doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Pressure measurement

The purpose of using crushers was to measure the pressure before piezo electric measurement devices were widely available for reasonable cost. So why is there a difference between LUP and CUP, etc? Do you mean to tell me that the engineers couldn't calculate the equivalent pressure for different materials? Or did they do it just to confuse?
I am an engineer and I do understand the principle.

Very briefly:

LUP (Lead Units of Pressure) and CUP (Copper Units of Pressure) are both crusher methodology units. Due to the different characteristics of lead and copper, LUP was used to describe relatively low pressure cartridges such as shotshells, and CUP was used for cartridges that generated higher pressures (e.g. hand gun and rifle cartridges).

The crusher methodology (regardless of whether LUP or CUP) provided a description of the entire area under the pressure curve.

Transducer (piezo electric) methodology provides a trace of the entire pressure curve, as measured at a specific point in the chamber. The single value commonly reported as the transducer peak pressure is the highest pressure in that curve, as detected at the point of measurement.

Crusher and transducer methodologies don�t really measure the same thing. That�s why crusher and transducer pressure measurements can be neither meaningfully compared nor converted from one to the other.

If you want more, see:

http://www.saami.org/specifications_and_information/index.cfm

P.S. I just have to ask. Of what are you an engineer?

As I understand it the lead & copper crushers were calibrated by use of a piston driven by a crank which crushed them while measuring the pressure necessary to crush them to various lengths. This was near to a static measurement of pressure. The duration of the peak pressure in the chamber is of such a short duration these crushers simply were not collapsed to the true extent of the pressure. Lead & Copper would not necessarily compress at the same rate so pressures would not read identical for the two components. The PE devices are fast enough to actually record the true pressure per square inch, thus if a difference is recorded between the methods the PE pressure will always be higher than the crusher measurements. I was just a lowly Machinist & can understand this.

I have read the SAAMI specs, and I still do not understand why they ever used or listed anything but PSI. With either crusher technology it should have been possible to calibrate the crusher to equivalent true PSI. I also suspect the higher instantaneous pressures recorded with electronics led to reducing the recommended charges in powder tables even though the previous weights had been proven empirically safe for years - in some cases decades.
I am a mechanical engineer. I was a licensed Professional Engineer for 36 years, but I recently let my license expire because I am going to retire and I am totally opposed to the what I consider stupid continuing education requirements. Physics has not changed. 99% of the licensees said it was not needed, but they don't take the State Board members out to lunch and who knows what else like the members of the sham education "industry" forking up ethics and other such trivia courses. That is the only significant change in the governing laws in decades.
I have run three machine shops. A good machinist is like gold in my book.

"A good machinist is like gold in my book." Soft, pliable and overpriced or a rare treasure beyond most mortal men? Thinking about my father, a machinist of almost seven decades, I pick the later description. A good machinist has more practical knowledge and more learned ways to get things done than most people ever understand. Books can only take you so far then you need to learn how to do it as my father told me many years ago.

When the crusher system was first placed into use the pressures were recorded as PSI as seen in old data. At this point in time they did not have the means of reliably calculating true PSI. When the PE system began to be used which recorded true PSI it was realized a distinction had to be made between the different systems so PE pressure took on the use of PSI & the others became either LUP or CUP depending upon the mat'l used for the crusher. There are several factors which affect the reading of the crusher measurements which ate not consistent from load to load so there is no absolute conversion from one to the other. The British did however by recording crusher pressures & PE pressures simultaneously work out a formula which would give close results for their crusher pressures compared to PE pressures. This formula was 1� times crusher pressure in tons, minus � ton = PE tons. Thus (3 x 1.5) - .5 = 4. As the British ton was the long ton of 2240 lbs then 3 tons (6720lbs) crusher pressure became 4 tons (8960lbs) PSI. This formula was good only within the limited range of LUP's normally encountered in measuring pressures of shotshells & no attempt should be made to use it for CUP as used for the higher pressures of rifles.

Miller's spot on. The terms LUP and CUP didn't exist until we starting using transducers to measure pressure--at which time it was determined that the old psi readings weren't accurate. In order to distinguish between the inaccurate crusher values and the new transducer values, the terms LUP and CUP replaced the old crusher psi values, depending on whether the measurements were made with lead or copper crushers. From then on, psi (at least in this country) meant only those values derived from transducer measurements. The problem comes when you read old literature, when crushers were still being used, and figures that should be interpreted as either LUP or CUP are expressed as psi.

L.Brown is correct, as long as we"old farts" are in the game, we remember the days that psi referred to cup or lup.We understand now that when the old data shows a load generates 50,000 psi(in cup), it would be more like 60,000 psi(pe).When an heir looks at the data to load ammo for one of the rifles we willed them, and see it "only" generates 50,000"psi",I hope they don't add more powder to the "old weak load" to bring it up to the "modern"60,ooo psi.It's complicated enough to explain it to us,imagine explaining it to someone that never lived without electronics and thinks the 10 commandments were written on a kindle.
Mike

Sorry Mike, it wasn't a kindle 'twas an iPad!

JC

FWIW, my old DuPont Smokeless Shotgun Powder booklets from the 1920s and 30s just says "pounds" as in the pressure of 28-grains of Ballistite pushing 1 1/4 ounce of shot in a 12-gauge was 12600 pounds.