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PSI VS CUP apples and oranges?
Discussion regarding the reloading of ammunition and tuning of loads for accuracy
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bodyalter
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PostPosted: Wed Feb 28, 2007 6:52 am    Post subject: PSI VS CUP apples and oranges? Reply with quote

so I'm looking through my 7-08 reloading manual and I'm faced with trying to compare pressure/recoil with these two different measurements. are they that different or is there a formula to convert one to the other?

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Morax
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PostPosted: Wed Feb 28, 2007 7:04 am    Post subject: Re: PSI VS CUP apples and oranges? Reply with quote

give this a try
www.shootingsoftware.c...ticle2.pdf
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bodyalter
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PostPosted: Wed Feb 28, 2007 8:33 am    Post subject: Re: PSI VS CUP apples and oranges? Reply with quote

thanks a lot morax. very good read even.

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PaulS
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PostPosted: Thu Mar 01, 2007 1:35 am    Post subject: Re: PSI VS CUP apples and oranges? Reply with quote

There is no correlation between CUP ans PSI because of the way that the two readings are taken and the sensitivity of each method.
The pressure piston method that is used to find CUP has a threshhold of several thousand pounds before anything registers. The longer it holds a pressure the more it deforms the copper "crusher". it measures a pressure over time - not peak, average or anyother kind just some amount of pressure and some period of time. It was used for years but when the pressure curve method was used, things changed.
The PSI instrument is a pressure transducer. The thing that makes it work is that when you put pressure on a crystal it produces a voltage. when you double the pressure you get 4 times the voltage. This method could not be used until computers got fast enough to measure the changes of the voltage in very short amounts of time. Once calibrated and hooked to a fast computer you can measure changes in pounds per square inch at a resolution of PSI/u seconds (1/ 1000000 of a second).
What that means is that as soon as you have 1 psi you get a reading and you continue to have a reading that is constantly changing all the way up to 1000000psi. Now you can read not only peak pressure, average pressure but you can also plot the actual time each pressure is exherted on the chamber. You get a curve that accurately defines the exact pressures over time. (exact means that you are within 0.5% of the maximum pressure recorded or 325 psi of 65000psi maximum)
Two very different loads can produce the same CUP reading and very different pressure curves in PSI.
You could get the same CUP reading with a 222 cartridge running a fast powder as you could with a 45-70 and a slow powder.
222 with a fast powder peaks very high pressure but holds it for a very short time while the 45-70 produces less pressure but holds it for a long time. The CUP might show both at a calibrated 30000 CUP but the PSI method would show that the 222 went to 60000 for .000004 seconds and the 45-70 only reached 25000 max but held it for .001 second. they both exerted enough force to squish the copper crusher to the same size but with very different real time pressure curves.
That is why there is no correlation between the two systems. if you want to see the real world facts send me an email at:
paulstephens2(AT)comcast(DOT)net
and ask me for the SAAMI pressures for rifles and pistols. I will send you a word document that list both pressures for the rounds that have been tested in both systems and then you can try to figure any correlation between one load that is 20000CUP and 45000 psi and two loads that have the same CUP and very different psi (one high the other low).
I'd be happy to share the files.

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twofifty
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PostPosted: Thu Mar 01, 2007 11:11 pm    Post subject: Re: PSI VS CUP apples and oranges? Reply with quote

Morax wrote:
give this a try
www.shootingsoftware.c...ticle2.pdf

I agree with bodyalter - a good easy read and a great intro to statistical methods and plotting. Makes sense too. But then so does what PaulS has posted.
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bodyalter
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PostPosted: Fri Mar 02, 2007 7:22 am    Post subject: Re: PSI VS CUP apples and oranges? Reply with quote

it would be nice if all the reloading manuals picked one of these methods and just used it.

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SwampFox
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PostPosted: Fri Mar 02, 2007 8:57 am    Post subject: Re: PSI VS CUP apples and oranges? Reply with quote

Now there you go again with wishing for common sense.....
Ed

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Bushmaster
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PostPosted: Fri Mar 02, 2007 10:14 am    Post subject: Re: PSI VS CUP apples and oranges? Reply with quote

If we had "common sense" in reloading, where would the fun be? And the challenge...Where would the challenge be? With "common sense" we wouldn't need these forums either...Just where would the fun be... Very Happy

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Morax
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PostPosted: Fri Mar 02, 2007 12:08 pm    Post subject: Re: PSI VS CUP apples and oranges? Reply with quote

bushy i use these forums for common sense i hit you guys up for your thoughts and ideas and to double double check my thinking so thats common sense to me!! but like you said pertaining to the reloading aspect, if we didnt have two trains of thought we wouldnt have someone out there saying ok well i can do this better, reloading for me is not all about accuracy, its a nessisary evil.. and i read what Paul had wrote and it does go a long way with the aspect of time versus pressures, but the only thing i have a question on is pressure at "X" for the time "Y" dont matter when too much of either goes boom in your face!! other than that I agree
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Bushmaster
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PostPosted: Fri Mar 02, 2007 12:11 pm    Post subject: Re: PSI VS CUP apples and oranges? Reply with quote

No arguement from me...And we do have an asset in PaulS...

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bodyalter
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PostPosted: Sun Mar 04, 2007 4:05 am    Post subject: Re: PSI VS CUP apples and oranges? Reply with quote

yes we do and I want to thank you PaulS for relating what the measurements mean before I went through my books converting everything. you more than likely saved me valuable time better used reloading

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PaulS
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PostPosted: Sat Mar 24, 2007 2:55 am    Post subject: Re: PSI VS CUP apples and oranges? Reply with quote

Been off for a while - duty called.
Thanks for your votes of confidence. If I had my way we would be using the PSI method because it tells you more about what is reaaly going on in the gun. It would be a lot harder to get a dangerous round into your gun. I have a hard time understanding why Hodgdon is staying with the older CUP method when it is so obvious that the PSI method is so much better.
PaulS

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Paul
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So and So's pages on the internet = NOT reliable loading data
Always check data against manuals
NEVER exceed maximum listed loads
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