What is differance between CUP and PSI?

Oh boy ... this is not a simple one.

CUP or "copper units of pressure" is a measure of pressure that was done by crushing a copper pellet in a special receiver (usually a Universal Bond receiver). This receiver has a chamber that is vented so that a rod crushes the copper pellet when a round is fired. The amount of crush is an indication of the pressure in the chamber.

I am not sure how they were originally calibrated for pressure, but CUP became the standard way of expressing the pressure of loads for a long period of time.

In the 1970's and '80s some research found that if a properly cut quartz crystal were compressed one could detect a signal that is linearly proportional to the absolute pressure on the crystal. This technology became known as piezo electric pressure measurement. It became commercially available and was traceable to absolute pressure in pounds per square inch.

The port is generally just into the barrel rather that requiring that the cartridge case be broached. Many pressure guns were modified to use these commercial sensors. Clerke introduced a simpler pressure gun using this sensor as well.

The sensor technology combined with a computer data acquisition system taking data at say 50 to 100 KHZ allows one to record the absolute pressure in the bore from the time the round is fired until the bullet exits the bore. This provides a pressure curve as well as bore residence time. Is really neat to use.

The relationship between CUP and PSI is non-trivial, and as far as I know has no general solution that applies to all situations.

CUP is generally lower and probably represents a flattened (excuse the pun please) response. PSI measurements are of course accurate if done properly, but require that we review our beliefs about pressure in firearms.

(Now this post should bring out the knowledgeable AND the trolls!)

A shortened version of the above: CUP is the amount of deformation measured on a copper pellet when exposed to the breech pressure of a cartridge fireing in a test reciever. PSI is the unit of measure of the amount of strain applied to either a crystal or gage.

There have been many heated discussions of pressure measurement on this forum. Here is my short answer.

CUP measures pressure by using a copper crusher. The technician measures how much the crusher has been crushed and supposedly that can be correlated to pressure. It's an old system that is gradually being phased out.

The piezo system uses a quartz crystal transducer to measure chamber pressure more or less directly in PSI. Strain gage systems also reports pressure in PSI but that system is an indirect measurement that depends on a predictable correlation between strain and PSI. In either case, it is very desirable to calibrate the instrument or at least do a sanity check with a known load.

The piezo system is generally more accurate and more repeatable and piezo data should be used whenever it is available.

Some companies like Hodgdon offer a mix of PSI and CUP data. Apparently, they buy piezo barrels for new cartridges but continue to use CUP barrels for the old cartridges. Pressure barrels are very expensive so industry has been slow to make the switch.

SAAMI gives most cartridges a spec in PSI and a different spec in CUP. Presumably, SAAMI has some way to correlate PSI and CUP but they haven't made this correlation available to the public.

It is possible to take this SAAMI PSI and CUP data and find a simple mathematical correlation. This was done by one Denton Bramwell and his formula is PSI = -17902 + 1.516 * CUP. It is valid only for bottleneck rifle cartridges and may give weird results if you try to apply it to pistol cartridges.

The Europeans use a different standard for pressure measurement that may be reported in PSI, however, the European PSI data is not interchangeable with the SAAMI PSI data.

Yes, it is confusing.

Popenmann,

It confused in the hodgdon magazine by listing it both ways.

quote:

Originally posted by popenmann:
...It is possible to take this SAAMI PSI and CUP data and find a simple mathematical correlation. This was done by one Denton Bramwell and his formula is PSI = -17902 + 1.516 * CUP. It is valid only for bottleneck rifle cartridges and may give weird results if you try to apply it to pistol cartridges. ...

Totally WRONG! This does not work to convert CUP to PSI. It is just another "denton doozie".

You can "guesstimate" the value as accurate as this joke will. And by guessing, you will not be mislead into thinking there is any useable accuracy involved in the end result.

These are the kinds of Reloading Fairy Tales that can mislead Beginners and Rookies into making significant mistakes.

As usual, you do not have to believe me. Just run a few examples using the "denton doozie" and you will quickly see that (like usual) the "denton doozie" gets blown out of the water.

