CUP vs PSI vs BAR confusion

Started by kombi1976, January 01, 2009, 06:28:53 AM

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kombi1976

I was reading about the new Baikal MP-221 Artemida SxS double rifle in 45-70 which another guy here in Oz had bought and the maximum operating pressure for them is 2200BAR or 31900HST.
Now since most of us depend on either CUP (copper units of pressure) or PSI (pounds per square inch) neither of the former measurements made a great deal of sense to me.
On running the 2200BAR rating through the convertor  at worldwidemetric.com I discovered that HST is the same as PSI.
But that didn't really solve the problem.
The pressure ratings given in manuals are in CUP, which from reading a number of sources, has little to do with PSI.
Further more there is no mention whatsoever of any correlation with BAR readings.
Can any of the gurus here shed some light on this subject as it is quite significant when reloading for a firearm like the Artemida.
Cheers & God Bless
22lr ~ 22 Hornet ~ 25-20 ~ 303/25 ~ 7mm-08 ~ 303 British ~ 310 Cadet ~ 9.3x62 ~ 450/400 N.E. 3"


gitano

#1
There isn't much light to shed after one recognizes that the relationship between CUP and PSI is very weak. I have a spreadsheet I can post that gives the best comparison I have seen, but I really have never used it.
 
Since "digital" has finally come to the firearms industry, CUP has seen its last days - at least for the most part. In the "old days" it was extremely expensive to set up pressure measuring equipment. And I mean hundreds of thousands of dollars kind of expensive. CUP and LUD (Lead based estimates) were relatively cheap in comparison. Copper crushers DO NOT MEASURE pressure, they approximate it based on a standard amount of pressure it takes to compress a cylinder of copper a specified amount. The trouble is, the amount a copper cylinder is compressed by a specific pressure is HIGHLY variable even under the most controlled environment. Variable like plus or minus 15%!
 
Today's micro strain gauges and digital instrumentation means that very precise pressure sensing equipment can be had for hundreds of dollars instead of hundreds of thousands of dollars. The variation for this 'stuff' around a given set of pressure conditions is on the order of 0.1%. About 150 times more precise than CUP-based methods. There's just no reason to use CUP except that for the longest time, most ammunition manufacturers were set up for CUP estimates, and didn't want to retool for the strain gauges and instrumentation. I feel confident in guessing that all major ammunition manufacturers are now using strain gauges and digital instrumentation.
 
Here's a piece of the spreadsheet. I didn't create it, and while I have no reason to doubt its content or accuracy, I am not responsible for either.
 
Paul
 
I think the Mpa figures are too large by an order of magnitude. In other words where it reads "2400" it should be 240. For example, 2400 megapascals = 348,091 PSI. :eek: Whereas 240 megapascals is 34,809 PSI. Mobetta. See what I mean about "accuracy and content". I never even looked at those figures before because I was only interested in the CUP vs PSI figures.
 
Paul
Be nicer than necessary.

sakorick

#2
Paul, you are my hero. This is one of your classic posts and I appreciate the info very much. That also explains why some of my old reloading manuals have some wild deviations when compared to say the new Sierra manual. I still think common sense and careful inspection are the cornerstone to safe reloading.

Thanks for asking the question kombi as until now, I hadn't given this subject much thought. Regards, Rick.
Talk to yourself. There are times you need expert advice.

gitano

Quotecommon sense and careful inspection are the cornerstone to safe reloading.
Right on.
 
Paul[/SIZE]
Be nicer than necessary.

mhlmvl

#4
Paul, sure would like to see the rest of the spreadsheet you posted above on this thread. I have been working on this Remington/Baikal SPR22 operating pressure question and value every bit of information I can get. I am convinced, however, that the SPR22 is safe with mid-30,000 psi loads in the 45-70 chambering. I am working on bolt thrust, chamber wall thickness, and reviewing many loads on various sites where people have experience with loads in this rifle/cartridge combo.
 
Of course, Remington is no help on this as the manual and the barrels have pressure ratings. The barrel says CIP 2200 BAR/31,900 HST. The owner's manual and their web site in the past has said SAAMI 28,000 psi/28,000 cup. The 45-70 just happens to be the same for both SAAMI operating pressures. But, I note the Marlin 45-70 1895 lever action, commonly loaded in the 40-45,000 psi range in real life, has the same pressure rating in their owner's manual, 28,000 psi. We now know through long experience that is bunk.
 
What we do know already is that the '06 chambering in the SPR22 handles SAAMI pressures, 50,000 cup, and 60,000 psi. With the chamber walls on the SPR22 substantially thicker than the 45-70 Marlin, and the '06 with far greater pressures, it seems logical to assume the SPR22 is capable of far greater pressures than the Trapdoor. And since the Hornady Leverevolution 325 grain 45-70 ammo is said on the box to be fired safely in any rifle originally chambered for the 45-70 and in good condition, I think that means Trapdoors as well.
 
Meanwhile, there is a lot of chatter on sites that the SPR22 is only capable of safely firing the same SAAMI pressures as the Trapdoor. I can only take that to mean they believe both of these, the SPR22 and the Trapdoor are similar in safe operating pressures. To those suggesting this, I propose they rebarrel a Trapdoor to 30-06 Springfield, take two aspirin, shoot that rifle and call me in the morning. Since the Trapdoor and the SPR22 are said to be equal in strength, and the SPR22 handles the '06, then I guess the Trapdoor will as well. It is a silly argument the detractors of the SPR22 make, is it not? But from the first, there has been a lot of animosity against this "poor man's Russian built double rifle." And yes, I'm old enough to remember when the AK 47 was Russian junk as well.

Jamie.270

This may help too,  It has the .45-70 listed:

http://kwk.us/pressures.html


Quotecommon sense and careful inspection are the cornerstone to safe reloading.

Certainly can't argue with that!
QuoteRestrictive gun laws that leave good people helpless, don\'t have the power to render bad people harmless.

To believe otherwise is folly. --  Me

gitano

mhlmvl - Gimme a day or two to resurrect it from my archives and I'll post the entire chart.
 
Welcome to THL.
 
Paul
Be nicer than necessary.

mhlmvl

Quote from: gitano;90801mhlmvl - Gimme a day or two to resurrect it from my archives and I'll post the entire chart.
 
Welcome to THL.
 
Paul

Paul, sorry to bug you again about this chart, but I'd really like to see it in its entirety. I request that if possible, you post it. Thank you.:help: mhlmvl

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