How do they measure cup pressure
- Thread starter Dano4734
- Start date
big al hunter
New member
C.U.P is measured with a copper crusher. Basically it is a hole precisely drilled into the chamber of the test gun. The hole is there to accept a copper pellet that is measured before and after the cartridge is fired. The amount of difference in length of the copper pellet is used to calculate the pressure. It should be noted that C U.P is quite different than P S.I.
ETA,. Each reloading company makes their own load data and testing. Each decide on the best way to determine max load data. Each lot of powder will give different pressures with the same charge weight, so different results from each company.
ETA,. Each reloading company makes their own load data and testing. Each decide on the best way to determine max load data. Each lot of powder will give different pressures with the same charge weight, so different results from each company.
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The Lyman #47 had an article with good illustrations depicting this, and I'm sorry it disappeared. If you are interested in the technical nitty-gritty, the SAAMI Centerfire Rifle Standard has a description starting on page 160 (172, as Acrobat counts pages), but it will be beyond people unfamiliar with basic statistics and QC practices.
Basically, a SAAMI standard test barrel for a particular chambering is mounted in a universal receiver that accepts all SAAMI test barrels and serves as the ballistic technicians switch-barrel gun. The test barrels are different for copper crusher and for conformal piezoelectric transducer testing, and the Europeans have their own standard employing a channel type piezoelectric transducer that is yet another configuration.
The copper crusher barrel has a hole in the chamber that is specified as to location and diameter in the SAAMI specification I linked to. A small piston is placed in that hole. The old system required drilling a hole in the case to match that location, but that was a bother and later made optional. A kind of saddle clamp anchors an anvil over the piston location in the barrel. A calibrated copper slug is placed between the anvil and the piston. When the gun is fired, pressure in the chamber drives the piston to crush the copper slug against the anvil. The slug is then removed and a micrometer is used to measure how much it was crushed. The amount of crush is then looked up on a tarage table—a calibration table provided by the slug maker—and the pressure is read off that table for the amount of crush that was measured.
This method used to be used to determine psi. However, when piezoelectric and strain gauge testing was used for comparison in the 60's, it was discovered the copper crusher system was non-linear and as the pressure got higher its readings were too low. However, it was still possible to use the instrument to compare a reference load to a new load being developed using the old apparatus, so rather than abandon it, SAAMI gave the old instrument its own unit called the CUP for Copper Unit of Pressure so it would not be confused with actual psi (pounds per square inch). Nonetheless, you have to be careful reading pressure data to be sure you are not seeing older data for which the copper crusher result was still reported in psi instead of in CUP. All the older military manuals, for example, still report copper crusher results as psi when they are actually CUP. Any military technical manual not published after about 2000 may have that confusing data. Older publications like Hatcher's Notebook (the last edition was 1961) still refer to copper crusher measurements as psi.
One other problem with the copper crusher, besides its non-linearity, is that it is less consistent than the other methods by about a factor of two, so even for comparison, it is a less desirable method of measuring. The military produced an improved M11 copper crusher tester in the 1970's or 80's that was apparently more consistent, but not adopted commercially, AFAIK.
The cause of the non-linearity is that the copper slug calibration is static in which the psi for the tarage table are measured crush produced by weights bearing on a piston like the ones in the crusher barrels. That is a static test that deforms the metal a bit more than happens in an actual test at the higher pressures. Those higher pressures have very brief peaks that occur so quickly the inertia of the piston and slug absorb some of the force before deformation progresses fully. The system could be made much better if dynamic calibration were done, but the lower consistency as compared to other methods would still be present. It is due, in part, to the fact no two ballistic technicians seem to have identical touch with the micrometers used to make the measurement. Maybe that could be addressed with a laser micrometer, but nobody would bother now. Everyone likes the complete pressure traces produced by the transducer and strain gauge instruments, and there is no way for the crusher to do that or to measure more than the peak value.
Basically, a SAAMI standard test barrel for a particular chambering is mounted in a universal receiver that accepts all SAAMI test barrels and serves as the ballistic technicians switch-barrel gun. The test barrels are different for copper crusher and for conformal piezoelectric transducer testing, and the Europeans have their own standard employing a channel type piezoelectric transducer that is yet another configuration.
The copper crusher barrel has a hole in the chamber that is specified as to location and diameter in the SAAMI specification I linked to. A small piston is placed in that hole. The old system required drilling a hole in the case to match that location, but that was a bother and later made optional. A kind of saddle clamp anchors an anvil over the piston location in the barrel. A calibrated copper slug is placed between the anvil and the piston. When the gun is fired, pressure in the chamber drives the piston to crush the copper slug against the anvil. The slug is then removed and a micrometer is used to measure how much it was crushed. The amount of crush is then looked up on a tarage table—a calibration table provided by the slug maker—and the pressure is read off that table for the amount of crush that was measured.
