In spite of what some will try to convince you there is absolutely NO CORRELATION between PSI and CUP or LUP. The methods used are totally different and you CANNOT convert one to another.
It varies from caliber to caliber and round to round and even at different pressure levels. Some times they are almost identical, at other times CUP is higher than PSI yet at other times PSI is higher than CUP.
Beware anyone who tells you it's possible to convert one pressure reading to another. Chances are there are other things they know nothing of they will also tell you wrong about.
As Graybeard said, NO CORRELATION. The two methods are so vastly different that there could never be any relationship in results. To over simplify--PSI measures firing stress' of the barrel steel over the chamber area and CUP measures how much the expanding gases crush a (regulated) copper plug by means of a piston drilled directly into the chamber wall. CUP is nonfunctional below the pressure range required to pierce the wall of the case. Told you it was oversimplfied.
You guys are great. Denton Bramwell shows there is a correlation in his article. Two different measurement systems measuring the same thing have to be correlated and there has to be a conversion between them. Its the error in both of those systems in this case that creates a problem. Reread the article and learn what correlation means.
The conformal transducer of a piezoelectric transducer system which yields results in PSI does not measure firing stress of the barrel steel over the chamber area. However, a strain gauge does.
Again, there is a correlation between CUP and PSI. However, as Ron Keysor states in his 1996 article in American Rifleman, Pressure Measurement, "shooters have been left to reconcile two pressure value systems that don't correlate in a sense we laymen can understand."
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