# Thread: Whats up with cup and psi...anyone know the difference..?

1. ## Whats up with cup and psi...anyone know the difference..?

If I have a load that shows 55,000 cup...what would that be in psi...???

2. http://how-i-did-it.org/762vs308/chamber.html

Try this, explins it pretty good.

3. It's not so simple as you would hope. This is a good read on the subject of correlating PSI and CUP: Attachment 65028

The authors web site (I have no affiliation, just giving credit where due): http://www.pmg.cc/

4. Thnx for the info...it helps

5. I was gonna post the same article that iofthetaiga posted. The math he uses to extract the equation at the top of page 4 is sound. PSI =55000cup*1.51586-17902 which ends up being about 65500 PSI. Thats on the hot side for any of the belted magnums (typical max is 64000 or 62000 psi). What cartridge is that load for?

6. Originally Posted by highestview
I was gonna post the same article that iofthetaiga posted. The math he uses to extract the equation at the top of page 4 is sound. PSI =55000cup*1.51586-17902 which ends up being about 65500 PSI. Thats on the hot side for any of the belted magnums (typical max is 64000 or 62000 psi). What cartridge is that load for?
.223/5.56 in a AR 15

7. Originally Posted by Oneriver
.223/5.56 in a AR 15
223's max pressure is 55000 PSI. Are you sure you're reading that right? 55000 CUP would blow your brass up pretty bad if that's really the load pressure.

8. You can't reliably relate CUP and PSI. As in what PSI equals CUP or vise versa.

To quote the article...

("there is no real relationship between CUP and PSI (beyond the fact that they both get higher as pressure goes up). Sometimes, they're the same. Most of the time, CUP is lower than PSI. As a rule, the behavior of copper under pressure is so quirky that there's no reliable way of translating CUP to PSI mathematically; you just have to measure both ways.")

Smitty of the North

9. So there I was de-capping and sizing brass for my daughters .243Win tonight. As I stared at the rifle leaned against the wall I realized that this particular 20" 9.5 pound Ruger Tactical with a forged stainless steel bull barrel and CF bolt action is much much stronger than the import, youth, light weight, tapered barrel, push feed, cheap rifle.

So now a question comes to mind...

If I develop a max PSI load in this heavy barreled rifle, keep in mind the chamber is in this heavy forged stainless bull barrel that is attached to a beefy action. If the strain gauge that is attached to the barrel steel shows 60,000 PSI which is max SAAMI PSI in the .243Win. What would that load read in a light weight, thin, tapered import barrel with the same gauge attached to it?

For those that may know the answer my question is, is there an established steel/action/receiver strength that is used as a standard to develop loads for PSI testing? It seems to me that the reading would vary from rifle to rifle with the same load being tested in each due to the variable strengths between the brands and materials used.

10. Originally Posted by Smitty of the North
You can't reliably relate CUP and PSI. As in what PSI equals CUP or vise versa.

To quote the article...

("there is no real relationship between CUP and PSI (beyond the fact that they both get higher as pressure goes up). Sometimes, they're the same. Most of the time, CUP is lower than PSI. As a rule, the behavior of copper under pressure is so quirky that there's no reliable way of translating CUP to PSI mathematically; you just have to measure both ways.")

Smitty of the North
To quote the other article:

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.

11. There is certainly a correlation between psi and cup but as is shown in the charts there is a noticable difference in cup vs psi any many of the cartridges listed. Many show the same cup with a noticable difference in psi and others show just the opposite with same psi and a quite different cup. So, although there is a correlation between cup and psi, the average R^2 of .927 is just that, an average. Converting a cup to psi within + or - 5.3% is definately a correlation but not what I would call a real accurate one.

I don't know exactly how the strain gages are configured to account for the differences in thickness of barrel around the chamber. I would think if you made a rifle barrel with a breach diameter of 10" chambered in 223Rem and put a strain gage on it, you would have trouble getting any reading at all. I would think some sort of accounting for barrel thickness and material would be necessary.

12. Originally Posted by rbuck351
I don't know exactly how the strain gages are configured to account for the differences in thickness of barrel around the chamber. I would think if you made a rifle barrel with a breach diameter of 10" chambered in 223Rem and put a strain gauge on it, you would have trouble getting any reading at all. I would think some sort of accounting for barrel thickness and material would be necessary.
rbuck351,

You have hit on the question that has been stuck on my mind. In your example I would be willing to bet that a load would appear to have low pressure. In reality it could be huge and we wouldn't know it.

There must be a standard in development that is used.