This method used to be used to determine psi. However, when piezoelectric and strain gauge testing was used for comparison in the 60's, it was discovered the copper crusher system was non-linear and as the pressure got higher its readings were too low. However, it was still possible to use the instrument to compare a reference load to a new load being developed using the old apparatus, so rather than abandon it, SAAMI gave the old instrument its own unit called the CUP for Copper Unit of Pressure so it would not be confused with actual psi (pounds per square inch). Nonetheless, you have to be careful reading pressure data to be sure you are not seeing older data for which the copper crusher result was still reported in psi instead of in CUP. All the older military manuals, for example, still report copper crusher results as psi when they are actually CUP. Any military technical manual not published after about 2000 may have that confusing data. Older publications like Hatcher's Notebook (the last edition was 1961) still refer to copper crusher measurements as psi.
One other problem with the copper crusher, besides its non-linearity, is that it is less consistent than the other methods by about a factor of two, so even for comparison, it is a less desirable method of measuring. The military produced an improved M11 copper crusher tester in the 1970's or 80's that was apparently more consistent, but not adopted commercially, AFAIK.
The cause of the non-linearity is that the copper slug calibration is static in which the psi for the tarage table are measured crush produced by weights bearing on a piston like the ones in the crusher barrels. That is a static test that deforms the metal a bit more than happens in an actual test at the higher pressures. Those higher pressures have very brief peaks that occur so quickly the inertia of the piston and slug absorb some of the force before deformation progresses fully. The system could be made much better if dynamic calibration were done, but the lower consistency as compared to other methods would still be present. It is due, in part, to the fact no two ballistic technicians seem to have identical touch with the micrometers used to make the measurement. Maybe that could be addressed with a laser micrometer, but nobody would bother now. Everyone likes the complete pressure traces produced by the transducer and strain gauge instruments, and there is no way for the crusher to do that or to measure more than the peak value.
CUP or Copper Units of Pressure was an earlier method of measuring chamber pressure before newer methods came about. There is also LUP or Lead Units of Pressure which was popular with shot shells. There is no direct conversion from CUP or LUP to PSI.
<EDIT> Wow I fed the dogs, came back and typed at my snail's pace and question asked and answered,
</EDIT>
Ron
<EDIT> Wow I fed the dogs, came back and typed at my snail's pace and question asked and answered,
</EDIT>Ron
big al hunter
New member
Correct psi and cup are totally different. This is part of the worry people have about shooting 5.56 in a 223 chamber. 223 standard pressure is CUP. 5.56 is measured in PSI.
Dano4734,
At 30,000 psi they are very close, but not identical. The non-linearity in the copper crusher readings is just starting to show at around that pressure. For example, the 45-70 is rated at both 28,000 CUP and 28,000 psi because, in that chamber, at that pressure range, the two measure the same. But if you look at the .308 Winchester, the maximum pressure is 52,000 CUP but 62,000 psi, so by the time you get to 62,000 psi, the copper crusher is already measuring 10,000 psi too low. But even that is too simple. Different cartridges don't have identically different maximum ranges, even up at that pressure. For example, 52,000 CUP is also the .223 Remington maximum, but it's maximum for the transducer is 55,000 psi, a much smaller difference. So it's complicated and affected by cartridge geometry. There isn't agreement using the CIP method in Europe, for which the copper crusher gave the .223 Remington a maximum of 53,700 psi and their channel transducer maximum gives 62,400 psi (both rounded to neares 100 psi), a bigger difference than our equipment produces.
I spoke at some length with Dr. Ken Oehler at the NRA Annual Meeting a couple of years ago, and he suggested the absolute accuracy of pressure measuring in cartridges was a ±5% precision proposition. I pointed out the Europeans claim ±3% for their transducer system and he sort of shrugged and said "yeah, maybe that's possible".
I expect you now are beginning to get some idea where the discrepancies in published load data come from, at least in part. There are other factors, too.
Big Al Hunter,
That difference is only for older tech data. The U.S. military started going to NATO style channel transducer measuring in the late 1990's and then in 2012, ATK changed them over to conformal transducers and adopting U.S. commercial pressure standards for .223 so now its all psi by the conformal transducer. ATK's thinking was that in time of war this made commercial ammo manufacturing facilities able to produce military specification ammunition more quickly and easily for backup resources.
