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I have a very simple request. Could somebody tell me what 45,000 CUP is equal to in PSI? If there is a simple formula for converting them back and forth I promise not to bother anybody else if somebody is willing to share it with me.:eek:

Thanks,
 

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My understanding is that there is no easy formula (or any formula as far as that goes). What I have read indicates that actual PSI can vary quite widely from CUP readings. It also seems that pressure comparisons of the two methods can vary between calibers or possibly even cartridges. The relationship in the two measurements may be closer say , in a 45-70 as compared to a 300 mag. FWIW.
 

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The Hog Whisperer (Administrator)
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Can't be done, don't try. The two systems do not measure exactly the same thing. Only PSI (piezo-electric) can measure actual peak presssure based on strain of the barrel. Neither CUP nor LUP is totally accurate with regards to peak pressures, but were useful systems until the industry had better tools available.
 

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Ditto, don't do it. If comparing pressure readings in loading manuals, just be sure to compare apples to apples.

SSB
 

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Agreed. And back when everyone was using CUP, the Americans, Brits and Germans used different setups, so you're still looking at apples and oranges. For example, the Lee-Enfield No.4 Mk.I was proofed to 18 1/2 tons per square inch. Even if that's long tons, and I think it was, that's 41440 psi, but the SAAMI spec is 45,000 CUP.

Bye
Jack
 

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Also, be aware that some of the earlier ballistic writings stated what was actually CUP readings in PSI which is erroneous.

Confusion set in as some true Piezo pressure readings were being done and expressed in "Real" PSI units, so then "CUP" was used to express the copper crusher method and not PSI.

There are pressure ranges where the PSI readings can be close the the CUP readings but it is really nothing to stake any accuracy on.

Also, the Piezo method for measuring chamber pressures on an oscilliscope, also measures peak pressure DURATION.

In the Speer manual, you will notice as a for instance, the 357 Mag data being reduced because of this fact as they are now using Piezo equipment for load development.

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BCstocker said:
My understanding is that there is no easy formula (or any formula as far as that goes). What I have read indicates that actual PSI can vary quite widely from CUP readings. It also seems that pressure comparisons of the two methods can vary between calibers or possibly even cartridges. The relationship in the two measurements may be closer say , in a 45-70 as compared to a 300 mag. FWIW.
With the old CUP measuring system the pressure could actually vary according to brands of brass as what was being measured was the pressure remaining after the piston had pierced the wall of the case.
 

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Dutch4122 said:
I have a very simple request. Could somebody tell me what 45,000 CUP is equal to in PSI? If there is a simple formula for converting them back and forth I promise not to bother anybody else if somebody is willing to share it with me.:eek:

Thanks,
I picked this up on another forum.

psi = -17,902 + (1.51 c.u.p.)

conversely

c.u.p. = (psi + 17,902)/1.51

I load for 30-30, 7mm Rem Mag, 300 Win Mag, 45-70, .357 Magnum, and 40 Smith & Wesson. I have run through these calculations upside down, forwards, backwards, and sideways, with the loads I use and the conversions all make sense. It is almost impossible to find any reference to the metrics but it occurs to me that if they measure the same basic physical phenomena they have to be interchangeable and correlated in some way. Since I am not a certified agency to make claim to the validity of the equations I would strongly encourage you to investigate it to your own safety and satisfaction.

It works for me.
 

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What is the point of converting psi to cup & vice verse? Is it just for general background knowledge? If you're a balistician at a balistics lab, you should already know to your own satisfaction.

If, on the other hand, you're using it as an excuse to exceed published loading data, DON'T! When, for instance, Hodgdon publishes
H4831 52.0 grains for 36,000 cup starting load and
H4831 60.0 grains for 48,500 cup maximum load
they mean just exactly that. Don't exceed 60.0 grains with a 130-grain bullet, under any circumstances. The people who publish loading data debate it a lot. "Do we tell them the pressure we measured and take a chance that somebody who thinks he knows more than we do will exceed the load we listed because it's under the SAAMI pressure limit?"

The pressure data is useful to us hobby handloaders from a few points of view.
#1: Is this likely to be an accurate loading when loaded to the maximum? Why? Because the balisticians who generate the data stop at some point where pressure from a significant portion of the cartridges will exceed the SAMMI maximum pressure. It's statistical. Low standard deviations and extreme spreads will allow them to approach the maximum more closely.
#2: Is this the maximum load because they couldn't physically put any more powder into the case?
There may be more uses I don't know about.

The old cup measurements were good enough until the better, more repeatable, precise, and accurate electronic methods (psi) came along.
Suppose you measured race times by averaging the results of three guys with stopwatches. You then converted to an automatic timing system. You'll probably find both random and systematic differences between the two methods. You'd probably find pretty good agreement for long distance races (corresponding to long barrels and slow powder) and more disagreement in sprints (corresponding to short barrels and quick powder). Does this mean there must exist some formula that will convert the stopwatch times into automatic timing times? Given the non-zero reflex time of the guys running the stopwatches, would it surprise you if people started taking slightly more time running sprints? Given the inertia and friction of the piston and spring of the copper crusher, would it suprise you that many pistols and revolvers would show higher psi than cup on the same load? Likewise, given the momentum of the piston once it starts moving, would it surprise you that other loads would show more cup than psi?

