Home Forums Hunting & Shooting Ask The Gunwriters Loading manual pressure info?

Denton,

Very interesting about Ken being given a flat 7 KPSI offset. I dunno how that would even be possible, given all the variables involved.

One of my old high school classmates retired from the computer industry in California a couple of years ago. He's now back in Montana, working on an easier-to-use strain-gauge system for average handloaders. He doesn't plan to include a "PSI" readout, because of all the potential problems. Instead it will provide comparative pressures (essentially "microstrains") so handloaders can compare the results in their rifles to factory ammo. We tested a prototype a few months ago and I should probably see how things are progressing.

Thanks to all for the info.

That's an excellent point. Squirrely things happen if you're too near to the receiver or the end of the barrel. I generally put my gauges a bit away from the receiver for just that reason.



Let me know if there is anything I can do to help. I hope this is a smart phone app. Something I can slip in my pocket would be super cool.

That sounds like a very interesting and useful tool. Please keep us posted.

32_20fan,

The brass is one of the problems the head ballistician from a major company stated was a real problem with strain-gauge pressure systems. The phrase he used, as I recall, was "all the layers" between the powder and the outside of the barrel each contribute minor problems in measurement.

As I'm sure you know, in piezo measurement the case wall itself activates the pressure reading, and the walls of that particular lot of cases are tested in a separate machine before the actual pressure test.

One of the labs I've visited frequently became interested in the difference in transducer location SAAMI and CIP, the European system. They build their own test barrels, so ran some experiments and came to the conclusion that the difference is around 1000 PSI.

denton,

I'll talk to him and find out. The prototype he demonstrated to me had a very simple, hand-size read-out system, rather than having to be plugged into a computer.

One of the problems, of course, is keeping the price down to where more handloaders feel they can afford one!

Steve Faber used to sell a unit like that. It just captured the peak microstrain reading. That's pretty straightforward to do, and can be done quite economically. I think mine is still kicking around somewhere. He did some good work on that product.

For whatever reason, the Faber unit had less precision than the PressureTrace. I think it probably related to circuit board layout and such. There are some tricks to excluding electrical interference, such as using a two layer board, with one surface being used just for ground.

You can get strain gauges for $5 if you shop. Some of those are not sealed, so the user will want to cover them in nail polish. But that's one way to keep cost down.

A good place to start is searching the SAAMi web site, they put out a lot of free info that isn't overreaching and will give you a good starting point.

One such of their pages;

Link

Phil

Good link, thanks.

thank you for sharing. although i may not understand all the technical aspects that you and john are mentioning. discussions like this are great added value to the campfire.

I was a die-hard Piezo person until I watched them done at the same time as strain gauge (Thanks, Dr. Oehler). That gave me much more confidence in the strain gauge system.

Interestingly enough, Piezo transducers are calibrated on a system that uses...a strain gauge.

It's interesting to think about the measurement chain for piezo and for strain gauges....

Peizo: The pressure in the cartridge is converted to force by the surface area of the rod. Force applies stress to a piezo crystal. The stress is converted to strain in the crystal by its physical properties. The strain is converted to charge on two opposing faces of the crystal, by the piezoelectric effect. The charge is converted to voltage by the capacitance of the crystal. The voltage is amplified and converted to PSI by the appropriate conversion formulas.

Strain: The pressure in the cartridge applies stress to the chamber. The physical properties and dimension of the chamber convert the stress to strain. The strain is converted to voltage by Ohm's Law. The voltage is amplified and converted to PSI by the appropriate conversion formulas.

The conversion chain for strain gauges is simpler than the chain for piezo devices.

Denton,
Your measurement chain discussion summarizes a lot. I might add that both systems start the chain inside the case and both see similar problems transferring the actual gas pressure to the chamber walls through the brass case. I regard the two systems as "equivalent", with accuracy ratings of roughly two percent. Those who say that accuracy is significantly better than two percent have probably never played the game of making two or more measurements of the pressure generated by a single shot.

The conformal piezo method adopted by SAAMI has the advantage of a using well defined procedures including calibration of the effects of the specific lot of virgin brass used in the loads. You have no calibration with virgin brass exactly corresponding to your load? You are right back to guessing if the calibration is applicable.

