April 14, 2006, 12:13 AM | #1 |
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CUP vs. PSI
For a given cartrigde loading, I'll see a max presure given in psi or cup. is there a diff. between the two ? If so what is the conversion ? thanks
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April 14, 2006, 12:23 AM | #2 |
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CUP vs PSI
Jibjab--According to my reading, yes there is a difference between the 2. You cannot compare a pressure reading in CUP (Copper Units of Pressure) directly with one in PSI (Pounds per Square Inch). Also, IIRC, there is no handy conversion factor to use, either.
They're arrived at differently: CUP as I expect you know, is measured by how much the gas from a given load of a given powder crushes a copper "crusher" fired from a laboratory test gun, under carefully controlled circumstances. PSI is measured electronically, with a "strain gauge" which can be attached--taped or glued I believe--to the chamber of almost any firearm, and again, for loading manual purposes, done under carefully controlled conditions. And they are not easily comparable. The earlier pressure test used a lead crusher instead of a copper one; I believe this is still done with shotguns because of their inherently lower pressures. In that case, the result is given in LUP, Lead Units of Pressure. Alas, things never get less complicated as time passes.
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April 14, 2006, 05:42 AM | #3 |
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converted
There is NO conversion; be careful (see, 'cause it only matters as one approaches maximums, ay?).
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April 14, 2006, 06:27 AM | #4 |
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A mathematically literate shooter derived an equation between CUP and psi with a correlation factor way up in the 90s, which is about as good as pressure readings from one lot of ammo to the next.
Still just good for curiosity, since as W says, if a manual lists SAAMI maximum pressure one way, converting it to the other isn't going to tell you much or give you an excuse to increase your loads. Unfortunately the .308 Win/7.62 NATO twins came out during the period when the changeover was starting and are dual, maybe triple, specified leading people to think commercial ammo is loaded substantially hotter than military. Factory psi readings are with a quartz crystal piezeoelectric transducer screwed into a test barrel, stick-on strain gauges are for hobbyists and small operators. |
April 14, 2006, 09:04 AM | #5 | |
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One slight correction to Smokey Joe's otherwise correct statements:
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April 14, 2006, 09:40 AM | #6 |
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One small addition. None of this is as exact as many like to pretend and it would not matter if it were. Different operators get different results, and none of the results are likely to coincide exactly with a given real world gun and load. There are simply too many variables at work.
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April 14, 2006, 09:49 AM | #7 |
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Very large and informative thread on this very subject over at Handloads.com, including some expert knowledge from Denton Bramwell:
Handloads.com - Topic: Correlating PSI and CUP
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April 14, 2006, 02:07 PM | #8 | |
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Thankyou Mal H!
For the 90% endorsement!
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Mia apologia, and I hope no one suffered losses due to that. Also thx to Jim Watson for clarifying the use of strain gauges.
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April 14, 2006, 04:10 PM | #9 |
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This is a fun and messy business. There are three Piezo methods, the copper crusher method and the strain gauge method. Of the three, the strain gauge method actually produces the lowest (best) sigma-E, a statistical projection of how likely results are to be accurate, and is the only one most amateurs can hope to afford. There were earlier problems with this method, partly to do with speed of analog elements of the measuring system and partly due to analog to digital pre-conversion filtering, but it has been improved upon since. The Piezo method, which had been presumed in the past to have best accuracy, has turned out to have some issues of its own.
Copper crusher and both SAMMI and CIP piezo transducers work by pressure applied to a piston through a hole in the side of the chamber. The SAMMI copper crusher and piezo transducers typically have the piston located in the side of the chamber 1” ahead of the base. My understanding is the CIP piezo transducer samples the pressure nearer the case neck. Even though both the SAMMI and CIP piezo systems have their transducers exposed to gas pressure at the moment of peak chamber pressure, the two don't get the exact same results from identically loaded rounds in otherwise-identical chambers. This is what clouded the presumed absolute accuracy of the piezo systems used here and there. This caused doubt that a single pressure sample location could be relied upon. Subsequently, conformal transducers were developed. These are made of a piezo-constrictive film that is actually wrapped around the case and its leads are run out to the instrumentation. The transducer is destroyed by heat shortly after bullet is away, so this isn't cheap to operate. It does, however, pick up the pressure actually seen by the chamber around the case wall. The strain gauge approach indirectly measures pressure by measuring its effect on the gun's metal at the gauge location (how much the metal is strained by the stress of the pressure). Regardless of obsolete arguments about its accuracy, what it certainly tells you is how hard the gun metal was worked by the chamber pressure impulse. That, to me, is what matters most to keeping the gun in one piece. If you have some commercial or military loads you know to be safe, and you use them as a standard in a gun with a strain gauge on it, you can safely load any other powder/bullet combination to the same peak pressure readings your reference ammo produced because you already know the metal can withstand being deformed that much. There was already a link to Bramwell's article on CUP vs. PSI posted by Nortronics. Bramwell’s correlation is linear, and the intercept for his line doesn't land on zero. In other words, 0 PSI does not equal 0 CUP on his graph, as we know would have to be the case in real life. Excel wouldn't curve fit his data to non-linear forms as he had it organized. I re-ordered it to be ascending in CUP, and then ascending in PSI where more than one point had the same CUP value given. This allowed non-linear fit algorithms to work with that same data. Bramwell's R² test of his linear fit equals 0.927. I was able to improve this to R² = 0.939 within his data range using an exponential fit. However, this fit didn't trend toward zero either, so I went to a power function regression getting R²=0.936 (close enough, dude) and trending toward 0 CUP = 0 PSI. The graph of a linear regression, giving the same result as Bramwell is below, plus the two non-linear fits mentioned above. A second order polynomial fit improved on the linear fit slightly, but not as well as the two non-linear functions, so I omitted it. Nick
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April 14, 2006, 08:49 PM | #10 |
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Ya baby, you guys rock thanks for the great info.
