Home Forums Reloading Big Game Rifles Pressure Trace measuring system

I'm about to spring for this system. I know several here have mentioned having one. Experiences? Pros and cons?

Bob, is that the same system Stan uses?

Also, I've lost track of his website and want to get his 30-06 manual... you have the contact info?

I hope you get the system!

Brad

It is an excellent system, and it does what it claims.

My main use for it is finding good loads for calibers that have relatively little decent loading information: 6.5x55, 8x57, 7.62x54R, 7x57. I also found it quite useful for resolving a large discrepancy between three loading manuals on the max load for a 100 grain bullet in the 243.

The nice thing is that you see what the pressure is in your rifle, not in a minimum cut test barrel.

Brad~
No, Stan used the Oehler M40 with the acoustic targets. Too much hassle for me. I don't have my own range, he did.

I haven't been in touch with Stan for quite a while. The last we communicated he was in ill health. His website address came up blank and I fear the worst. He posted here very rarely and it's been a couple or three years at least since I've seen anything from him anywhere. I E-mail you his last address.

Likely I will get the system but with a snotty reply to a question I had I'm looking at the Oehler again.

Thank's Bob.

Who was snotty? About what?

Did they ever resolve the "pressure spike" at the muzzle as measured with pressure trace?. It sure looked like a barrel bending moment, and how would muzzle pressure be measured by a strain gage at the breech?.
Good luck!

The PressureTrace is not the only device that sees that so-called pressure spike. It is a real event, but it is not real pressure. It happens primarily with light bullets and long barrels, and I can easily make loads that have it and loads that don't in the same gun. My best guess is that something that happens in the barrel momentarily acts almost like an obstruction, near the muzzle. Charlie Fisk has blown the muzzle off three barrels by deliberately choosing loads that create the effect, and I know of at least one benchrest rifle that got a very distinct ring near the muzzle.

So it's a real event.

Pressure is measured at the breech because as the bore is pressurized, the barrel stretches outward, and the amount of stretch is linearly proportional to pressure. Since we know the dimensions and material constants of the steel, pressure is directly calculated from strain. Locating the gage at the breech allows you to look at practically all of the pressure curve.

Traces still appear from time to time. This is a trace of a Hornady 139 gr SST over a load of IMR4350 I was working with last Monday. The "spike" as you can see excedes the breech pressure. The rifle is a 24" Walther BBL`ed M700 in 7-08 with a Answer brake on it.
(I know some others thoughts about brakes, but I won it in a contest and the rifle was already at Answer being barreled so it was elected. BTW the brake DOES do a very fine job of reducing recoil!)

I don`t know what causes these events but I will change powders when I find this instead of adding to the load as Sisk did.

As you work up the load, the "spike" will first appear as a small flat spot in the pressure decay. As you increase the charge, the flat spot will become a one-cycle damped sine wave. As you increase still more, the bottom portion of the sine wave will hit zero (PT can't go below that), and the positive portion will continue to grow. Taken too far, the positive excursion will grow to the upper limit of the PT at the top of the screen.

The event does not represent real chamber pressure. Even a full charge of powder does not contain enough energy to pressurize a whole barrel to 60 KPSI. Further, the spike makes no difference in muzzle velocity, as it would have to if it were real pressure.

So, it's a real event, you can vary the charge and make it come and go, and it can destroy a barrel. Other than that, it is still pretty much unexplained.

denton-
A question: If you were to lengthen the barrel, would the pressure spike occur at the same time, or would it be moved out on the curve?

I think this may have been addressed in a previous thread, but I cannot find it.

Thanks.
--Bob

That's a good question, and I don't know the answer.

My present guess is that it is something that acts a little like a barrel obstruction. If that is true, it probably means that a "donut" wave is being launched from wherever that happens. It would also mean that we are seeing the effect delayed by its transit time.

I have a way figured out to answer your question, but it involves hauling a real dual trace analog oscilloscope, a pair of differential amplifiers, and a scope camera to the range.

With a gage at the breech, and one down the barrel 16-18" or so, you can see the effect in its true time relationship to other events, and determine whether it is traveling down the barrel, as I think it might.

denton-
I appreciate your answering, and I hope you will manage to collect some useful data on this phenomenon.

Have you any idea about whether it's been observed in the testing of loads by the powder and bullet makers? I assume they use similar strain gauges in collecting their data. With tens of thousands of readings made with a variety of powders, bullets and cartridges, the secondary spikes ought to have been observed and noted as unusual by the "pros".

Thanks again.
--Bob

Denton, are any of the powder manufactures exploring this? One would think they would be very interested in what is happening. The possiblity of damaged rifles with loads that appear to be of reasonable pressures is something they should definately explore.

As time permits, I'll be doing some experiments to see if I can unravel the riddle.

Yes, you would think that the professional labs would worry about this. I do know that the Oehler professional piezoelectric units cut off data collection after the pressure wave gets down the slope a bit. You have to specifically design the electronics to do that. I don't think it is unreasonable to believe that something similar might be happening there, and the output suppressed.

I haven't tested any plinker loads, but from what I've seen, they might well show this effect... or not. I wouldn't assume that a 150 grain cast bullet over 15 grains of AA#5 is free from it, until I checked it.

You state that the spike becomes an "undamped sinewave", and the electronics cut off the negative signal. If it is a real measurement wouldn't indicate a vacuum?. Hard to imagine.
The spike certainly looks like a barrel moment. The fact that it occurs when the bullet leaves the muzzle is interesting. As the barrel shortens it has been reported that the spike moves back. My bet has been for a while that it is barrel moment caused by muzzle blast recoil. It does need resolution, what about the people that make Pressure Trace ?.
Good Luck!

hawkins...

Good thoughts...

It does often happen before the bullet leaves the muzzle.

The electronics often DO indicate a vacuum for a very short time before the positive excursion.

The PT people definitely know about the issue, and are as curious as the rest of us.