CUP vs PSI

[frnkeore]

A resent thread has brought up this subject and I can't remember a discussion regarding it but, I thought it might be important for reloaders to gain a better understanding of CUP/PSI.

Reloading manuals don't generally talk about it but, just list their data as either PSI or CUP. As yet there hasn't been any understandable overal correlation between the two measurements. It is said that the one (and only that I know of) known study has been debunked. But, I have never heard of who or what source debunked it, does anyone have info on that?

If you read the Denton Bramwell's study, you'll find that it is done with all the availiable data at the time and that a pattern does exist, at least enough to give the average reload a since of what to expect, if using one measurement vs the other.

There are people that try to suggest that CUP isn't to be trusted but, it was used as the standard for at least 70 years and there were no safety issues that I know of. I have seen (in NRA) tests CUP results, in bolt action rifles of over 60,000 CUP, they were'nt recomended results but, they didn't say they would be unsafe, either, not even the mention of pierced primers.

The reloading public has gotten by with CUP as well as measuring case heads and the use of primers for safe pressure indicators all these years but, it seems that some people don't think that is relevant anymore.

There may be more concerns at the lower end of the scale since it's much harder to get accurate indications of pressure from case head, primer condition but, CUP is a long standing safe standard in the cartridges at this end of the scale, also.

For experienced reloaders, does anyone feel uncomfortable using CUP pressures over PSI? Are there any reloaders out there that don't know that there is a difference between the two?

I'm hoping that this thread will help clear up some of the confusion in this area and help new reloaders know the difference between them.

Hear are three sources of info on bramwell's work.

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

http://kwk.us/pressures.html

http://www.shootingsoftware.com/tech.htm

Frank

[Harter66]

I am aware of them and would like to throw another bone on the pile just to make sure there is plenty of food for thought that would be LUP or lead units of pressure which also do not match up to PSI but are both used in shotgun load data w/similar numeric values,much as CUP/PSI is.

I believe my current data from the 47th and 48th LYMAN books shows 50 kpsi and 52kCUP for maximum loads in the 06' . I don't recall ever seeing,that I paid any attention too,308 data showing 60k + of any scale. IIRC it has a 65000 PSI SAMMI limit . Also I recall somewhere (Handloader maybe) an artical that stated to the effect that the CUP/PSI scale curves crossed but were not "parallel " or interchangable . In example only not intended to reflect actual values as I do not have that document in hand and I've been very wrong before, 10k was correct on both but PSI gave a higher number value above that and a lower below but the scale crossed again and reversed around 25000 .

[frnkeore]

One crossing point is 40,000 PSI/CUP

[Rocky Raab]

Denton is a good friend of mine. I can't even begin to touch his knowledge of mathematics, but I intuitively disagree with his hypothesis that CUP and PSI can be correlated. My doubt stems from one simple fact: CUP wasn't that good a measurement system to begin with. The results it gives are less than trustworthy. That (I suspect) is why even Denton's system has so many exceptions, variances, and places where things just don't "jive."

CUP was in use a long time because it was the best they had. But then again, so was the telegraph.

[Wayne Smith]

Frank, I believe either are adequate safety measures. Which ever one is used in the manual doesn't bother me, I load a lot of older rounds not attracting much attention currently. We all need to remember that there is no, and I mean NO, reliable continuous correlation between the measures. Think of them as separate and different technologies, different processes, attempting to accomplish a similar goal. Then you should not be tempted to compare them looking for relationships.

[Multigunner]

Theres some information on the various chamber pressure testing methods in use when many common milsurp cartridges were developed in this book.

http://archive.org/details/gunitsdevelopmen00greerich

Occasionally in old books you'll find the chamber pressure in CUP of a cartridge otherwise only listed in Metric Atmospheres or in Long Tons as measured by other methods. This can give you a ball parks estimate of the differences.

The radial crusher gun was first used by the French (don't know if they invented it), then adopted by Germany and the U S.
German pressures were CUP but expressed in Metric Atmospheres which is analogous to the method used by the British these days. Both are multiples of the average atmospheric pressure at sea level.

U S Army Propellent procurement standards list most rifle cartridge pressures in both CUP and in PSI EPVAT, only M118 Long Range or Special Ball is listed only in CUP.

[frnkeore]

I do believe that there has to be a correlation, pressure is pressure. CUP depends on actually deforming a piece of copper and that takes pressure, now I can't say how pure the CUP samples of copper were and I'm sure that they did not all come from the same foundry in the same melt, just as we do not know the specifics of the manufactor of the strain gages, or if they all came from the same ACTUAL source.