Psi and cup are two very different ways to measure the same thing. Attempts to reconcile them are futile. Compare oranges and pumpkins instead.

-91
 

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91Carcano:
Well put...makes no difference how you measure, the limit is still governed by the chamber/barrel of the gun at hand....just becasue the book said 60.0gr. was a safe load, doesn't mean that YOUR rifle will stand 60gr. Manuals can measure it in parts per million of Arrdvark flatulance per cubic meter of German potato salad (and we'd be looking for a conversion to egg salad); it's still not a measure of YOUR chamber pressure.
 

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91Carcano said:
What is the point of converting psi to cup & vice verse? Is it just for general background knowledge? If you're a balistician at a balistics lab, you should already know to your own satisfaction.

If, on the other hand, you're using it as an excuse to exceed published loading data, DON'T! When, for instance, Hodgdon publishes
H4831 52.0 grains for 36,000 cup starting load and
H4831 60.0 grains for 48,500 cup maximum load
they mean just exactly that. Don't exceed 60.0 grains with a 130-grain bullet, under any circumstances. The people who publish loading data debate it a lot. "Do we tell them the pressure we measured and take a chance that somebody who thinks he knows more than we do will exceed the load we listed because it's under the SAAMI pressure limit?"

The pressure data is useful to us hobby handloaders from a few points of view.
#1: Is this likely to be an accurate loading when loaded to the maximum? Why? Because the balisticians who generate the data stop at some point where pressure from a significant portion of the cartridges will exceed the SAMMI maximum pressure. It's statistical. Low standard deviations and extreme spreads will allow them to approach the maximum more closely.
#2: Is this the maximum load because they couldn't physically put any more powder into the case?

There may be more uses I don't know about.

The old cup measurements were good enough until the better, more repeatable, precise, and accurate electronic methods (psi) came along.
Suppose you measured race times by averaging the results of three guys with stopwatches. You then converted to an automatic timing system. You'll probably find both random and systematic differences between the two methods. You'd probably find pretty good agreement for long distance races (corresponding to long barrels and slow powder) and more disagreement in sprints (corresponding to short barrels and quick powder). Does this mean there must exist some formula that will convert the stopwatch times into automatic timing times? Given the non-zero reflex time of the guys running the stopwatches, would it surprise you if people started taking slightly more time running sprints? Given the inertia and friction of the piston and spring of the copper crusher, would it suprise you that many pistols and revolvers would show higher psi than cup on the same load? Likewise, given the momentum of the piston once it starts moving, would it surprise you that other loads would show more cup than psi?

Psi and cup are two very different ways to measure the same thing. Attempts to reconcile them are futile. Compare oranges and pumpkins instead.

-91
I don't pretend to understand all the reasons for the original question but I think you missed the point. I assumed the thread was started because you can get different information from different manufacturers (guns, powder, bullet, etc) where you don't have both sets of information, psi or cup, you get one or the other. I had the very same question when I found out gun manufacturers were providing psi data establishing the woking pressure limt for their gun. How does that equate to cup data that is published by component and ammunition manufacturers and vise versa? It's only prudent and responsible for the handloader to want both sets of data so he/she knows they are operating in the safe zones. It would be nice if the original poster would weigh in on this.

No offense, but your analagy of time keeping is badly flawed in that it is a subjective correlation, not objective, who pushes the button first or last is a matter of reflex, pressure is a phenomena, it has tangible parameters, time keeping in a race can be improved just like you suggest but it is not a good comparison to pressure. I don't believe it's a futile effort to draw correlation between the two metrics. I would like to see manufacturers in industry provide both sets of data to remove any ambiguities that arise when you only have one set of information
 

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The Hog Whisperer (Administrator)
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Flashhole,

There isn't any need to correlate the CUP/PSI data (or even the european CIP data), because you shouldn't exceed the published data in any case. Doesn't matter how it was developed - it is published to establish a safe range of working pressure for the particular gun/ammo combination. Frankly the units of measurement are of no consequence whatsoever.

There will always be some variation in published load data, due to different guns, and different lots of components being used.

It may be interesting to try to correlate the pressure systems, but it's an exercise in triva, and provides no useful purpose in terms of developing your own load data.

Exceed or experiment at your own risk....
 

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MikeG said:
Flashhole,

There isn't any need to correlate the CUP/PSI data (or even the European CIP data), because you shouldn't exceed the published data in any case. Doesn't matter how it was developed - it is published to establish a safe range of working pressure for the particular gun/ammo combination. Frankly the units of measurement are of no consequence whatsoever.

There will always be some variation in published load data, due to different guns, and different lots of components being used.

It may be interesting to try to correlate the pressure systems, but it's an exercise in trivia, and provides no useful purpose in terms of developing your own load data.

