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Converting CUP to PSI

5mmgunguy
How do you convert CUP pressure readings to PSI pressure readings?
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Comments

10 comments

Date Votes
  • perry shooter
    They don't really convert Higher is higher and lower is lower But you cant say one CUP = 37PSI
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  • shoff14
    We had a big argument about this last time it was asked. [:D]
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  • Tailgunner1954
    Denton Bramwell has a formula that shows the correlation (not a conversion) between the 2. IIRC 42,000=42,000 but above that PSI is higher than CUP and below that PSI is lower than CUP

    Note that the relationship is non-linear and there are a few samples that don't agree with each other IE: 2 with the same CUP rating, but one has a higher and one a lower PSI rating and others that have the same PSI rating but one a higher and one a lower CUP rating.
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  • nononsense
    5mmgunguy,

    There is no direct conversion but there is a proposed correlation as demonstrated by D. Bramwell. He has written a very excellent paper on this correlation which is worth a read at the very least, agree or not.

    I have sent you a link to the articles that have been made available on the internet by e-mail.

    Best.
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  • Maalox
    Interesting article from Wikipedia with some charts correlating the two. The Bramwell article referenced in the other posts.

    http://www.shootingsoftware.com/ftp/psicuparticle2.pdf
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  • shoff14
    quote:Originally posted by Maalox
    Interesting article from Wikipedia with some charts correlating the two. The Bramwell article referenced in the other posts.

    http://www.shootingsoftware.com/ftp/psicuparticle2.pdf


    I would just like to see some type of regression rather then linear.
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  • partisan
    Sounds like too many experts in the kitchen.
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  • v35
    My understanding of CUP or copper units of pressure is there isn't an accurate conversion factor between that method and chamber pressure determined by the later strain gage method.
    In the '80s I had a CUP fixture built and bought copper crusher pellets from White Labs. They came in two diameters to cover the pressure range of small arms ammo. Each batch of calibrated crusher cylinders come with a "Tarage" chart giving deformation vs pressure. Crushers were measured after firing and corresponding pressures found on the Tarage chart.
    In the CUP method, a plug of brass case is blown out of the case to start the piston moving to compress the calibrated crusher cylinder. Therein lies some of the error.
    Young's Modulus states that stress is proportional to strain up to the elastic limit for steel and strain is measured directly by the later method.
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  • mcaso
    Anyone have any graphs? It seems to me that if both are measuring the same thing then there has to be a formula to convert one to the other. If the conversion can't be made then the two are not measuring the same thing. So there must be at least one measurement that both are not measuring.
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  • Tailgunner1954
    Mcasco
    CUP/LUP is measuring plastic deformation in compression of a metal (copper or lead, with lead being used in ultra low pressure systems like shotguns). This is not a completly linier process because as the metal deforms, it's cross sectional area is also changing.
    PIEZZO systems use a crystel that generates a voltage signal that is proportanial to the force applied (same crystels are used in the "snap igniter" on your BBQ grill)
    Strain gauge systems use the hoop expansion of the chamber to measure the pressure.
    The "factors" your missing are SENSITIVITY and TIME, Piezzo is more sensitive than copper, and the SG systems have the advantage of not only being less expensive but they can moniter pressure over time not just the microsecond of peak pressure.
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