CUP, psi & Reloading Data
January 04, 2011
Here is the piezo-electric conformal transducer. The threaded cylinder with the contoured tip is the sensor.
Photo by PCB Piezotronics.
My 20-year tour as a reloading-manual developer began during the transition from copper crusher pressure testing to modern piezo-electric testing, and what good timing that was. Learning both let me understand the effects of each on published load data and gain confidence in the loads derived by each method.
Copper Crusher Basics
Crusher testing is a very old technology and is strictly mechanical. It is so simple and low-tech that you could measure cartridge pressures on a desert island with no electricity or computers as long as you had a decent micrometer and some place to write the results.
A crusher-type test barrel is pierced or ported at or near the chamber in a spot precisely called out in the cartridge's standards. Once a cartridge is loaded in the barrel, a soft copper gascheck is pushed into the port against the case, and a special steel piston is inserted and snugged against the gascheck. The gascheck prevents gas escape that could erode the expensive barrel.
A copper crusher is placed on the upper end of the piston. Crushers are made to high material and dimensional standards, and each lot is calibrated to ensure consistent deformation. To provide the resistance that ensures the "crush" and captures the crusher, the pressure barrel has a heavy yoke--basically an inverted "U." The yoke attaches to the massive frame of the universal receiver above the barrel. A large anvil screw in the top of the yoke lets the operator secure the crusher.
Pressure against the piston shortens the crusher. We measure the crusher and consult a table of pressure values provided with each crusher lot. Starting at 0.499 inch, the table lists decreasing lengths in thousandths of an inch and, beside each length, the pressure in copper units of pressure (CUP) that it takes to shorten that much. The system must be reset after each shot, requiring that you remove the piston, knock the gascheck into the fired case (which now has a hole in it), extract the case, and start over. It is a tedious procedure, but it served well for years.
When modern electronics and sensing methods became more prevalent in the last 30 years of the 20th century, ballisticians found a way to improve the speed and accuracy of pressure measurement. The heart of the system is a little gadget called a piezo-electric transducer. Each one contains a tiny crystal that emits a small electrical signal when compressed. The signal is directly proportional to the force applied and is linear, making it an excellent method for testing pressure.
If you replace the crusher piston with an electronic sensor, you avoid the need to reset the barrel for every shot. But you need potent electronics. The tiny signal output from the crystal must be amplified and then fed into a computer that applies all the initial settings derived for that barrel and transducer to yield an accurate result.
In Sporting Arms and Ammunition Manufacturers Institute (SAAMI) protocols, the transducer's sensing surface is contoured to match the chamber's radius and taper. The device "conforms" to the case wall, giving this type of sensor its name--the conformal transducer. A small metal bracket ensures the sensor is installed with the taper in the proper directions. When properly installed in a pressure barrel, the transducer is almost invisible to the casual observer. The transducer can leave a slight mark on a fired case but is otherwise like using any firearm in that you do not have to reset anything other than the breech between shots.
A PCB transducer is fitted to a Krieger pressure barrel. The bracket ensures the contoured sensor is in the proper orientation.
The Big Difference
Being strictly mechanical, the crusher system records only peak pressure--nothing else. It's like a black and white snapshot taken with an old box camera. By comparison, transducer testing is a high-definition, full-color video. The new system has a time basis and records from the point the fired case contacts the transducer to the point where the bullet exists the test barrel.
Just as you can derive more information about an event from a video than a single still photo, modern electronic testing opened new doors for those of us who develop reloading data. We can see when the peak pressure occurs (time-to-peak), how long it is maintained, and even analyze primer performance--something hard to detect with a crusher system.
In addition, transducer testing gives higher resolution. The nature of crusher calibration creates large increments of pressure up to several hundred CUP per increment, but a transducer technically can read to 1 pound per square inch (psi). I found this higher resolution let me confidently produce a more consistent data set for each cartridge and better judge which propellants were best suited to that cartridge.
It's About The Units
People seem to think there is some absolute relation between copper units of pressure and the pounds per square inch results of transducer testing. There isn't. CUP is a special unit of measure defined only for ballistic testing; psi is a standard unit of measure for all pressure testing in the English system. Whether you are testing radiator caps or valves for a nuclear submarine, psi is the same. CUP has no other use.
There is about a 96 percent correlation between assigned psi and CUP for rifle cartridges, with the psi value being higher. However, that means that 4 percent of the time, you can't make it work. That's because of some rifle cartridges whose psi assignment is lower than their CUP assignment. It's even more prevalent with handgun cartridges. When you convert inches to feet, dividing by 12 always works. Not so for ballistic pressure units.
