From: Norman Johnson <firstname.lastname@example.org>
Subject: re: Can someone provide a definition for a "Copper Unit of
Date: 3 Sep 1999 22:49:50 -0400
#Can someone provide a definition for a "Copper Unit of Pressure"?
#I've been trying to find out exactly what this is, in terms of PSI of
#Pascals, and can't find anything.
#All I found was the revelation "unit of pressure, dervived from
#crushing a copper cylinder" on a miliary website somewhere.
The process is such that there is no conversion from one to the
other. The CUP method uses a "standard" copper slug that is
compressed according to the pressure CURVE of the tested round.
It is a reliable way to control ammo pressures because it is
repeatable within the limits that must be maintained to
manufacture safe ammunition. As you may know, this is very
roughly equivalent to psi but not close enough to assume that they are the
same. To do so would be foolhardy. I will attempt to explain why.
Strain gages are used to measure pressure in psi. The PEAK
value is that which is published but the whole pressure curve is
revealed on the oscilloscope. Herein lies the inconsistency of the two
Lets see if I can explain that inconsistency. Assume two pressure curves
having the SAME AREA UNDER THEIR RESPECTIVE CURVES, one with a SPIKED (high
"Q") pressure curve, the other with a FLATTER (low "Q") curve.
Because the pressures are such that the copper slug is in plastic
deformation, the area under the curve is associated with the slug
deformation. Sort of like punching a block of modeling clay with your fist
versus holding your fist against that block with the same force for an
A strain gage would indicate a higher pressure for the spiked
curve and a lower pressure for the flatter curve while the copper
slug might have the same deformation and thus indicate the same
CUP measurement for the two loads.
In summary: Two loads having the same area under the pressure
curve might show similar or same CUP measurements, while the one
with the sharp spike would show significantly higher pressure
(psi) using the strain gage.
Clear as mud?
They both work - but one does not translate to the other.
Because there is much more information made available by use of
the strain gage, and because it is far less time consuming, it is
slowly replacing the copper deformation method.