At 30,000 psi they are very close, but not identical. The non-linearity in the copper crusher readings is just starting to show at around that pressure. For example, the 45-70 is rated at both 28,000 CUP and 28,000 psi because, in that chamber, at that pressure range, the two measure the same. But if you look at the .308 Winchester, the maximum pressure is 52,000 CUP but 62,000 psi, so by the time you get to 62,000 psi, the copper crusher is already measuring 10,000 psi too low. But even that is too simple. Different cartridges don't have identically different maximum ranges, even up at that pressure. For example, 52,000 CUP is also the .223 Remington maximum, but it's maximum for the transducer is 55,000 psi, a much smaller difference. So it's complicated and affected by cartridge geometry. There isn't agreement using the CIP method in Europe, for which the copper crusher gave the .223 Remington a maximum of 53,700 psi and their channel transducer maximum gives 62,400 psi (both rounded to neares 100 psi), a bigger difference than our equipment produces.
I spoke at some length with Dr. Ken Oehler at the NRA Annual Meeting a couple of years ago, and he suggested the absolute accuracy of pressure measuring in cartridges was a ±5% precision proposition. I pointed out the Europeans claim ±3% for their transducer system and he sort of shrugged and said "yeah, maybe that's possible".
I expect you now are beginning to get some idea where the discrepancies in published load data come from, at least in part. There are other factors, too.
Big Al Hunter,
That difference is only for older tech data. The U.S. military started going to NATO style channel transducer measuring in the late 1990's and then in 2012, ATK changed them over to conformal transducers and adopting U.S. commercial pressure standards for .223 so now its all psi by the conformal transducer. ATK's thinking was that in time of war this made commercial ammo manufacturing facilities able to produce military specification ammunition more quickly and easily for backup resources.
For anyone who wishes to really get into it, Absolute chamber pressure in center-fire rifles is a pretty cool white paper on the subject. Like many university white papers it can be a dry read but something to note is when the paper was done and the supporting grant monies. Around page 30 they mention several of the causes of error in the CUP method. Also pretty interesting is the equipment used and the limitations of precision measuring equipment around Circa 1965.
Ron
Ron
Carriertxv
New member
So is this why newer loading manuals have lower loads in them compared to the older ones?
Ron,
That 1965 paper by Brownell was instrumental in influencing the separation of CUP from psi. He's got a curve of the error in the .30-06 cartridges he was testing on page 43. Unfortunately, it is somewhat truncated in the scan of the copy used for that file. I've got a hard copy I acquired fifteen years ago that shows it a little more completely and gives a little room for projecting it. Overall, lots of information on the effect on pressure of bullet jump, gas bypass and all sort of goodies.
Carriertxv,
Most of the old manuals didn't have their data pressure-tested by anybody. They just worked up watching for pressure signs in the brass and primer and did it in a production gun whose chamber sizes and overall strength may or may not be a match to other guns out there. Really too risky to do in today's litigious environment. Today the bullet companies often still develop loads in production guns, but get their top load tested and bring them down when they go over SAAMI max, as a lot of the old data did. The powder companies all seem to use SAAMI standard pressure-test barrel results now and rarely use production guns except for odd calibers they don't have a test barrel for.
Another factor is that a lot of powder companies have changed hands and sources since the old data was developed and the powder isn't always quite the same as it was back then.
Still another factor is that because of using pressure test equipment that graphs the whole chamber pressure from start to bullet exit, they have been able to identify pressure anomalies in some of the old data that statistically could lead to very occasional over-pressure events that might be dangerous, and they have adjusted or eliminated some data because of that.
That 1965 paper by Brownell was instrumental in influencing the separation of CUP from psi. He's got a curve of the error in the .30-06 cartridges he was testing on page 43. Unfortunately, it is somewhat truncated in the scan of the copy used for that file. I've got a hard copy I acquired fifteen years ago that shows it a little more completely and gives a little room for projecting it. Overall, lots of information on the effect on pressure of bullet jump, gas bypass and all sort of goodies.
Carriertxv,
Most of the old manuals didn't have their data pressure-tested by anybody. They just worked up watching for pressure signs in the brass and primer and did it in a production gun whose chamber sizes and overall strength may or may not be a match to other guns out there. Really too risky to do in today's litigious environment. Today the bullet companies often still develop loads in production guns, but get their top load tested and bring them down when they go over SAAMI max, as a lot of the old data did. The powder companies all seem to use SAAMI standard pressure-test barrel results now and rarely use production guns except for odd calibers they don't have a test barrel for.
Another factor is that a lot of powder companies have changed hands and sources since the old data was developed and the powder isn't always quite the same as it was back then.
Still another factor is that because of using pressure test equipment that graphs the whole chamber pressure from start to bullet exit, they have been able to identify pressure anomalies in some of the old data that statistically could lead to very occasional over-pressure events that might be dangerous, and they have adjusted or eliminated some data because of that.
