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Offline simplicity

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psi vs cup
« on: January 06, 2011, 02:23:58 AM »
I have been noticing in manuals that some loads are measured in CUP and some in PSI. The CUP is usually about 10,000 less then the psi. I'm wondering is there a formula to calculate cup to psi or vice versa? Also how does CUP and PSI comepare to each other is as in 40,000 cup to 40,000 psi?
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Offline SHOOTALL

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Re: psi vs cup
« Reply #1 on: January 06, 2011, 02:41:09 AM »
I don't think there is a reliable one. If you check out some older manuals when cup was the only test used and compare the new editions when the manual was updated using psi mesurements you will often notice a reduction in max load data. for safety reasons. Some loads exceeded safety levels but the cup mesurements did not reflect it.
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Offline Scibaer

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Re: psi vs cup
« Reply #2 on: January 06, 2011, 02:56:50 AM »
This topic gets brought up and asked about from time to time. if you do a search, you should find a few threads on it, but.. CUP and PSI are very different ways to measure chamber pressures and there is no cross over formula. i have looked, and i have tried to use algebra to make one without success.
some reloading manuals have explanations of how PSI and CUP are derived and Wikipedia has them too, and you can see that even though they both run out 40,000 CUP and PSI, they vary in their incremental spacing and do not correlate to each other.
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Offline wncchester

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Re: psi vs cup
« Reply #3 on: January 06, 2011, 05:52:38 AM »
PSI is pretty linear, CUP/LUP is not, can't make a math comparision between a constant and a variable.
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Offline LONGTOM

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Re: psi vs cup
« Reply #4 on: January 06, 2011, 07:14:10 AM »
Don't beat the dead horse.
I tried this a long time ago on here.
There is no way to compair the two.
Some are close to what you say while others are way off.
There are a few rounds where PSI runs less than the CUP.
Just can't be done.
I, along with others have tried.
Each is it's own realm!!!



LONGTOM
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Offline stimpylu32

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Re: psi vs cup
« Reply #5 on: January 06, 2011, 09:20:08 AM »
Quote from: Scibaer on January 06, 2011, 02:56:50 AM
This topic gets brought up and asked about from time to time. if you do a search, you should find a few threads on it, but.. CUP and PSI are very different ways to measure chamber pressures and there is no cross over formula. i have looked, and i have tried to use algebra to make one without success.
some reloading manuals have explanations of how PSI and CUP are derived and Wikipedia has them too, and you can see that even though they both run out 40,000 CUP and PSI, they vary in their incremental spacing and do not correlate to each other.

That about sums it up , kinda like Apples to Oranges , their both fruit that grows on trees , but other than that they have nothing in common .

stimpy
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Offline quickdtoo

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Re: psi vs cup
« Reply #6 on: January 06, 2011, 09:57:05 AM »
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Offline Larry L

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Re: psi vs cup
« Reply #7 on: January 06, 2011, 10:08:38 AM »
Rather than eating up bandwidth with a detailed explanation, here's some reading for you that fairly well covers the subject.

http://www.chuckhawks.com/pressure_measurement.htm

The military has been using the Piezoelectric method of pressure testing since 1978. What you may find interesting is that the military uses the the head pressures in the 7.62x51 cartridges but uses the neck pressures in the 5.56 ammo. Readings/pressures are considerably different.
http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=ADA433154
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Offline simplicity

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Re: psi vs cup
« Reply #8 on: January 06, 2011, 10:59:30 AM »
Wow thanks for all the replys. What brought this question on to myself was that I have just started loading for a ar10, in the few handloads I have try'd bullets running the same speed yet different powders some batches cycle fine yet one or two it'll hang up so often the only difference is the powder used IE bl-c2 and h335 for example. What I was thinking (I find myself getting way to technical and sometimes at work I have a bit to much time on my hands) is to run a bit more pressure aka a bit more gas to push the bolt back with just a bit more force aka slightly slower powder etc. The loading data I can find on hand  for the powders I have are mixed as far as the pressure designation goes (some are in CUP and the others in PSI) So hence my question of how to compare the two.
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Offline simplicity