Yes, there is such a thing as reference ammunition. It is a valuable tool. The "assessed value" of a particular lot of reference ammo is the average value of the pressure observed by several labs, using their barrels and transducers previously calibrated using SAAMI defined procedures. The reading from each lab is their best estimate/measurement of the average pressure actually generated by the ammo in their lab. The "assessed value" is the "average of the averages" and represents the average pressure of that ammo when fired in a mythical "average" pressure barrel. The pressure correction obtained through the proper use of reference ammo is often referred to as the "barrel correction", and the corrected number is a better estimate of the pressure that might be seen when the ammo is fired in that imaginary average barrel. Simply using reference ammo is not a valid procedure for transducer calibration. Barrels are not equal, even if they are "identical"; that's why we have reference ammo.

The 7000 psi offset mentioned earlier in this thread was not forced on me. It is just my best guess. The offset for the strain gages is very similar to the offset for the conformal piezo transducers. Over the years I've seen many offsets for different brass. They usually vary between 5000 and 12000 psi, and I've been unable to establish any reliable system of predicting the offset for any specific case. My thinking was that 5000 is probably too low for a typical case and 12000 was too high. I just called it 7000 and tried to not outsmart myself. The guess now has morphed into a scientific fact.

Either the conformal piezo or the strain gage can give comparative or relative values of the pressure. (So can any unit that measures only peak microstrain over the chamber.) When you are looking at comparisons only, the offset really doesn't make a significant difference. You get into trouble when you are working near failure pressure (probably brass or action and not the barrel) and try to compare your estimated pressure to the pressure guaranteed by the gunmaker. I don't know any manufacturer who is willing to guarantee that their guns are "good to XXXX psi", and I'm no longer excited by trying to wring the maximum velocity from my loads.

Back to my cave where I've been neglecting pressure measurements and working to improve first-round hit probability at extreme ranges.

Thanks to everyone who has contributed to this discussion.

I too would like to thank the smart fellas...

I'm glad there are smart people working on this. I'll stick to using their insights, as published in manuals, and load to a point where the powder weight and velocity are reasonably inline with what they publish (factoring in barrel length), and the primer looks 'normal', the bolt opens normally and their are no excessive extractor marks, shiny bolt face, etc. Any user friendly tools to better understand pressure in my rifles would be welcomed!

I knew a lot more about "Traditional Pressure Signs" before I began to conduct Piezo pressure testing, or thought I did.

One day we pressure tested some Proof loads. "Traditional Pressure Signs" would have indicated that the Proof loads were not only safe but could probably be increased a little bit. I went home, that night, and pulled bullets from my favorite 25/06 load (which was above book max).

There is no such thing as a free lunch. You won't get appreciable increases in velocity without corresponding increases in pressure.

Get yourself two chronographs or one that reads velocity twice (the genius of the Oehler 35P). If the two readings agree, you've got confidence in the number.

Load to reasonable velocity and you'll get reasonable results.

My new 25/06 load worked just as well as the hotter load...and was a lot safer.

Been quietly sitting in the back of the classroom enjoying the discussion but you hit on something that I've really taken to heart these last several years.


As I understand it, striving for those last 50-100 fps might cost as much as another 5,000 to 10,000 psi. One might be safely at 60 kpsi and 3050 fps for some particular rifle, trying for 3100 could send you to 70 kpsi - speaking in broad general terms, of course.

I used to try for that last 50 fps, just gotta have it! But then it dawned on me the reverse could be used to advantage. You need give up only 50 or 100 fps and perhaps gain a good 5,000 PSI or more safety margin in the process. The animal won't know it and the trajectory won't be meaningfully affected until ranges get way past my comfort zone, but you will increase safety by a goodly amount. Brass lasts longer, bolts don't suddenly get sticky on a particularly hot day, the barrel steel isn't stressed by a few more microstrains (had to google that to enjoy this thread ) per shot - all kinds of good things come from a slight reduction in speed vs. all the nasty things that can happen striving for that last minor gain.

And if someone really needs those last fps just get a bigger combustion chamber.





"No, son, let's walk down there and **** them all!" makes a lot of sense.

Thanks gentlemen great info.

...and to think company's like Sierra publish loading manuals without any pressure testing equipment at all.

I contacted Nosler and Hornady a few years ago and asked why they did not list pressures. Their answers were on par with Johns comment.

If there is a gap between published pressures and SAAMI MAP far to many handloaders would view that as a green light to exceed published max.

Jim,

Homer Powley's rule with single-based powders was that pressure doubles at twice the rate of velocity.