Unclenick:
Yeah, really a pretty cool paper. I liked the reference to some of the test measurement and diagnostic equipment which at the time was the latest and greatest. I am sure you are likely familiar with the Tek 545 Scopes. Not that I would date you of course. Note the reference to the Polaroid film. Think I used a camera on a 545B scope till we could no longer buy the film.
Absolutely that paper was instrumental in the separation of CUP verse PSI. I like how they used a "standard cartridge" in the M1903 action to get a baseline or calibration. There is some pretty cool stuff in that paper. I have the .pdf on my system but wanted to find a usable link to post.
Ron
Yeah, really a pretty cool paper. I liked the reference to some of the test measurement and diagnostic equipment which at the time was the latest and greatest. I am sure you are likely familiar with the Tek 545 Scopes. Not that I would date you of course.
Note the reference to the Polaroid film. Think I used a camera on a 545B scope till we could no longer buy the film. Absolutely that paper was instrumental in the separation of CUP verse PSI. I like how they used a "standard cartridge" in the M1903 action to get a baseline or calibration. There is some pretty cool stuff in that paper. I have the .pdf on my system but wanted to find a usable link to post.
Ron
"So is this why newer loading manuals have lower loads in them compared to the older ones?"
Am not even going to make the assumption a lot of older data, some of which may still be in print, even used cup. Am guessing the powder companies were much better at using measured pressure testing, instead of reading primers/brass etc. Would also guess there is not a definitive cut off point when loading manuals made the switches between types of pressure guesstimation or various forms of pressure testing.
Am not even going to make the assumption a lot of older data, some of which may still be in print, even used cup. Am guessing the powder companies were much better at using measured pressure testing, instead of reading primers/brass etc. Would also guess there is not a definitive cut off point when loading manuals made the switches between types of pressure guesstimation or various forms of pressure testing.
I really don't see either as more exacting. Looking at most loading manuals they point out how their data was obtained. Just as an example Hornady 9th Edition 308 Winchester they point out Rifle: Winchester Model 70, Barrel: 22" 1:12 Twist, Case: Hornady / Frontier, Primer: Federal 210, Bullet Diameter: 0.308", Max Case Length: 2.015", Case Trim Length: 2.005". My Speer #12 list much different test conditions and has different results. If we leave the bullet smiths and look at the powder guys the same sort of results will apply. All of this simply means that on a given day under given test conditions, methods and procedures here is the data we got. Two identical rifles will likely give different results to a point. So I really do not see either the powder guys or bullet guys being more exact than the next. I do like to know how their data was obtained but that is more just a me thing.
Ron
big al hunter
New member
Unclenick,
I have had conversations with folks that misunderstood the CUP vs PSI that told me the pressure max for 223 was 52,000 and the max for 5.56 was 55,000. I had to explain that two different pressure measurement systems were involved. The PSI max for both 223 and 5.56 is 55,000 PSI. I had to explain that the only difference is in the chamber of the rifle, and it's affect on pressure before they understood the issue with the ammunition.
One of the gun rags did an article a while back that did pressure testing in rifles with both chamberings and with both types of ammunition in each. The results were less than conclusive. But the interesting part for me was that some of the 5.56 ammo shot in a 223 chamber was lower pressure than the same ammo shot in a 5.56 chamber. The lesson I learned was that the individual rifle has more to do with pressure than the stamp on the cartridge head.
I have had conversations with folks that misunderstood the CUP vs PSI that told me the pressure max for 223 was 52,000 and the max for 5.56 was 55,000. I had to explain that two different pressure measurement systems were involved. The PSI max for both 223 and 5.56 is 55,000 PSI. I had to explain that the only difference is in the chamber of the rifle, and it's affect on pressure before they understood the issue with the ammunition.
One of the gun rags did an article a while back that did pressure testing in rifles with both chamberings and with both types of ammunition in each. The results were less than conclusive. But the interesting part for me was that some of the 5.56 ammo shot in a 223 chamber was lower pressure than the same ammo shot in a 5.56 chamber. The lesson I learned was that the individual rifle has more to do with pressure than the stamp on the cartridge head.
FrankenMauser
New member
I know it's only a minor addition to this thread, if I can even find them, but I have some .444 Marlin cases somewhere around here that were subjected to copper crusher testing. If I can find the cases and get a good photo of the ring left by the hole in the chamber, I'll post it.
Reason for sharing (if I can find them):
Seeing the small portion of the case used to measure CUP can be enlightening, when compared to the concept of using strain gauges and/or piezo transducers to measure deflection of the actual steel around the chamber.
Reason for sharing (if I can find them):
Seeing the small portion of the case used to measure CUP can be enlightening, when compared to the concept of using strain gauges and/or piezo transducers to measure deflection of the actual steel around the chamber.