PSI and CUP


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ArchAngelCD
July 3, 2007, 05:44 AM
Some reloading manuals have mixed data. They will rate some powders in PSI and others in CUP. When you are trying to compare powders for velocity and pressure it's very hard to choose because of the mixed data. How can I convert one to the other so I'm comparing the powders in the same way? :banghead:

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RecoilRob
July 3, 2007, 07:06 AM
This has been discussed before and the short answer is normally you cannot. SOME powders and cartridges will translate with a mathmatical formula but many, many will not.

Problem is the method of measurement, not the units used. CUP crushes a copper slug and actually gives you an 'average' of forces applied. Sharp but brief peaks are not discernable as they are in a piezo-strain gauge system.

If it really is an issue, and I guess any responsible reloader should be concerned with this, I'd suggest using only the latest strain-gauged data. It will catch those spikes that can weaken a firearm over time much better than the older CUP system.

Ol` Joe
July 3, 2007, 08:21 AM
The data isn`t acually mixed if one conciders both methods are showing what the load developers decided were max loads. It matters little how the pressure is measured, or by what standard. It is kind of like measuring speed in MPH or Kilometers/hr, top speed or presssure is still max.

Bad Flynch
July 3, 2007, 04:55 PM
Well, the several pressure measuring methods are not translateable, one into another by simple means; many are not correlatable at all. The problems arise because the pressure methods are very different. Some give a measurement more like an average and some give a measurment that reflects more like a true peak. It is just like the difference between the old psig and psia measurements, but more complicated than that. In additon, the places on the case where the measurement is taken, are different and affect the readings, too. All of the measurements are greatly affected by the measuring tool, too.

Best bet: max is max in any system, just as others have written here.

CZ57
July 3, 2007, 09:08 PM
AACD: Do you have time to make a graph? The max pressure rating of the .45 ACP is rated at 21,000 PSI and 19,900 CUP. If you have established equivalent pressures for an older and lower pressure cartridge, you can set a lower baseline. Those pressure ratings have never changed so that is what I used to combat this conundrum originally. The .270 Winchester is rated 65,000 PSI and 54,000 CUP. Use these as your low and high for PSI and CUP, then create increments of 100 on each line. The PSI line will be diagonal in relation to the CUP line. You can then plot horizontally from the CUP line to the PSI, or vice versa. For a midpoint reference check, 35,000 PSI should equal 33,000 CUP. Those are the current pressure ratings of the 9 X 19mm in either system. Use the former 9mm rating of 35,700 CUP and you'll find it's nearly identical to the current SAAMI rating of 38,500 PSI for 9mm +P. Not exact enough to bet your life on, but within a couple of hundred PSI, and most variables in handloading easily exceed that!;)

ArchAngelCD
July 4, 2007, 06:55 AM
I appreciate the answers and they were what I was expecting. I know the data isn't "mixed." What I meant was it's hard to evaluate 2 or 3 different powders when the method used to report max pressure is different.

Thank you for your help.

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