I have an Oehler model 35 chrono and am very happy with it. Brent has the entire Model 43 system which enables him to measure pressure. I hope he will come over here and tell us a few things about the system.
How does the unit determine the pressure? I understand it references off of a factory round where one assumes the pressure is a certain amount known to be standard for that cartridge. Please explain that, Brent.
My next question has to do with wildcats. When you can't fire a factory round first to establish a reference point...how do you determine pressure?
Finally, how much correlation do you see between the pressure readings you get and the classic signs of higher pressures we all look for such as ejector marks and flattening primers? We ordinary people don't have one of your Model 43 systems to use...or your technical skills in interpreting the data!
Re: Oehler Model 43 chrono and pressure reading unit
All SAAMI test barrel bores are on the tight side, as well as their chambers. This simulates a “highest” chamber pressure situation a manufacturer might chamber a given cartridge for, and ammo manufacturers might make ammo for, or provide us loading data for, just to keep us on the max end, or less, of what SAAMI rates the specific cartridge for.
The various ammo manufacturers send their ammo around in a circle to every other member of SAAMI to test the ammo for maximum pressure in each test barrel. The maximum pressure arrived at in the one test barrel that shows hottest will dictate how hot this load can be loaded, a ceiling if you will. Testing of loads in one barrel at one facility at the place of manufacturer is the starting point, and then they make the rounds to be tested at the other facilities. This is what the ballistician at Federal Cartridge Company has told me.
What we end up with, is our rifle will produce at, or less pressure with SAAMI approved commercial ammo when manufacturers use barrels and chambers within SAAMI specs. Most often factory ammo produces less pressure, and thus MV than it would in the rifles barrels and chambers were held to the tightest of specs.
When we chamber for a factory or wildcat cartridge and want to know the chamber pressure that OUR barrel/chamber combination is producing, not what SAAMI’s tight spec barrels would be, we can glue on a strain gage over our rifle’s chamber. This will measure the stretch of our barrel in order to calculate the peak pressure as well as the entire pressure curve from ignition to bullet exit.
A strain gage which is super glued over our chamber at the mid-point between the face of the receiver and the shoulder of the cartridge as it sits in the chamber will change (increase) electrical resistance as the thin wire circuit inside is stretched when the chamber expands upon firing.
We have two wires that attach to the strain gage, one on each end of the circuit and run to the a device which measures the voltage change when a trigger level has been reached upon firing. The unit captures multiple data points during the approximate two millisecond firing cycle and a pressure/time graph shows us this plotted pressure curve.
Converting voltage change to chamber pressure is not difficult; it is really two rather simple formulas. A Thick Wall Pressure Vessel and a Thin Wall Pressure Vessel formula are used to calculate the pressure contained in the two vessels, the barrel chamber and the brass chamber.
Barrel steel of known thickness and diameter will expand a specific amount with a specific pressure. Brass of a known thickness and diameter will contain a specific amount of pressure before it yields to cause further expansion upon the chamber wall. Together these two formulas determine the total expansion we will measure at a specific chamber pressure. We measure this expansion with a calibrated gage that will produce a specific voltage change with a specific change in length. The barrel expansion causes the gage bonded to it to expand with it and increase in length, thus expansion is measured.
With PressureTrace’s super fast processor, it collects and plots 100 data points (micro strain) per millisecond on a graph, which is sensitive enough to capture even the smallest changes in pressure along the pressure curve.
When comparing commercially available factory ammo for a reference, hand loads can be held well in the safe zone if careful measurements are taken and you stay below the reference ammo’s measured pressure. Reference ammo is not “needed” but it does help in determining if you’ve made a mistake with barrel measurements or bonding the gage over the chamber.
Of every chamber I’ve done testing on, I have never found a factory load that has exceeded SAAMI maximum, ever. Usually they are just below, some loads are a few thousand PSI below. I get about what I’d expect in my hand loads for MV considering barrel length at the indicated pressure, and also pressure signs on the cases when I exceed them by very far.
