Copper Unit of Pressure (CUP) Defined by Hodgdon
When you look at a loading manual with load data, you will usually see pressure ratings for stated load. Sometimes these are listed in PSI numbers, which most people correctly understand to be Pounds per Square Inch of pressure. However, powder-makers also commonly list pressure in CUP numbers. CUP stands for Copper Unit of Pressure. You may be asking — “What exactly is a CUP, and what is the origin of that unit of measurement?” You may also be wondering — “What’s the difference between CUP pressures and PSI pressures?” On Hodgdon’s Facebook Page, you’ll find answers to these questions.
Q: What is CUP?
A: Copper Unit of Pressure (CUP) is a measurement used in the ammunition industry to determine the chamber pressure created by a cartridge load. Originally, a precisely formed copper slug was placed in a fixture over the chamber. When the cartridge was fired, the amount of crushing measured on the slug allowed engineers to determine the pressure.
These days, modern electronic transducers provide faster, more accurate measurements of chamber pressures in pounds per square inch (PSI). CUP and PSI are measured to different scales and are NOT interchangeable.
Hodgdon Reloading Data Center Sample
Similar Posts:
- CUP vs. PSI — What’s The Difference in Pressure Measurements
- Know Your Terminology — CUP vs. PSI
- CUP vs. PSI — Do You Understand the Difference?
- Why Does Load Data Vary Between Reloading Manuals?
- Variances in Load Data — Why Load Manuals Don’t Always Agree
Tags: Copper Units of Pressure, Cup, Hodgdon, Reloading
CUP needs to go the way if the dodo bird. You can’t convert/correlate from psi or MPa to CUP anyway.
So just give the pressure in PSI and quit making an issue of the data!
Two main reasons.
1) Much of the SAAMI cartridge spec data is still in CUP.
2) Replacement of copper crusher gauge (CUP) test equipment for piezoelectric based systems may be prohibitively expensive.
CUP-based setups tent to be mechanical only, so do not require any form of computer or electrical supply. Even the measurement of the crushed gauges can be done with a mechanical micrometer.
Anyway, at least its rare to see LUP (lead units of pressure) now. So we don’t have yet another measurement system to worry about!
Being in the UK, I stick with CIP data, which is in Bars, easily converted to MPa or PSI, and testing is undertaken with piezo-based system.
CUP is of no relevance or importance to end user customers, which makes it useless. Adherence to CUP data simply “dates” the manufacturer and does nothing for credibility. As for the cost of piezoelectric measurement, it’s now affordable to most shooters in the form of the Pressure Trace II system. Manufacturers can do better. Hornady et al, should consider joining the 21st century.
Steve,
The CUP measurement method is a pretty basic system in modern times, giving only the peak force on the crusher, but then again its simple!
Although I haven’t been working in the ballistic instrumentation and photography field for a few years, copper crusher gauges were still used for large caliber weapons systems when I was there. We are talking artillery and tank guns here. They were mainly used as multiple gauges could be placed within the charge and inside the chamber of the weapon system. Though simple, calibration of the copper slugs or balls crushed in the gauges required a piezo measurement system. The piezo gauges calibrated beforehand using high-pressure (upto 10,000 bar, 145,000 PSI) Budenburg oil and dead weight calibration equipment.
As to personal use of pressure testing equipment, piezo-based systems are simply not cheap, nor is modifying the firearm beforehand. Also the Pressure Trace II system you mention is not a piezo-based system, it uses strain gauge technology. Though useful as a guide, the problem with this measurement method is calibration. For accurate results you need the chamber material properties and dimensions along with the strain gauge properties, in addition the chamber and outer wall temperature. If you don’t do this, the best you’ll get is a pressure guesstimate. Even if you use factory ammunition to calibrate the system before hand, do you know the actual pressure produced by the factory round, unlikely. All you’ll know is that you’re ammunition does not exceed the factory ammunition peak pressure, what that pressure actually is, well we are gussing again.
Neil
Is there significant relevance to the hand loader using established tables published by reputable sources? Speer #14 etc.?
Why is CUP,LUP,PSI important when the average loader has little if any access to the resources to measure these? Explicitly follow well publicized guidelines based on fire arm and desired effect. In the meantime, put your dick in your pants.
Be careful what you all say!
To Steve Hurt: Nearly all of the ammunition test barrels we make for Hornady are the Piezo transducer. So they are in the 21st century.
Second it is not necessarily up to the ammo maker/firearms manufacturer which system to use. In some cases the standard for a given caliber might be CUP testing. With that being said we make very few CUP style pressure test barrels anymore but every once in a while we do and usually it’s for a odd ball caliber.
Until the standard is changed for a given caliber it’s the way it is. With that being said also some makers do buy a Piezo style test barrel along with CUP style barrels for testing of the same caliber.
Later, Frank
Thank you Neil/Frank, your points are well made.
I should apologise for any slight implied.
To me though, talking in CUP and comparing it to PSI is like having a Milradian optic whilst thinking in MOA, whilst trying to shoot at long distance. The maths does the head in. (Not a perfect analogy!)
The problem in making any comparison in ‘quality’ load manual data is still not quite so simple, and the unit of measure differences simply complicates an already challenging issue.
We all know that load data is based on certain standards (SAAMI & CIP) as well as manufacturers tolerances. Further, as most would know, the influence of temperature can vary velocity (and therefore obviously pressure as a result) quite widely. I want to know by how much and whether these fluctuations are within safe limits by comparing it to something I can make sense of. It is impossible for any printed load guide to address all the variables, to cover all possible scenarios. Being within the published velocity bounds can be a bit of a guide, but then …
Load manual data, regardless of unit of measure, is published based on specific standards, as they should be (for temperature, air pressure et al) but these are only sometimes published and I for one shoot in an environment that is way outside some of the standards I know about.
A strain gauge ‘guestimate’ is as you say fraught with some potential error and perhaps not even a ‘standard’ at all, but under the circumstance, better than nothing. Comparing it to CUP data is impossible and pointless. If only PSI is used, that’s at least one issue out of the way. The strain gauge yardstick may be cut from a tree branch, but it should at least be roughly in the ball park. Published load manuals published in CUP using standards the vary from my actuals by more than 60 degrees F, well quite simply, I have no yardstick by which to value such data at all.
Steve, Wasn’t taken in a bad way so I apologize if it came across harsh.
I agree for the most of us CUP and PSI is like comparing a MIL/MOA scope adjustments. Sometimes it just doesn’t work.
Later, Frank
Bartlein Barrels
Only thing CUP or PSI means to me is if the case is limited on volume for said powder or peak pressure is created before said case is filled.
What???? Pull the trigger and gun goes bang.