13. ## the real difference between 7.62 NATO and .308 Win

A bit off topic, but a very significant difference I found between the two cartridges that 7.62 NATO can be fired in a .30-06 military chamber while .308 Win cannot. In the assortment of rounds I have that shoulder diameter of the 7.62 is smaller than the .308. When the 7.62 was developed there was a very real need to have the new cartridge work in the existing weapons - some rifles were converted using chamber adapters. The .308 Win cartridge avoided the liability of being fired in .30-06 based chambers like the .270 Win by enlarging the shoulder diameter so the .308 won't chamber.

You can try this in your own weapons- I tried it in quite a few of mine and found the results agree with I've stated above. One exception may be foreign 7.62 rounds - they may not be exactly to US specs.

Originally Posted by Daveinthebush
http://how-i-did-it.org/762vs308/chamber.html

Try this, explins it pretty good.

14. ## should read the same

IF the strain gage setup is calibrated correctly the readings should be the same. The thicker barrel will expand or deform less but a correctly calibrated gage will take that into the calculations. There are other factors such as taper etc which will also affect the readings on either. Calibration is the other issue- getting an accurate and known pressure level to calibrate the instruments.

However, in general a thinner barrel is more desireable than a thick one as the deformation is larger and easier to measure. Load cells, for example, use a thin linear section to get the best accuracy and widest range.

Originally Posted by marshall
rbuck351,

You have hit on the question that has been stuck on my mind. In your example I would be willing to bet that a load would appear to have low pressure. In reality it could be huge and we wouldn't know it.

There must be a standard in development that is used.

15. This tends to make me think that unless you have access to some very high dollar testing equiptment, you would still be getting a number that is not really accurate even if you have a strain guage. Apparently you need a known pressure in your barrel to properly set the strain gage. So, how would you develope a known pressure in any particular barrel. A round of 308 fired in one barrel would more than likely show a different pressure if fired in a different barrel. And 2 supposedly identical 308 rounds would probably show different pressures if fired in the same barrel. I think what we are working with is averages and possibly close estimations rather than an exact formula. So X = Y + or - some percentage. All I can say is 55,000 cup is probably a pretty stiff load, but that too is just a guess.

16. ## inaccuracy

You are correct in your suspicions - to make any sort of accurate measurement with a strain gauge you need to calibrate the installation by pressurizing the barrel up to known accurate pressure levels. This would involve capping the barrel at both ends and then using some sort of a pump or dead weight tester to raise the pressure in the barrel up to 50,000 pounds or so. Obviously this isn't an easy or cheap thing to do. Even after calibrating the gauge you won't know if you were at the correct place to measure the peak pressure - choked flow and other issues may cause the peak pressure to be elsewhere in the barrel or chamber.

So while you can compare two or more loads as to pressure generated at a given point or points you are making a lot of assumptions if you try to determine the absolute pressures.

Also - while there are numerous ways to measure pressure strain gauges are not normally used in commercial applications where high accuracy is required. There are lots of other technologies that give more accurate results and avoid the issues of temperature etc. affecting the measurements.

Originally Posted by rbuck351
This tends to make me think that unless you have access to some very high dollar testing equiptment, you would still be getting a number that is not really accurate even if you have a strain guage. Apparently you need a known pressure in your barrel to properly set the strain gage. So, how would you develope a known pressure in any particular barrel. A round of 308 fired in one barrel would more than likely show a different pressure if fired in a different barrel. And 2 supposedly identical 308 rounds would probably show different pressures if fired in the same barrel. I think what we are working with is averages and possibly close estimations rather than an exact formula. So X = Y + or - some percentage. All I can say is 55,000 cup is probably a pretty stiff load, but that too is just a guess.

18. Thank you all who have helped to make this a very informative thread. 50 years of reloading have given me a pretty good idea as to how to keep my body parts intact but I have never tried to put any real numbers on the pressures of my loads. I don't even have a good idea of what that would cost but I'm sure it would be a lot higher than my buget would allow. Thanks again Buck

19. Thanks IO, I guess.

rbuck and tv:

It was my understanding that the strain gauges had to be calibrated using special ammo, or perhaps using Factory Ammo.

Smitty of the North

20. Because each chamber, throad, leade, freebore, exact bore size and smoothness of barrel is different from gun to gun, it would be hard to say what the pressure would be with any round from one gun to the next. Tv has brought up some points that I hadn't given a lot of thought to but I believe he is right in saying it is difficult to apply a specific pressure (say 50,000 psi) to any given barrel and know that 50,000 is what you in fact applied.

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