All things being equal, I look at it this way........... the CUP will give a good reading for high average pressure, the PSI will give the highest pressure spike that occurs. If side by side tests were actually done with certified pure of copper and if the strain gage is installed and certified as well as the gaging instruments for both, a correlation could be done. Back in the day, I used H.P. White lab results as my best indicator of pressures.

Frank

Frank

[felix]

Yes, you can adjust the bias point each (electronically, of not mechanically) of a piezoelectric crystal and a copper cup to make their output values match, provided both devices are attached to the same barrel at the same time of fire. That would be the ONLY time I would trust any correlation between them of being truly representative of what is going on. The piezo measures vibrations generated via the pressure, whereas the copper expansion measures the pressure directly. This gives the true reason why the measurements are apples and oranges and MUST be calibrated to what is desired. As per another thread answered today by me, it is all in fun and games what these measurements represent. ... felix

[Rocky Raab]

Each lot of copper slugs came with a tarrage (correction factor) table unique to that lot. The technician meausred the compressed slug after firing, and applied the tarrage factor to that measurement to get a peak pressure estimate.

Possible errors could accumulate with whatever test was used to determine the tarrage factor, typos in the table, inclusion of the WRONG tarrage table (as happened with one lot of slugs in the Speer #8 book!), measurement error by the technician, math error by the technician, and typos in reporting the results. Then there are the intrinsic errors such as inertial lag in the copper slug response. And that's just a partial list.

One could compile a similar list of error points for piezo transducer or strain gauge testing. The bottom line is that neither system is foolproof - far from it. But the modern ones are much less prone to error and give a full pressure curve profile, not just a peak pressure estimate.

[felix]

All true. Correction factors is another name for bias points. As a side note, Rocky, a strain gage made out of piezo crystals would be almost perfectly linear in response, but more sluggish. Moving coils would be more sensitive, but hysteresis must be compensated for and most likely that would not be linear. Therefore, various bias points would have to be applied during the couse of a single fire. In other words, pick your poison. ... felix

[Larry Gibson]

Denton is a good friend of mine. I can't even begin to touch his knowledge of mathematics, but I intuitively disagree with his hypothesis that CUP and PSI can be correlated. My doubt stems from one simple fact: CUP wasn't that good a measurement system to begin with. The results it gives are less than trustworthy. That (I suspect) is why even Denton's system has so many exceptions, variances, and places where things just don't "jive."

CUP was in use a long time because it was the best they had. But then again, so was the telegraph.

I have to concur. Also I haven't found any particular case where "(t)here are people that try to suggest that CUP isn't to be trusted". CUP was/is still trusted and used. Piezo-transducer and strain gauges (currently listed as "PSI") have/are replacing CUP pressure measurement because they give a much more complete picture of what is occuring through out the pressure curve, CUP does not. What is not to be trused is any correlation between the two. Too many "exceptions, variances, and places where things just don't "jive" as Rocky mentions.

BTW; SAAMI lists CUP and PSI pressures for most cartridges

Since I started measuring psi with the M43 on 20+ cartridges I have tried correlating CUP to psi and it just doesn't work. I've a copy of Denton's article in the same binder as SAAMI's CUP and PSI MAPs along with how to correctly do CHE and PRE. CHE also does not work out as well as claimed by some. PRE is reliable at duplicating factory pressures (it doesn't "measure" them) but must be done correctly.

Larry Gibson

[Multigunner]

I have to concur. Also I haven't found any particular case where "(t)here are people that try to suggest that CUP isn't to be trusted". CUP was/is still trusted and used. Piezo-transducer and strain gauges (currently listed as "PSI") have/are replacing CUP pressure measurement because they give a much more complete picture of what is occuring through out the pressure curve, CUP does not. What is not to be trused is any correlation between the two. Too many "exceptions, variances, and places where things just don't "jive" as Rocky mentions. back in the pre WW1 days they tried determining the exact pressure at every point in bullet or shot charge travel using a test barrel with seperate crushers every few inches. I'm pretty sure this was just experimental and intended to better understand the power curve of the still young smokeless propellents.


BTW; SAAMI lists CUP and PSI pressures for most cartridges I've been told that SAAMI uses a slightly different tranducer set up, and PSI readings vary a bit from those obtained by the method used by the US Army and NATO. Don't know if thats true or not.


