How to measure pressure (PSI &/or CUP)

[RamSlammer]

How do ammo manufacturers test for pressure of various loads? I'm looking for more data on my reloads than possible with just a chronograph. Looking to experiment with powders to achieve longer burns at lower pressures versus short high pressure burns.

Any and all information would be useful . . . thanks!

[FALPhil]

They use piezoelectric transducer. There is a specific protocol for placement of the transducer and a specification for the platform it is attached to. If you don't follow the exact methodology, your results will be different. This is why SAAMI and CIP can fire the same ammo and get slightly different pressures.

For the hobbyist, piezoelectric strain gauge monitoring equipment will set you back about $400 - $500, plus a supply of strain gauges (reusable, but permanently attached to your firearm), plus the cost of a computer to analyze the results.

The only organization that I am aware of that still uses the copper crusher system is the US Army. They publish in c.u.p. which does not translate well to PSI, so if you see Army numbers in PSI, you must realize that those numbers came from a lookup table based on the theoretical modulus of compression for the batch of copper pellets that they had on hand at the time the test was run. It has caused major confusion among firearm enthusiasts with regards to military ammo pressures vis-a-vis commercial ammo pressures.

See my article at
http://thefiringline.com/forums/showthread.php?t=326877

[dahermit]

deleted

[Jim Watson]

Most industrial setups use piezoelectric transducers in pressure test barrels on universal receivers, not something you are going to set up at home.

A strain gauge is not the same as a piezoelectric transducer. A piezoelectric transducer generates a tiny voltage as the chamber pressure deforms a quartz crystal. A strain gauge is a pattern of fine wire whose electrical resistance changes as the chamber pressure bulges the barrel minutely and stretches the wire.

There are a couple of sources of consumer grade strain gauge chamber pressure instruments as FALPhil describes. Cement a strain gauge over the chamber of a conventional firearm and get a reading off the instrument. It is a fairly arbitrary number but you can compare it to factory loads. I don't know where to buy the official reference ammunition that a SAAMI member would use.

Some things to be aware of when thinking about a home strain gauge setup. The gauges have to be very solidly cemented to the barrel, on a spot sanded to bare metal and thoroughly degreased. So you are going to lose a patch of finish on your barrel. The gauge has to be directly over the chamber to read the chamber pressure. Some gun's receiver shape won't let you get right over the chamber. Going to be tough to get one on an autopistol barrel or revolver cylinder. The usual hobby solution is a Contender barrel with strain gauge.
Speer once built a one-shot "cylinder" with piezo transducer to go in a revolver frame so they could get pressure readings that took into account the barrel-cylinder gap. But they had company resources for parts, machinist, and money.

[FALPhil]

Jim, I'm sorry; you are right. I have strain gauges on the brain right now because I am shopping for one to do some definitive testing.

Transducers are what SAAMI and CIP use in their testing. I just had brain flatulence. I am amending my OP.

[BillCA]

Just another tidbit on the differences between a CUP and PSI reading.

The CUP (copper crush) method records only peak pressure. Using modern equipment the PSI can be measured not only for its peak pressure, but how long that pressure is sustained over time.

In otherwords, if the same cartridge is fired, the CUP method will only tell you that the cartridge reached a certain pressure. The PSI readings can tell you how long the peak pressure was maintained over the firing event and how quickly pressure built up and dropped off.

[Jim Watson]

Ihave a little book The .30-'06, A Source Book for the Advanced Handloader by W.L. Godfrey. In 1975 he set up with some strain gauges and an oscilloscope to read chamber pressures. His confidence limits on pressure readings are wide, but if you want to know what you could expect from full charges of Bullseye in a .30-06, this may be the only place to look. (24 gr B.E. + 150 gr JSP = 50,500 psi, 2353 fps. With a very steep slope on the pressure versus powder charge weight graph. Y'all be careful, now, you hear?)

[wncchester]

What all this means is, we can't do it at home. Not at all.

Even the Ohler system was calibrated against a factory cartridge, not a true pressure standard, so all we can get even with his system is a comparision of our loads vs. the factory load IF we do everything right. That means it's no more "scientific" than measuring case head expansion, flat primers and primer pockets, case life, etc.

[RamSlammer]

Wow. Does seem difficult if not impossible for someone to test for this in other than a big laboratory. Guess I can mark that off the project list as undoable.

[crowbeaner]

FWIW; the readings between PSI and CUP tend to be the same around 28000 CUP or about that of trap door rifles. After that they go in different directions, and there is no available formula to convert one reading to the other. This has caused quite some concern to handloaders, and is the main reason I have a variety of reloading data sources from the 60s to the present day. More references, more discrepancies and confusion. I tend to stick with familiar powders and reloading manuals to glean the best loads and combinations for the guns that I own. If I use an unfamiliar powder or component bullet that I haven't used before, I ALWAYS start low and work up until I see signs of pressure or difficult extraction and back the charge back down. Many times these signs appear before the maximum charge, and therefore dictate using discretion with different types and LOT#S of powder.

[Unclenick]

I have the RSI Pressure Trace. There is a learning curve, but it is usually pretty close to predictions by QuickLOAD. The calibration depends on measuring the wall of the chamber under the strain gauge and knowing the modulus of elasticity of the steel (fairly constant) and estimating an allowance for the cartridge brass. I would suppose it is typically within 10% of absolute. Using it in conjunction with the QuickLOAD internal ballistics program offers some confidence when they agree well.

[FALPhil]

Unclenick, I have been looking at the RSI Shooting Lab. Has it met your expectations?

[Unclenick]

It has done pretty much what I've wanted. Usually I compare the result to what QuickLOAD predicts because if I get a wild difference in pressure reading or muzzle velocity that don't match, I know there is a bug somewhere. With an accurate chronograph and some understanding and a little adjustment of powder burn rate for your lot in QuickLOAD, you can get very accurate modeling and even tweak the barrel settings in the Pressure Trace with it and know pretty certainly what it is you really have. As I said, it usually gets with five or ten percent anyway.

I've never understood the standard pressure load calibration that Oehler uses. That standard load will only hit a certain pressure in a standard dimensioned chamber that your gun won't likely have. If the case, under pressure, stretches any larger in your chamber than it would have in the test barrel chamber, the real peak pressure you get will be lower. Ditto if your throat is longer. So it's only a way to match SAMMI standards, not provide absolute values in your gun.

Strain gauges are not cheap and it takes care to get the gluing right. The glue does not hold up forever, so it is good to plan what you want to get done with each gun you gauge ahead of time and do it. I recommend you go to Fastenal or some other Loctite dealer and order in the original Loctite 401 superglue they recommended. I like it better than what they ship in the little tubes with the gauges now.

I've had a bit of trouble with the connector getting loose and added a clip to help keep contact. But overall, I've been satisfied with the tool.