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Re: psi vs cup
« Reply #9 on: January 06, 2011, 11:04:36 AM »
Also i'm kinda dealing with what brass was fired in what chamber before hand, all get resized in a small base die. just ones that have beeen resized in my dillon won't reliably feed and ones resized in my bonanza will. The difference the dillon shell plat e is a bit thicker and is holding the die about .020 higher. It's a bit hard to explain but it's another variable I have on my mind.
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Offline BCB

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Re: psi vs cup
« Reply #10 on: January 07, 2011, 12:13:20 PM »
I am going to disagree, but only slightly and I can’t find the proof I need…

Some years ago, I had an article by someone—I don’t remember who—but it had a method of comparing cup to psi or visa versa…

Don’t ask me where I found it, but it was from a reputable source if I remember correctly…

I will continue to look for it, but I have my doubts…

Yet, there is something out there somewhere that indicates there maybe be a correlation—or at least in the author’s mind and he seems to prove it with mathematics…

That is all I can add to this and really, without the article, it might be considered worthless!...

But, I just want to let that thought out…

Good-luck…BCB
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Offline quickdtoo

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Re: psi vs cup
« Reply #11 on: January 07, 2011, 12:36:52 PM »
That's probably Denton Bramwell, see the Shooting Software .pdf file I posted.... Correlating PSI and CUP

It's also discussed in this article. http://kwk.us/pressures.html

Tim

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

Quote
Conclusions

1. PSI (correct use) is highly correlated to CUP. Evidence: R^2 = .927 makes it
impossible to successfully argue otherwise.

2. CUP is mainly an indicator of peak chamber pressure: Evidence: The way that
piezoelectric systems are commonly used, they report purely peak chamber pressure.
The CUP system is highly correlated with the piezoelectric system. If the “off-peak”
deformation of the copper pellet were large, the correlation to the piezoelectric
system would be poor.

3. SAAMI did a pretty consistent job of setting maximum pressure limits in both
systems. Evidence: The two are highly correlated. Basically, they got pretty close to
the same answer both ways.

4. You can convert from one system of measurement to the other. Evidence:
Definition of "correlated". Basically, correlated means that you can estimate one
variable from the other. The opposite of this is "statistically independent", which
means that you can't.

5. The formula for the conversion is the one shown above. Evidence: Produces the
"least squares fit" for the two systems, and it produces an R2 of .927. You can test the
formula by plugging in any of the CUP numbers shown above. The formula will give
you back a PSI number that is close to the one shown in the table.

6. Work remains to be done in refining the SAAMI conversion. Evidence: An R2 of
92.7% is produced, leaving 7.3% of the variation to be explained. Measurement
system error probably sets the limit of the R2 that can be obtained at around 98%.
That leaves 5.3% of the variation unexplained. Perhaps someone can discover what
the unaccounted for variable is.

7. The first example of something disproves all claims that it does not exist. The
formula exists, and it works. So all claims that it does not exist cannot be true.
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Offline BCB

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Re: psi vs cup
« Reply #12 on: January 07, 2011, 12:42:07 PM »
I knew someone else had heard of it and I wasn't just making things up that appeared in a dream!!!

I think I will read it and research it a bit more than I did years ago...

The second link you list is the one I read some years ago.  Best get your asbestos underware and get ready as he says!  And, people just parrot what they have heard in the past and really have no knowledge of the subject at all.  They say what they have heard.  (This could also be true of the S.E.E. subject!  I don't think that exists, but we will let that go for now!!!)...

The linear regression plots really show the picture...

Thanks quickdtoo...

BCB
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Offline Scibaer

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Re: psi vs cup
« Reply #13 on: January 07, 2011, 03:03:54 PM »
well ok i read thru that stuff, im not real sure what kind of math you call that, but anyhow.. do we as reloaders really need to delve into math like that or even attempt to correlate CUP and PSI ? i think the way things are right now have been and will continue to serve us safely
its hard for me to feel comfortable working around linear regression plots, or even feel the need to when working up a good load.
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Offline necchi

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Re: psi vs cup
« Reply #14 on: January 07, 2011, 08:18:37 PM »
I'm with ya Scibaer,
 I kinda just plain gave up worrying about a conversion for the two. I guess for me, when I look up the data, either CUP or PSI  works for me as it applys to my gun and load.
I could understand the need for more if I was wildcatting,,but min is min and max is max either way. (shrug)
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