Location, Location, Location
The location of the pressure sensor is a major factor in the relation between psi and CUP values. The reason that 96 percent of rifle cartridges have a reasonable correlation between the two systems is that the sensor location is similar. The sensor locations can be very different for handgun cartridges, especially the short ones. Because the yoke and anvil assembly on the universal receiver must be a minimum distance from the breech end to create a safe and secure contact, many handgun cartridges were set up with the crusher sensor at the case mouth. The shortest centerfire cartridge, the .25 Auto
, has a max case length of 0.615 inch, but the anvil assembly means the crusher port is 0.698 inch from the breechface. The accompanying chart shows how much the sensor position varies for the .25 Auto, 9mm Luger, and .38 S&W Special.
The transducer is secured to the barrel, so it can be positioned much closer to the breech, usually mid-body. This means a transducer can detect pressure events that occur very early in the pressure cycle--things a crusher at the case mouth may fail to detect.
Effects On Load Data
First, let me repeat that when the sensing ports for the two systems are similarly located, the ballistician will not see much difference in final charge-weight assignments. The newest cartridges no longer have crusher standards, so this is a moot point for them.
For me, the two handgun cartridges that highlighted the effects of sensor location were the 9mm Luger and the 10mm Auto. When we started on Speer Reloading Manual #12 (the first Speer manual to incorporate transducer testing), we were using crusher data from older Speer manuals as starting points. Most of the loads compared well within the safe zone, but some propellants that "shot spec" on crusher were exceeding the transducer limit of 35,000 psi by up to 7,000 psi. Retesting on crusher showed them still normal on the older system.
After firing, there is a faint trace from a conformal transducer on this .44 Magnum case fired in a modern pressure barrel.
Before we used this barrel, we received the complete SAAMI technical data packet for the 10mm, and there was only a transducer standard. We built another barrel, this time a transducer version. Because we had inquiring minds, we did parallel testing with each system. Most loads compared well, but again, we found some propellants that posted 36,000 CUP shot nearly 45,000 psi, well over the 10mm's 37,500 psi limit.
In both cartridges, the propellants that did not "line up" between the two systems were the fast-burning handgun propellants. By the time we adjusted their loads to read in-spec on transducer, velocities were falling off.
My immediate thought was, "Are crusher loads unsafe?" We faced one big, obvious fact: The "old loads" had not damaged any cartridge cases or firearms when used as prescribed. Why not?
The engineering director nailed the answer: Some potentially dangerous things are of such short duration that they do not have time to produce damage.
Look at fire walkers. They routinely walk on coals hot enough to burn skin, but their technique avoids staying in contact with the heat long enough to suffer burns.
Fast-burning propellants can create pressure effects early in the firing cycle that a crusher at the case mouth will never detect. They last such a short time that even the soft brass case suffers no ill effects. It is a credit to the sensitivity of the conformal transducer that it picks up these early effects and includes them in the total picture of the event.
We seldom saw these effects in rifle cartridges. Most standards for bottleneck cartridge chambers place crusher and transducer ports much closer, at least as a percentage of total chamber length. We shot transducer pressures for .30-06 loads with the various styles of Speer 180-grain bullets, data for which had previously been developed on crusher only.
The crusher loads were set to max between 47,000 and 48,500 CUP, just under the SAAMI max of 50,000 CUP. Transducer testing showed that the same loads fell between 56,700 and 58,500 psi, just under the 60,000 psi limit for the same cartridge. No changes in published data were needed. Across the range of modern rifle cartridges, we did not find any surprises when we added transducer data to our existing files of crusher data.
So the handgun reloader stands to be most affected, but is this a serious problem? No, it's not. The lesson I see in this is for the reloader to take advantage of the wide range of handgun propellants we enjoy today. Don't try getting high-velocity loads with the quickest burning propellants. In spite of some wonderful mid-rate handgun propellants on the market, some reloaders are still trying to get top velocities from Bullseye, W231, AA No. 2, and other very quick-burning handgun propellants. Save those fine quick-burners for your target loads.
Transducer testing provides a direct payoff to the hobby reloader: volume of data. We had to budget 50 percent more time when developing loads that still used crusher standards. Transducer testing let my team greatly expand the coverage in the latest Speer manual.
I'm a real traditionalist--blue steel, case-coloring, and fine walnut still speed up my pulse. However, for pressure testing, give me the high-tech conformal transducer.