Testing so many different ones this far, I see no problem with the measurements given without reference ammo; I rarely even buy reference ammo any more FWIW. If I wasn’t confident as a result of testing so many loads the last few years I still be using the factory stuff for a reference, it has always seemed a total waste of money and time it’s so predictable though. When I jumped into the 30-338 Lapua Imp wildcat, I had no reservations what so ever about not having factory ammo for a reference…
Hope that helps some. 70,000 psi is a good point most classic pressure signs begin to be observed, generally not any lower than about 68,000 psi. I'll post a little more when I get back.
Re: Oehler Model 43 chrono and pressure reading unit
I have a problem with looking at flat primers; how flat is flat? That’s the problem I see and have such a hard time with. I know the pressure is getting up there, but how far… A primer don’t tell you that, case head expansion doesn’t, stiff bolt lift might, heavy extraction might, MV might. They all can tell you something, but what?
We as savvy hand loaders use ALL indicators of pressure to warn us of serious pressure. What we want to know is how close/far off we are and what change, and how much of one is taking place that might be helpful to know.
Getting up near the 70,000 psi mark, brass begins to yield and flow from the heat and extreme pressure generated. Brass will flow into the ejector slot/bore and primers will begin to crater as pressure extrudes it into the firing pin bore around the primer pin, the worse the pin to bore fit is and/or the higher the pressure, the higher the crater’s rim becomes.
Three things can cause stiff bolt lift;
1) Bolt lug flex
2) Receiver ring flex
3) Shaving of brass extrusions on bolt rotation
Bolt lug flex is caused by high thrust loads on the bolt’s locking lugs due to excessive chamber pressure. When the bolt’s lugs flex under extreme pressure, the case also stretches getting longer as it follows the bolt further to the rear under pressure. When the pressure subsides, the bolt springs back forward, as the lugs did not reach the point of yielding. This now leaves the case under compression between the bolt face and the shoulder, a negative headspace condition, and now the bolt is heavy/stiff to lift up to the camming point.
The other cause of this is along the same line, only this is not due to a longitudinal tight condition but rather radial expansion, a result of the receiver ring stretching too far. The receiver ring will stretch and the brass will follow. The brass will spring back some, but when the receiver ring stretches, the brass springs back from the largest diameter it took the form of during firing. The steel receiver springs back more, that is the bottom line, and it leaves the case tight along the body and mostly near the base where it begins to get quite thicker, referred to as the pressure ring. Excessive “pressure ring expansion” (PRE) will also cause another condition that excessive bolt lug flex and a negative headspace condition will not, described next.
The bolt handle contacts the extraction cam on the rear receiver ring at the top of the lift stroke. If radial expansion was just too great and this left brass in the chamber in a press fit condition, the bolt will ride up the extraction cam and continue to be stiff instead of just popping open. You may even find it is so tight that the case is still sticky trying to withdraw it beyond what the cam does for you, especially if the case design has very little body taper to it. This indicates the most extreme case of high pressure, i.e. bolt all the way open and cases are still not wanting to come out.
Now being able to see the actual chamber pressure along with these pressure signs developing to that level is the only way I have learned other methods to evaluate pressure more reliably than I had before. In my mind, I have associated certain aspects discussed here to certain pressure levels with the use of Oehler’s M43 and Southwest Products’s PressureTrace, much like one would learn to better judge range by the use of a laser rangefinder over time. No, my guestamates are not set in stone, perfect by any means, just a much better idea because of the associations made with very good pressure measurement equipment.
You can see how the very stoutest built custom actions can mask excessive pressure signs better than the remchesters. The cartridge case remains the weakest link in the system and can contain a considerable amount of pressure beyond what is considered a max load in some cases, but the steel the action is made of has a life span measured in cycles at a given pressure. The higher the pressure is, the lower number of cycles it will survive before it yields, then fails. Fortunately, steel has the outstanding characteristic of surviving what is an infinite number of cycles at max load levels but once you exceed a certain level of pressure, the cycle life of steel falls more than exponentially, like a rock, it’s called the knee curve. I am not exactly sure where that level is, but I sure don’t want to be operating there and suddenly find out I reduced the cycle life of my rifle to a couple hundred rounds, or a few rounds and find the last one it finally fails on.