Since I started measuring psi with the M43 on 20+ cartridges I have tried correlating CUP to psi and it just doesn't work. I've a copy of Denton's article in the same binder as SAAMI's CUP and PSI MAPs along with how to correctly do CHE and PRE. CHE also does not work out as well as claimed by some. PRE is reliable at duplicating factory pressures (it doesn't "measure" them) but must be done correctly.

Larry Gibson

If a rifle/cartridge combination has its proofing standards and max working pressure standards expressed only in CUP then pressures expressed in CUP is what I look for in reloading data.

PS
On another board someone had gotten the idea that the 8mm J cartridge could be safely loaded to 58,000 CUP.
The proof marks for these rifles give a bullet weight and powder charge weight along with the proof test pressure.
While reading Greener's book "the Gun and its Development" I found information on the German proof cartridges. They used a special powder formulated to mimic the effects of powder degraded by poor storage under tropical conditions. The charge weight and volume were the same as the service powder charge but the proof cartridge gave the much higher pressure of 58,000 CUP (translated from Metric Atmospheres, which is equivalent to the present British "BAR" pressures).
They had earlier tried simply using a heavier charge of the service powder, but found this caused very erratic and dangerously high pressures that damaged or destroyed many rifles that would have normally passed proof.

[Gtek]

OK, let me make sure I have this straight. The Thingamabob is good, but only for Thingamabob readings, kinda, cause sometimes the Thingamabobs are not the same. The Hoogies, while being the latest only gives a Hoogie reading that is not the same as the Thingamabob reading measuring the same thing. Clear as mud- Got it. But I bet you veterans that I have been tracking for some time, still check just about every case from nose to butt before it goes back in the recycle box- Don't you? Gtek

[Rocky Raab]

You bet we do, Gtek. There are things that a half-century of reloading tell you that simply can't be measured - by thingamabobs or hoogies. Those things don't fall into an easily communicated "if this then that" methodology, though.

As just one teensy example: It is far simpler and even more "accurate" to run the pad of your finger across the case head to see if the primer is seated too high or too deep. Your finger doesn't "measure" it - but it knows!

[oldred]

No need for sarcasm, this has been a very informative topic and has answered a few things I have wondered about, even if a person does not fully understand the technical aspects of both systems one thing should be perfectly clear and that is using PSI and CUP interchangeably can be dangerous. Sometimes even info published by a supposedly knowledgeable source can be confusing, an example is the published pressure limits for some Pedersoli rifles. They give test pressure charts in both BAR and another column listed as CUP-PSI, using CUP and PSI for that particular numbers column as if PSI and CUP means the same thing! Above that they give as a basic guideline that,

"when loading full case charges of the correct grade of BP behind a bullet of normal weight for that caliber, you will not exceed a pressure of around 22,000 to 26000 psi (or CUP)",

They added the "(or CUP)" just like that after PSI, using PSI and CUP as if they are simply interchangeable then they go on to use this line,

"Modern made Pedersoli Trapdoor 45-70 rifles are safe to the higher 29007 PSI/CUP pressure",

Again using PSI and CUP as if they mean exactly the same thing!

[Gtek]

I have been for a long time also, sometimes the finger saw way before the eye. I have been a fan of Mr. G and Mr. F and others since I came on board and we know who the players are. Sometimes when all the stars are in alignment and they cannot hold back, we get about God knows how many years of bang out of them in a four inch run, you know of what I speak. I have reached that age where I have had to say goodbye to several of my mentors including my father. This place keeps that warm spot going, I would like to thank you all! Gtek

[Larry Gibson]

Multigunner

“back in the pre WW1 days they tried determining the exact pressure at every point in bullet or shot charge travel using a test barrel with seperate crushers every few inches. I'm pretty sure this was just experimental and intended to better understand the power curve of the still young smokeless propellents.”

Yes that was a very difficult set up and a tedious procedure. Using a CUP fixture is a tedious procedure at best, slow too. Piezo-transducers test fixtures are must faster and give a picure of the time/pressure curve. Strain gauges are even faster as the give the same time/pressure curve data but can be used to test load development or production ammunition in factory production firearms.

“I've been told that SAAMI uses a slightly different tranducer set up, and PSI readings vary a bit from those obtained by the method used by the US Army and NATO. Don't know if thats true or not.”

Original 5.56 and 7.62 NATO ammunition was developed using the then industry standard CUP method. However, most all of it is now done with a gas piezo-transducer at the case mouth instead of at the center of the case body as SAAMI does it. It’s why we see widely different 5.56 and 7.62 pressure MAPs because some are CUP, some are SAAMI spec and some are military spec measurements.

