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.

Piezo-Electric Basics
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.