CUP vs. PSI ?

[hylander]

So from everything I have read HP38 and Win231 are the same powder.
Why the different measurement and what is the difference ?

Looking at my Lee 2nd Edition I see:
.45ACP, 200SWC

HP38 5.6gr., 914fps @ 16,900 CUP
Win 231 5.5gr., 910 @ 19,600 PSI

[missionary5155]

Good morning The Lee #2 Loading manual you have explains all this. #1 you made the right choice getting a manual #2 I would STOP reloading and take the time to READ all the information in the front of that book. Your weapon, vision, hand, and those standing nearby... not to mention FAMILY would prefer you in 1 (one) normal piece.
CUP... Copper Crusher used in pressure testing that gets measured after shot thus indicating the pressure...

[Wayne Smith]

There is no mathematical relationship between CUP and PSI. There is no way to compare them reliably.

[hylander]

There is no mathematical relationship between CUP and PSI. There is no way to compare them reliably.

That is what I was wondering.
I was just wondering why Lee would measure one load in CUP and the other in PSI.

Missionary5155:

I have been reloading for over 27 years and have several load books.
I just don't understand why Lee would measure the same load only a different powder with two different measurement methods.

[MT Gianni]

Lee states in their book that they didn't measure anything just compiled data from those who did.

[.45Cole]

hylander, there should be a way to convert from one system to another, because both systems measure a physical property, and can give precise results. I would post a link to someone who does, but i'm a little lazy today. find a load that has both and plot the data against eachother and the trendline will be the conversion from the y axis to the x axis. Psi is a little better because it can pick up on the spikes, where cup only tells the max pressure.

[fourarmed]

Older pressure data was measured in special guns with a piston in the chamber that compressed a copper pellet, or "crusher." The measurement in CUP was fairly repeatable, but hardly what could be called precise, and nobody ever pretended that there was a simple and accurate conversion to PSI. Piezoelectric transducers mounted on test guns measure minute expansion of the gun itself by generating voltages that are read in real time by computers. There are imponderables involved in interpreting those results too, but that is where the PSI data comes from.

[mousegun]

A definitive lay work on CUP versus PSI values is the Bramwell paper:


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

The problem with regression fits of CUP against PSI lies in the methods used. Piezo relies on elastic response to applied pressure forces while the CUP (and LUP) rely on the response of materials that are stressed pasttheir elastic region to derive the data. This renders the CUP/LUP data more nonlinear and more subject to the effects of alloy, sensor position and geometry, etc. The upshot is that a better correlation between the two data sets is to be had from their slope (first derivative) rather than the values themselves. Error propagation for this exercise is complex and even if successful won't give reliable data as long as data from both methods with concurrent testing isn't available (data derived from crusher and piezo methods during the same firing series.)

Does all this hooha mean there's no value in comparing the two methods? Nope. Not for me anyway. From the data I've seen there are some observations that seem to be valid.


There is an approximate equality between the two methods somewhere between 33,000 and 42,000 psi/cup.

The CUP method becomes nastily nonlinear and inaccurate somewhere below 25,000 PSI, which is why LUP (Lead Units of Pressure) takes over.

Above about 35,000 psi the CUP values become quite linear and thereby a relatively accurate gauge of comparative pressure behavior within that range.

Values in the region of 50,000 CUP correspond roughly to values in the range of 56,000 to 59,000 PSI.


Actual conversion of PSI/CUP isn't practical with the published data. CUP/PSI data is, however, useful if the above points are observed.

[anachronism]

The CUP is most likely old data. Alot of the bullet obturation formulas use CUP. The PSI loads are more modern, and are more precise. Wayne the Shrink covered the interchange nicely. LEE "cherry picks" data from other companies manuals. If you want first hand data, buy a Lyman manual. The last I knew, they did their own testing. One other thing, any change in components from the test sample, including (& especially) alloy hardness negates the value of the pressure data. Just like with jacketed bullets.