“If a rifle/cartridge combination has its proofing standards and max working pressure standards expressed only in CUP then pressures expressed in CUP is what I look for in reloading data.”

That’s the correct and smart way to do it; keep the apples with the apples, keep the oranges with the oranges and stay away from fruit cocktail.

Larry Gibson

[tonyjones]

Does anyone know whether slight variations in strain gauge attachment have any influence on the results obtained?

Thanks,

TJ

[Larry Gibson]

Does anyone know whether slight variations in strain gauge attachment have any influence on the results obtained?

Thanks,

TJ

The M43 PBL program has a "systems" and gauge check that is run before each test string. If anything is wrong with the gauge attachment or connections it tells you. Also the testing/checking is continual; if something goes wrong yo can not test. If the attachment breaks lose there is an obvious anomoly in the pressure curve of that shot. I can show you one if you want..........

It took a bit of a learning curve even following the excellent directions that came with the M43 on attaching the gauges. I strive to locate the gauge over the center of the case as per SAAMI specs. Not loacating the gauge over the case can definately skew the results. I also run a "check" with each cartridge/barrel/attachment using known factory ammunition as "reference" ammuntion.

Larry Gibson

[41mag]

Like most I have kept the two separate when working with a printed load. I usually try to run a box of factory ammo as a base load through my chronograph. While it isn't as precise as the Oehler, it at least gives me a base line for a given lot of ammo, charge weight, and bullet weight. I can then at least work up to this known level with my handloads and be reasonably sure I am still within the safe portion of the ball park. This however has always been when using jacketed loads.

Where I have found issues has been when looking at mixed data and trying to decide which loads will work the best for my newly cast boolit and which alloy. While I am sure this is where the topic wsa pointed, it still does have influence on the J-words as well.

I have dome extensive measurements with mic's and such on a couple of wildcat calibers I load for while working up loads. During this I have found that it works right along with watching the primers and the velocities as I inch up the load in small increments. This however can a bit flawed if you are testing several brands of brass at once One brand might hold up very well into the top end pressures while the next brand may not, even if the internal capacities are quite similar. Also throw in a difference in bullet material, primer make up, and general ambient temps slowly climbing while your testing and you can easily find some weird readings.

I do however enjoy reading the information provided and do my very best to avoid the loads which are known to produce pressure issues in my own firearms. It for sure isn't a simple pick a powder, bullet, case, and primer and have the ideal load for sure. This is also why it takes me a year or more to finally settle on a particular load for a given firearm.

OK now carry on with your educated discussion, I will bow out and read for a while.

[Rocky Raab]

The European (CIP) test method has the transducer at the case mouth. I presume that this is to eliminate the problem of the cartridge not fitting the chamber. With the sensor over the cartridge body, there's a lag as pressure builds enough to expand the case until it touches the chamber wall. And there's another gap as the case shrinks again during pressure decay. That latter happens quite a bit after the bullet leaves the bore, so it's less onerous, of course.

Our military probably went to a similar setup in order to assure consistent readings between us and our allies. Neither system is greatly superior to the other, by the way. Both show good trace data for the majority of the pressure curve, almost from primer blast to bullet exit. For very short cartridges, I've had to attache the strain gauge ahead of the case mouth simply because the entire case is inside the receiver. I still could get readings of almost the whole curve. I suspect that the barrel actually starts to expand before the bullet gets to that point, because the steel reacts faster than the bullet can initially accelerate. Even with the gauge mounted ahead of the case mouth, I could clearly see a spike from the primer, a very short decline in pressure as the powder begins to burn, and then the rapid climb to peak.

[tonyjones]

What I'm curious to know is does the strain gauge function correctly by merely making contact with the chamber/barrel or do variations in this contact influence results? My background is in structural engineering and industrial construction and know from considerable study and experience that enough variation exists in high strength bolted connections to understand that we are not counting on exact science when making these connections and rely to a considerable degree on precedent and design safety factors.

I realize that bolted and solder connections have little in common. I'm just wondering if variations in how firmly in contact the strain gauge is with the chamber/barrel influence readings.

Regards,

Tony

[Rocky Raab]

The strain gauge has to be firmly and intimately attached. It works (this is the short and unscientific description) by stretching along with the barrel steel. So unless it is tightly attached, it just won't work right.

I've applied more than one that tested "good" as far as conductivity was concerned, yet failed to report any pressure reading whatever at the shot. Because removing them destroys them, that's a $30 loss every time. It's doubly frustrating because you can't test them without shooting the gun, and that means hauling the gun, the system, and a laptop to the range - only to get nada.

