Since there is no such thing as an NIST standard pressure, how does SAAMI standard pressure ammo get calibrated? There is no standard pressure artifact for SAAMI to directly compare with.



Is SAAMI standard ammo uncalibrated then, as Hot Core says it must be? No. It's calibrated.



Actually, what they do is follow the same procedure I have outlined, from NIST. Force on the rod is cartridge pressure times piston area. The induced charge on a piezoelectric crystal is proportional to force exerted by the rod. Voltage, in turn, is proportional to charge if C is fixed and leakage is small. Voltage is amplified, and either the curve is displayed, or the peak voltage is captured. So, for a batch of ammo, we have peak preassure as a function of piston area, crystal sensitivity, and amplifier gain. From that, we calibrate the batch of ammo. The function, and the input variables are different, but the process is identical with the process for calibrating a strain gage.



And, while you're at it Hot Core, show us your EE isn't a complete fabrication. Answer the questions. Some of them are repeats from another time you were exposed for claiming a phoney credential. If you had paid attention then, you'd have half the answers this time. At least show us that you actually know what a strain gage looks like.

Somehow I know I shouldn't have opened my big mouth...

Denton, how about a quick run of the error calculation in the SG system pressure calcs. Say someone can't hit the same point on the outside of the chamber that they are measuring on the inside and gets an error of +- half the last sig. digit (the 'ol standard error of a measuring device assumption), and the device happens to be a pair of dial vernier calipers (on my desk) with marks in increments of .001". So that makes me capable of getting numbers within .0005" due to measurement uncertainty, and add .001 for user error. +- .0015"...sound reasonable for an 'amateur do it yourselfer'?

HC, gonna have to disagree with you. If PRE/CHE can compare to factory ammo, so can HSGS. I've never claimed it gives accurate numbers to the n'th digit, just that I see it as being much more ***repeatable and sensitive***. My own experiments with PRE showed massive random variation, and that is NOT a good system. And yes, after more time that I care to remember working in a trace metals lab (uranium mine-paid for college in that job not fun!), I know how to make accurate measurements and work with controlled experiments.

Read the posts again...you aren't convincing because the physics are against you. PRE/CHE works...sort of...but it isn't the VERY BEST...just another possibly useful but hard to use tool.

Hot Core,

I've no doubt that you have had good success with the PRE method you recomended. However, your attempts to denounce pressure readings calculated from strain gauge measurements are almost comical. You essentially admitted they are as useful as PRE in your last post (for relative pressure measurements........not absolute)

You recommend a system that requires an accurate measurement down to 1/10,000 of an inch (not easy) and denounce a system where mismeasuring by 0.001" has little affect on the accuracy of the pressure reading as the error is less than 1%. If you are a double E, do the math.

If the individual doen't have the proper tools to measure the chamber ID, it's a 1 time trip to have this done. What is so difficult about that?

Another issue: factory ammo is not always safe.......an example is some factory Weatherby loads. I've shot factory loads from Winchester for a 270 Win that blew primers. How would this mislead a beginner using PRE? (remember beginners are the guys that stuff over book max loads into cases and believe chronograph readings at half what they should be) Couldn't this same "beginner" then use the PRE method to load all his loads to the point they blow primers based off "safe" loads from PRE measurements?

An idiot is an idiot. An idiot can abuse/misuse any method. If traditional pressure signs are noted, one has to question his tools and methods: are my batteries good, is my micrometer calibrated, am I using the micrometer properly, etc. Point is that I would trust the Pressure Trace reading over the PRE for identyfing a hot load.

I've no problems believing the Pressure Trace is accurate to the nearest 1,000 psi. A guy that posts over on 24hourcampfire has done a lot of work with strain gauge measurements. (See his website here: OK Shooter )

He has ran them side by side with piezo systems and has verified their accuracy. I've no reason to doubt him.

CDH...



The strain gage calculations simplify down to a constant times the ratio of ID to OD.



If your OD is truly 1.200", and your ID is truly .400", and you did the absolute worst case things, you could get a ratio as large as 3.0151 or as small as 2.9851. The delta is almost exactly 1%. That is the maximum possible possible output error that could be induced by the measurement errors you stated... about plus or minus 300 PSI for most rifle loads.