Formula for converting CUP to PSI

rwsmith

rwsmith

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For you internal ballistics nuts, I just read an interesting article by Denton Bramwell telling how he made this conversion that is usable in rifle cartridges. There's no reason why the same exercise can't be done with pistol cartridges for anybody that cares to do the math and plotting. It's copyrighted so I'll just give the link to the article.

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

Even if I don't use it to actually convert numbers, it's a good read and tells why there are differences between the two systems. Inquiring minds want to know.
 
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I always read there wasn't a direct conversion? Most cartridges have a higher PSI than CUP representation, but some have the same, or higher, CUP than PSI. How can they convert? I skimmed the article but I didn't see that mentioned? Am I miss something?
 
a number of years ago Ken Oehler had some articles on pressure testing in the "Shooting Times" magazine. he said there was no formula that could be used to convert cup to psi and I would tend to believe what he said since he manufactures pressure testing equipment for the factories
 
There is no reliable formula for converting CUP to PSI, because they measure a different amount of the peak of the pressure curve.

While it is possible to plot points and do a regression equation for a particular load in a cartridge, using that regression equation for another cartridge is dangerous. If the shape of the pressure curve is faily close to the other cartridge, the results will be sorta similar, but not exact, and the more different the cartridge, the more different the results.

The short answer is, no matter what someone claims to have "discovered", DON'T trust any conversion chart or formula to go from CUP to PSI. They are not measuring the same amount of the pressure curve.
 
A direct conversion is simply not possible.
That is an approximation and should never be considered safe at anywhere near max. levels.
 
You're right

OKFC05 said:
There is no reliable formula for converting CUP to PSI, because they measure a different amount of the peak of the pressure curve.

### He explains this

While it is possible to plot points and do a regression equation for a particular load in a cartridge, using that regression equation for another cartridge is dangerous. If the shape of the pressure curve is faily close to the other cartridge, the results will be sorta similar, but not exact, and the more different the cartridge, the more different the results.

### That''s exactly what he did. The data was not totally linear and he points out that some cartridges just don't fit the curve at all. He does the same using the European method and it falls into line a lot better than the Saami data.

The short answer is, no matter what someone claims to have "discovered", DON'T trust any conversion chart or formula to go from CUP to PSI. They are not measuring the same amount of the pressure curve.

### Again, he explains that. He only found a correlation with the data between about 22 rifle cartridges.
Click to expand...

Don't worry, I'll not be doing this to extrapolate loads, it's just nice to see how the correlation lines up.
 
Some years back Speer had a very good and detailed article on their website on the difference between the pressure measurement systems and how Speer used the systems.

They did note that there appeared to be some stable relationship between CUP and PSI in rifle cartidges. They also noted that NO SUCH REALTIONSHIP EXISTS AT PISTOL PRESSURES! This may, in fact, have more to do with the characteristics of most pistols powders rather than a fluid dynamics kind of thing involving pressures in the abstract.

Amuse youself all you like, but so long as the official word is that no such relationship exits, I'd stick with that line. The body parts you save may be your own.
 
There isn't really a direct conversion, but you can extrapolate per each individual cartridge & get close. Regardless, you should be loading to the book max, not trying to guess pressures.
 
fredj338 said:
There isn't really a direct conversion, but you can extrapolate per each individual cartridge & get close. Regardless, you should be loading to the book max, not trying to guess pressures.
Click to expand...

Unfortunately, some handloaders do guess at pressures. If the manual lists 18 grains of WhizBang-X Powder yields a pressure of 35,000 CUP but the SAAMI max is 40,000 CUP, the clever handloader figures he can bump the max load up by 10% or so. Because guns are usually built with a large safety factor, he will probably get away with it.

Most of the time anyways.
 
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