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April 14, 2006, 11:12 PM | #11 |
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Well after coming home from work and giving the replies a quick go over I was pretty excited . Now after carefully reading the info you all have shared, I was a little surprised at the complexity of my question.
This will take a little time to digest, I hope this thread lives on a bit longer for some more insight. Sometimes seeing a point from a different angle makes all the diff. Nortonics post makes the most sence to me , for giving a workable conversion. Whoops sorry Unclenick, I'm hereing ya |
April 15, 2006, 05:29 AM | #12 |
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Saving the best for last
As cool as all this science is, it is worthless for determining what happens in MY gun when it's fired.
Fact: each and every firing event is unique and non-repeatable. Fact. However, I can determine possibilities when using MY gun for testing. Just can't get inside that chamber and case to poke around during the 'event'....... I once converted my 44 Redhawk into a service need; does that compute?
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April 15, 2006, 01:08 PM | #13 |
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Weshoots,
Git chew wun o' dem strayn gayjes. Nick
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April 16, 2006, 10:20 AM | #14 |
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I am NOT gluing stuff to my guns
I am NOT buying TC barrels in each chambering :barf:
I am NOT 'in the business' so much these days I have learned more about MY maximums; far below my guns Besides, I got heavy 45 Colt bullets that don't need more than 1K to penetrate longways anything I'll ever meet up with, ay?
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April 16, 2006, 10:22 AM | #15 |
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and
MY "strain gauge" is behind me
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April 16, 2006, 09:28 PM | #16 |
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Weshoot2,
There just has to be a song in there somewhere? FYI, the Pressure Trace instrument sold by RSI allows you to purchase plain gauges that will actually glue to a revolver cylinder and fit between the frame and the cylinder. They use a super glue, so you don't have to remove any finish, and if you are careful you can get them off with acetone and reuse them. Be a nuissance to use the revolver, what with having to open the cylinder and back the gauged chamber up for each shot, but it can be done. I just stick with the factory loads of H110 and 296 for heavy revolver loads, myself, but thought I would throw that possibility out there for you; just in case you wake up one morning and decide you haven't seen enough strain in life. Nick
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April 20, 2006, 08:58 PM | #17 |
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Exact conversions! No, but close and fuzzy maybe, try these formulas from Denton.
CUP = 14395 + .612 x PSI and PSI = -17902 + 1.51 x CUP Using some of my old math skills, I converted the first formula back to PSI and came up with PSI = (CUP - 14395)/.615 I pointed this out to Denton and he said it is not perfect for a number of reasons. Denton says the formulas produce fair approximations and that the CIP converts piezo pressure measurements by math to crusher pressure values. Best-o-Luck |
January 11, 2010, 07:21 PM | #18 |
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Bringin' this back from the dead!!
I was doing an internet search on this topic and came across this Saami publication:
It lists pressures for all the calibers in CUP as well as PSI, for those of us who are wondering if they convert or not. Below is the data for centerfire pistol and revolver http://www.saami.org/Publications/205.pdf
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January 11, 2010, 08:50 PM | #19 | |
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January 12, 2010, 12:06 AM | #20 | |
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I have some reading to do |
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January 12, 2010, 10:23 AM | #21 |
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Let me first say that Denton Bramwell has never measured pressure with a strain gauge with any useful absolute accuracy in his life.
