ClarkEMyers Active Member

    Oct 3, 2003
    A stone's throw from Henry's Fork
    You don't.

    You don't.

    It's not all that difficult for a restricted range where firing data exists and intervening values or slightly outside the range values can be interpolated but

    psi, sometimes psia, typically expresses peak pressure as measured with electronics (although the electronics may be capable of measuring a full curve and so used) and copper units of pressure is by definition given by comparing shortening of the copper with the test firing in a pressure barrel to tables of similar copper slugs shortened under controlled pressure. The copper is exposed (through a piston typically) to all the pressure under the curve. The peak pressure and the copper pressure measure different things - like converting degree days to daily high temperatures - there is a relationship but they don't convert.

    And the question has been done to death with long detailed explanations and links to very long detailed explanations all over the place.
  3. Father Knows Best

    Father Knows Best Senior Member

    Apr 14, 2005
    Minneapolis, MN, USA
    True. The analogy I use is that PSI is like a race car's top speed on a given track, but CUP is the elapsed time it takes to do one lap of the track. Higher top speed may lead to faster lap times, but not necessarily, as lap times are really a function of average speed, not top speed at one isolated point. A car that tops out at 150 mph on the straight but sustains a higher average speed may well be faster around the track than one that hits 170 mph on the straight but is a lot slower through the corners.
  4. wolfe28

    wolfe28 New Member

    Aug 16, 2005
    Between Maid-Rites and Maple Syrup; North of Sweet
    Here's the best I can do

    Being somewhat of a science geek, I was able to come up with this link that has a fairly good explination of how CUP, LUP and PSI are related, different, and why we still use CUP to measure chamber pressure.


    Hope this helps.

  5. Ol` Joe

    Ol` Joe Senior Member

    Feb 25, 2004
  6. Clark

    Clark Senior Member

    Jan 3, 2003
    Where I5 meets the rain forest
    CAUTION: The following post includes loading data beyond currently published maximums for this cartridge. USE AT YOUR OWN RISK. Neither the writer, The High Road, nor the staff of THR assume any liability for any damage or injury resulting from use of this information.

    1) In the category of gun that is stronger than the brass, I do not care what the pressure is in PSI or in CUP. I do not care what was the length of duration of pressure.
    I simply work up until the brass takes some damage, and then back off a safety margin so I know I will not have brass failure when a shot is important.

    2) In the category of gun that is weaker than the brass, I can Super Glue on a strain gauge and measure pressure, or follow a load book recipe for that cartridge.

    What does it all mean?
    Most of the time I don't care what is the pressure in psi or cup, but I care what are the effects of pressure.
  7. Steve C

    Steve C Senior Member

    Jan 5, 2006
    The reason there's no direct mathematical conversion from CUP to PSI or vise versa is that CUP is not a mathematically related scale while PSI is. For example, if you double the pressure you double the PSI but the CUP's will not necessarily be doubled.
  8. brickeyee

    brickeyee Senior Member

    Feb 25, 2005
    The copper crusher system has significant mechanical momentum and does not measure peak pressures as accurately as the piezo system.
    Strain gauge systems suffer from the assumptions in converting strain to stress (load imposed to deformation) and variation in the material making up the chamber (they are 'calibrated' using factory ammo).
    There are conversion factors between PSI and CUP, but they have a serious problem in not converging to 0 PSI at 0 CUP.

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