Exceed or experiment at your own risk....
How true. If you want to give yourself a headache, just look at manual with PSI and CUP ratings. Look at the low pressure rounds, they are almost the same, so from that one can mistakenly conclude that PSI and CUP are interchangeable. But look at high pressure rounds, they are nothing alike, and the deviation from equal to unequal is not linear so no correlation between the two can be made. CUP measure the shortening of a copper disk, PSI measures the stretch of the barrel wall.
As Mike said, you can forget about the CUP and PSI reading and reload safely. Those listings are FYI as none of us basement reloaders has the equipment to measure accurately CUP and PSI, not even the Oehler unit will do so with complete accuracy unless you calibrate it with SAAMI calibration rounds. Just NEVER, EVER exceed the listed maximum load and get it into your skull once and for all that the listed MAXIMUM is just a reference, NEVER to be exceeded, but to be APPROACHED with CAUTION. In 35 years of reloading I have reached MY maximum well before the published maximum was reached too many times to count. There are too many variables from steel strength, chamber size, primer and powder burn properties, to atmospheric considerations for a reloading manual to be considered a Bible.
 

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The Troll Whisperer (Moderator)
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Big Bore's last sentence says it all - use reloading manuals for their original intent. That is, a guide and not the cast-in-concrete final word on safe loadings.

A sane, reasonable person will review several manuals to get a general feeling for load development, begin at the recommended start levels and progress up in slow increments toward the maximum, using a chronograph and critically examining the fired case and firearm operations at all stages.

I too, have often puzzled over the apparent conflict between CUP and PSI and how to interpret the differences. Not gonna lose any sleep over it though, because my practice is to do the above and recognize the accuracy/max velocity/max pressure indicators when they appear.
 

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MikeG said:
Flashhole,

There isn't any need to correlate the CUP/PSI data (or even the european CIP data), because you shouldn't exceed the published data in any case. Doesn't matter how it was developed - it is published to establish a safe range of working pressure for the particular gun/ammo combination. Frankly the units of measurement are of no consequence whatsoever.

There will always be some variation in published load data, due to different guns, and different lots of components being used.

It may be interesting to try to correlate the pressure systems, but it's an exercise in triva, and provides no useful purpose in terms of developing your own load data.

Exceed or experiment at your own risk....
It may be interesting .... I disagree. Roll back the calendar to 1998 when Marlin brought out their model 1895 chambered in 45-70 Government. The action was much stronger than the older springfield models. AT the time there was no published load data for ammo for the new gun, just a pressure rating (psi) for the chamber. Since then there has been a lot of load data published but to your school of thought you should never have exceeded the load data for the original 45-70 Government even though safe (emphasis on safe) loads at significantly higher pressures were under development by hand loaders as well as component manufacturers. Correlating the chamber pressure data served a very useful purpose - it gave hand loaders the info they needed to develop safe, high pressue, loads for that caliber well in excess of any load data in print at the time using any data they could find.

This isn't my thread. I've said all I have to say on the topic. I was just trying to help answer a question I had myself.
 

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The Hog Whisperer (Administrator)
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flashhole said:
It may be interesting .... I disagree. Roll back the calendar to 1998 when Marlin brought out their model 1895 chambered in 45-70 Government. The action was much stronger than the older springfield models. AT the time there was no published load data for ammo for the new gun, just a pressure rating (psi) for the chamber. Since then there has been a lot of load data published but to your school of thought you should never have exceeded the load data for the original 45-70 Government even though safe (emphasis on safe) loads at significantly higher pressures were under development by hand loaders as well as component manufacturers. Correlating the chamber pressure data served a very useful purpose - it gave hand loaders the info they needed to develop safe, high pressue, loads for that caliber well in excess of any load data in print at the time using any data they could find.

This isn't my thread. I've said all I have to say on the topic. I was just trying to help answer a question I had myself.
Not to drag this out forever, but high-pressure data for the Marlin lever guns has been published for years. Ken Waters did this soon after the .444 Marlin came out, noting basically that the actions (of the .444 and .45-70) were the same, and therefore, felt that they could be loaded to similar levels.

Now.... am I calling Ken a fool? Certainly not... once he published the data, and lead the way, then we all had something safe to work with, and the load manuals followed suit.

I'm just not going to be the person blazing the trail when it comes to new load data at pressure levels not yet run through a lab :) Hope you understand, and take my response as concern for the eyes and fingers of all forum members, and not a lecture.

Anyway, it is an interesting discussion of the history and mechanics of CUP/LUP vs. PSI vs. CIP, etc., but the knowledge gained does not allow us to load higher with impunity.
 

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CUP vs PSI

Having calibrated lots of brass balls and cylinders for CUP measurements, let me explain. A copper cylinder or brass ball is put in a fixture, attached through a port to the barrel of a gun. When the gun is fired, a piston in the fixture crushes the cylinder or ball and the crushed length is then measured to give a relative pressure reading. The lot of crushers has been calibrated by measuring samples after applying known loads in a tensile-test machine, so that the length is related to the load. This is not a true pressure measurement because peak pressure happens very quickly, while the friction and inertia in the crusher fixture slow down the response. CUP is just as useful as pressure in a relative sense though, as either way, a reading that is too high is going to be related to failure of the brass and the gas seal. I hope that answers your question.

wilsondoc
 
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