He is a sincere, honest, and nice guy, but has shortcomings on this topic. OTOH, he has tried to make money off this subject and has not tried hard to understand his critics, like me. He seems to be able to learn processes, but has trouble reasoning or understanding purpose. He needs a chief engineer supervising his work, one who will impose common sense. +++++++++++++++++++++++++++++++++++++++++++++++ If there are two methods of measuring pressure, they had better have a linear relationship. If CUP and PSI measurements are measuring pressure, they had better be related with a simple y = mx or y=mx+b relationship. But on rifles, strain gauges and Copper crushers are not measuring pressure. They are measuring something more complicated, with pressure being one of the inputs. ++++++++++++++++++++++++++++++++++++++++ Compounding the problem, pressure does not matter. For handloading in strong rifles, what matters is short brass life. The brass life is a function of many variables, including brass hardness, brass hardness in the time domain, and another function of pressure and time. A peak pressure of 100,000 psi for .01ms will not affect the brass, as it cannot get through the delay line of the flash hole. "It takes time for gas to accelerate." As an ironic twist, the time it takes to crush Copper introduces what seems to be an error, but simulates the time it takes to get plastic deformation of a brass cartridge case. |
January 12, 2010, 11:07 AM | #22 |
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The problem I see with these conversions
is that they are NOT using "data points" taken from acutal pressure tests, but rather are comparing the SAAMI STANDARD PRESSURES that were ADOPTED buy SAAMI. So, the curves are ALL fit to a bunch of numbers that are NOT actually separate MEASUREMENTS of the same pressure tests.
I have not done this myself, but my understanding from people who have is that firing identical cartridges in two pressue barrels that are as identical as possible except that one uses crushers and the other uses piezo, the comparison between the two measurements depends a lot on specifics of the load and cartridge. So, say for .357 Magnum, the relationship of CUP to PSI would not be the same for a 110 grain bullet load with Unique and a 180 grain bullet load with H-110. So, SAAMI tried to make a PSI standard that "sort of" approximated CUP with PSI over a range of relationships for each cartridge. EXCEPT that they also took the opportunity to de-rate some cartridges and up-rate others when they changed the standard. These de-rates and up-rates are treated by Bramwell and Denton as if they were actually the same pressures, so that tends to throw-off their derived relationships. Anther sobering factor is that the actual trace of pressure as a function of time is NOT the same in a standard SAAMI test barrel as it is in a revolver. The long "free-bore" in a revolver cylinder, followed by an over-bore ring in the cylinder gap and forcing cone areas, followed by the beginning of the rifling, allows revolver bullets to accelerate at lower pressure, initally, then get slowed when they engage the rifling while part of the bullet is not supported on the gap/cone region. That slowing can make soft bullets "rivet" out into the unspupported area, which then makes them harder to swage back down where they engrave with the rifling, so the pressure jumps more than if they were supported their entire length. That is why bullets for very high-pressure revolvers like the .454 Casull and .500 S&W are made with hard lead cores and there are warnings to NOT use regular bullets of the same diameter in max loads for those guns. The "rivet" effect would greatly increase pressures with the softer bullets. But, SAAMI does not directly measure that effect when they test bullets in standard barrels that do not have cylinder gaps and forcing cones. SL1 Last edited by SL1; January 12, 2010 at 11:41 AM. |
January 12, 2010, 11:26 AM | #23 | |
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Quote:
A piezo strain gauge is mounted in a similar manner to a copper crusher. A hole in the side of the case allows the chamber pressure to bear directly on the small metal tip and transfer the pressure to the piezo element ibn the sensor. The system has a far lower mass than a crusher system, allowing it to respond faster and actually provide the pressure as a function of time. A crusher system provides a fraction of the peak pressure only (reduced by momentum effects and other systematic errors like elasticity of the copper). The values can be correlated, but not directly converted. Strain gauge systems must be calibrated with known pressure loads (from a piezo system) to remove their uncertainties, like steel variation, wall thickness, etc. Both piezo and strain systems can provide relative pressure as a function of time. Getting absolute numbers form them is a little harder. If you have access to QuickLoad you can get a good idea of the shapes typical of pressure vs. time. If you adjust some of the parameters in QuickLoad you can even get it to align with measured pressure vs. time data. |
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January 12, 2010, 04:02 PM | #24 |
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With regard to 7.62 nato vs. .308 win. I have read that 50,000cup is roughly equivalent to 58,000psi but I don't have a source to quote on that.Hope this helps...
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January 12, 2010, 06:28 PM | #25 |
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According to this: http://www.saami.org/Publications/206.pdf
page 16 and 21: .308 win max CUP= 52,000 and max PSI= 62,000 If you go to www.saami.org/publications/ you can find the pdf files which list pressures in CUP and PSI for basically all pistol, rifle, and shotgun loads.
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