[Larry Gibson]

Rocky

The transducer at the case mouth eliminated the variation of case strength, i.e. the case does not have to expand before pressure is applied to the transducer. Extraction of the firedcase is also easier and thus testing goes quicker. Dr Oehler conducted a test simutaniously testing each 30-06 cartridge fired with CUP, SAAMI transducer, case mouth transducer and strain gauges to a M43 and a M83. The results were quite interesting and demonstrated the validity of transducer and strain gauge testing.

Larry Gibson

[Larry Gibson]

Tony

As Rocky mentions the strain gauges are attached very firmly and permanently to the barrel. I use a very high grade of adhesive recommended by the makers of the strain gauges. Strain gauge use is common in many industries where pressures need to be measured. There are many different types of strain gauges for many different uses. The technology on applying them is known and proven.

If not applied correctly I suppose variations can occur. However as mentioned the M43 tests the functioning and monitors it. It won't test if there is not a good connection. It will show an anomaly if there gauge is not correctly and firmly attached to the barrel.

I always test the gauges on my barrels at home with the M43 before going to the range. They always work....well they do now because I learned to follow the directions on applying the gauge precisely. In my early testing i had a couple gauges come lose o connections broken and as Rocky says it is very frustrating.....so I learned to apply the gauges correctly and test before leaving home and haven't had that problem again. However, as i said, if something is wrong with the gauge connection or the wire connections the testing stops.

Also, just like the big boys "reference ammunition" should be used. It does not have to be the SAAMI reference ammunition as that is very, very expensive. Factory ammunition works quie well as it gives a quick base line. The factories psi testing equipment do not agree with each other BTW. That's why the use the reference ammunition to get an offset to adjust their own psi measurements. It is erroneously called "calibrating" but in reality they do not "calibrate" their systems, they just use the offset.

Larry Gibson

[tonyjones]

Larry & Rocky,

Thanks for the responses and explanation.

TJ

[frnkeore]

The transducer at the case mouth eliminated the variation of case strength, i.e. the case does not have to expand before pressure is applied to the transducer.
Are you saying that the barrel will react to pressure before the case will?

Also, just like the big boys "reference ammunition" should be used. It does not have to be the SAAMI reference ammunition as that is very, very expensive. Factory ammunition works quie well as it gives a quick base line.

Are you saying that you use "factory" ammo to calibrate or check your M43?

What does Oehler give as tolerance for the accuracy of the M43?

Frank

[Larry Gibson]

frnkeore

The transducer at the case mouth eliminated the variation of case strength, i.e. the case does not have to expand before pressure is applied to the transducer.
Are you saying that the barrel will react to pressure before the case will?

No, with a transducer located at the SAAMI specified middle of the case the psi has to over come the strength of the case and expand it before it can apply pressure to the transducer. With the transducer located at the case mouth there is no case to expand.

Also, just like the big boys "reference ammunition" should be used. It does not have to be the SAAMI reference ammunition as that is very, very expensive. Factory ammunition works quie well as it gives a quick base line.

Are you saying that you use "factory" ammo to calibrate or check your M43?

Frist, perhaps you failed to read that "reference" ammuntion is not used to "calibrate" (as in resetting the instrument so the it reads what the reference ammuntion psi is supposed to be.) neither the M43 or the psi measuring instruments used in ammuntion factories. Yes there have been several mistatements made that it is "calibration" but it is not. Transducer are calibrated as are the strain gauges but t is not done with "reference" ammunition. What is obtained with the "reference" ammuntion is an offset figure that is used to "correct" the instruments reading. Example; if the reference ammuntion is supposed to be 50,000 psi and the test instrument measures 48,500 psi with it then 1,500 psi (or more precisely the % that 1,500 is of 48,500) is the offset figure. This added or subtracted to the other test psi measurements to "correct" them.

I use factory ammuntion that I bout and supplied the factory techs with the lot #. They graciously supplied me with the "corrected" psi that they had measured that lot to be. They also said their own strain gauge device (M83 Oehler BTW using the same strain gauges I use) showed a 2,000 - 5,000 lessor psi when tested in factory production rifles given their generally larger chamber and bore tolerances. Thus it was the same with my M43.

What does Oehler give as tolerance for the accuracy of the M43?

I don't have the manual for the M43 here but you can probably get that from Oehler, Inc. I also can send you Dr. Oehler's comparison test report if you'd like?

Larry Gibson

Frank