Best of luck to all the Beginners and Rookies who have to wade through the denton ignorance.
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By the way, since this came up, here is a link to The World's Most Ignorant Reloading Suggestion which is another "denton doozie". By reading it, you will understand that denton doesn't have a clue concerning Reloading.

Hotcore is right!
The mathamatical formula gives a ROUGH approximation. Kind of like taking a french curve through the data points. Most of the dots on the paper will be somewhat close to the interpolated line. Some will not be that close, and a few quite far away.

BUT when it comes to small bombs going off under my nose I want better than a ROUGH APPROXIMATION. And as HotCore points out, using the formula can lead one pretty far astray without warning.

muck

While I don't necessarily think the "denton formula" should be used as a "golden rule" I have used it to assist in understanding when a company publishes data in CUP.....

I relate to PSI and not to CUP.....so I convert the data as reference only for my personal understanding.

There are a number of cartridges that are seriously underloaded to their potential when used in guns that are stronger than much older models they were qualified for.

Example: If I looked at loading data for the 6.5 X 57 I might wonder if it's reloadable for better performance and if the data is listed in CUP then I convert it and can consider if it's worth chambering for or if it's likely maxed out "as-is"

For my purposes the denton formula is a helping tool.....frankly, it's not at all relevant if it's right.....I've discovered that it helps me better understand a cartridge when looking at it's loading data.

Or...in other words.....what are we using it for?

quote:

Originally posted by vapodog:
...Or...in other words.....what are we using it for?

Hey VD, Exactly correct.
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quote:

Originally posted by Ed:
What is differance between CUP and PSI?

Hey Ed, If we knew "what" you wanted to do with the information, perhaps we could actually give you an answer that will help you.

The CUP or PSI values really have very limited value to ANYONE outside a Certified Lab that has "Test Equipment Calibrated to SAAMI Test Ammo".

You simply use the information in the various Powder and Bullet Manufacturers Manuals to get a starting point to begin Load Development. Then you work up until the actual Pressure Indicators tell you to STOP, back off a couple of grains(for all kinds of variabilities) to the best Barrel Harmonic, and you are ready to go.

I got a question on pressure. I've been reloading for about 40yrs and in all that time have never had any idea how much pressure any of my loads developed other than safe or to hot. What difference does it make that we know the pressure developed by any round if its something we can never measure at home?

CORNERSTONE: as others have said, CUP measures pressure using little cylinders of copper. These are calibrated by subjecting them to a fixed psi and measuring the shortened length of the crushed cylinder. The crushed length can then also be used to give an indicated psi reading. However, in a gun firing, the pressure spike is so brief the crusher can not accurately track it; the crusher doesn't deform fast enough to read the true peak psi. That's why SAAMI began using a term such as "CUP" to distinguish this indicated psi from the more accurate psi readings obtainable from piezo crystals and strain gauges. Both of these latter two techniques go back to the early part of the 1900s; crushers go back to the late 1800s. You can read more here, near the bottom of the page.

Don Fischer: the reason the pressure is important is that loads that don't break your gun today might do so tomorrow. Guns are proofed at pressures well above the normal operating load. Guns that pass a proper proof and are then operated at their design working pressure will have long, safe working lives. Guns that are hot rodded will not break right away, but their working life will be shortened. How much shorter is a function of how much over pressure they are operated. Modern bolt guns are usually strong enough to take quite a bit of abuse and still last a long time. Some of us, though, don't own modern bolt guns.

I have to agree with Hot Core. The difference is only trivial, and the only time it ever comes up in conversation is either over a beer, or on the forums. I (an average Joe Schmoe) really could care less what the actual CUP or PSI is, as it doesn´t concern me in the least. I just load up for what the manuals reccomend.

I think that as long as I know that ´the higher the number, the more pressure´, that suits me fine. I never really say ´that old mauser 98 is one I load only to XXXX cup, so I better back off of the N´-150´.

In fact, now that I think about it, I wouldn´t know what high cup numbers would be anyway.

It´s really a question for a game of trivial pursuit rather than a question used down in the basement at the rockchucker press.