Correlating PSI and CUP, Article

[OnHoPr]

Maybe an interesting read for a number of reloaders/shooters.

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

[Bzcraig]

Thanks, downloaded to read later.

[44MAG#1]

This is just something for an egg head to have to pass the time.
If a loading company gives CUP for a cartridge deal with CUP while loading their data and if it is in PSI deal with PSI while using their data.. it is silly do have to figure some obscure figuring to try to see how they correlate.
They are both figures for obtained pressure of a cartridge for heaven sake.
If I want to see how they relate I just simply go to SAAMI and look at the cartridges for their standard on that cartridge. On a lot of cartridges they give both. Some even the +P figures.
How simple is that?

[Larry Gibson]

That is an old article and while a good effort it does not really "correlate". Any ballistician will tell you there is only a very narrow band of cartridges with certain powders where there is, if any, any "correlation" between CUP and strain gauge/peizo transducer pressure measurements.

Dr. Oehler has conducted many side by side tests and comparisons including using a CUP test Universal Receiver additionally equipped with 2 strain gauges (for M42 and M83 BLs), 2 PC Model 117B31 conformal transducers and PCB Model M165A01 Case Mouth transducer. With that test fixture then he could obtain 6 pressure measurements on the same cartridge........no comparison of pressure measurements from different cartridges but 6 pressure measurements of the same cartridge. By doing such Dr. Oehler was able to develop correlation coefficients. But again those coefficients are applicable only to a narrow spectrum. Dr. Oehler states in his report on his testing; "The use of copper crushers with undrilled cases is still recognized by SAAMI. Pressure measurements made in this fashion do not correlate well with other pressure readings."

As an example; the difference in figures for the .308W between CUP and PSI measurements run usually around 10,000 (52,000 CUP and 62,000 PSI). When you get down into standard pistol cartridge pressures in the 14,000 to low 20,000 the CUP and PSI measurements can be close to the same. In these comparisons there are no absolutes that should be assumed in making any comparison between CUP and PSI measurements. Only the actual measurement of pressures will provide an answer. That is why it is best to follow the admonitions by those who do have the proper pressure measuring equipment and publish that obtained data in all reloading manuals; "Stay with the start and maximum loads and work up the load stopping at any sign of pressure". If one is experienced one can work up loads with wildcats and/or components not listed in manuals or for use in firearms that can take higher psi's than the original cartridge generates. However, it is still easy to get into trouble if one is not careful. Based on my own experience the last 10 years measuring the pressure of thousands of cartridges I also suggest any attempt at correlation between CUP and PSI measured pressures be skeptically viewed.

Larry Gibson

[35remington]

When the CUP measurement "numbers" can be a little higher than the psi numbers, the same as the psi numbers, a little lower than the psi numbers, a lot lower than the psi numbers, and considerably lower than the psi numbers given his data set, well.....

I'd also say there is little basis for correlation. Treat the article as being of little use, because it is.

His formula for "converting" between the two is poor. Scan his list of cartridges and easily pick out the ones it does not work on. He must have skipped those. This makes for bad "science."