Hi guys,

I am starting to look in earnest for a universal receiver for pressure barrels for ammunition testing. If anybody knows of anyone who might be looking to retire, sell out, etc., please let me know.

Also, if you or someone you know has some knowledge about the various models of universal receivers, I would appreciate the opportunity to ask a few questions.

Many thanks in advance.

LD

I've never had one or even seen one, since they are spendy.

I think Oehler (sp?} makes them.

Thanks for the idea about Oehler.

I have bought ballistics hardware and software from them in the past.

HS Precision makes receivers for ~ $6,800.00, and I have heard that Bill Wiseman does that as well. I can't find an internet addy for Wiseman.

Heck, the receiver isn't that spendy. What adds up is buying one pressure barrel and one accuracy barrel for testing each caliber. Yikes!!

Oehler does not make or sell URs.

We have one with a collection of barrels and transducers that we use for our tests.

Expect to pay approximately $2K per headstamp for the transducer equipped barrel for each new cartridge, and also add the price of a new car for the equipment required to do the required calibrations.

Cost is the main reason we went to strain gages and Model 43 for our own tests.

KenO

lawndart, at a previous employer I worked with the Oehler 43 and the strain gage system. Excellant results and relativly inexpencive to use. Its also easy to set up a barrel for the test and you can use a stock firearm.
I highly recomend this system over the older UR and pressure barrel method.

So how much are strain guages? Is there anything out there that the home reloader can use to reliably measure pressure without spending a fortune?

I'll give Ken a plug.
http://www.oehler-research.com/wizard.html

KenOehler, are lab strain gauge set-ups calibrated, or are the properties of the barrel, brass, and strain gauge generally regular enough to allow pressures to be calculated directly, without an independent calibration?

In our experience it is sufficient to supply the M43 with inside and outside diameter of the barrel at the strain gage along with the gage constant supplied by the gage manufacurer. We suggest using 7K psi offset for chrome-moly barrels and 5K psi offset for stainless barrels. The process seems relatively independent of differing lots of brass.

We've done a lot of testing over the last fifteen years and have gained confidence that the Model 43 is even better than our original claims. Note that our experience has been only with the Model 43 used in accordance with our instructions and not with any other units.

In our humble and unbiased opinion, the Model 43 is the only practical solution for the individual to measure chamber pressure with reasonable accuracy and at a reasonable price.

Short of a M43, we feel that the typical reloader is best served by limiting velocities to factory levels, limiting charges to book maximums, and not seeing any of the classic pressure signs. In reality, the M43 is a research instrument and not simply a reloading accessory. (The Universal Receiver is even more dedicated to lab testing and is farther from your typical sporting rifle.)

Extra strain gages are approximately $10 each.

KenO

quote:

In our humble and unbiased opinion, the Model 43 is the only practical solution for the individual to measure chamber pressure with reasonable accuracy and at a reasonable price.

I was a part owner in one for years. At the time I was doing a lot of full wildcat work. It scared us showing just how hot you had to be to reach some of the AI loads floating around. Like Ken said often so called pressure signs didn't show up until you were way past my comfort level. We didn't have the downrange target. I got to use one of those about 5 years ago. If I had my own range it would be a must.

KenOehler, thanks for the reply. I'll follow on with a related question. First though: I don't mean to try to pry loose any trade secrets.

I was looking at equations for strain in a pressure vessel, and I see there is quite a difference (about 15%) in the relationship between pressure and strain depending on whether one looks at the barrel's chamber as a closed ended or an open ended pressure vessel. It seems to me the .458 Win will look rather like an open ended vessel, and a .25 WSM will look more like a closed ended vessel.

Does the Model 43 compensate for this difference by comparing caliber to chamber I.D. and O.D.? From what I can tell, another brand of software isn't set up to compensate for this, but using the closed ended solution gives one a conservative estimate for pressure in the straight walled cases, so I imagine that is what they are using.

In both cases, recoil will have a small effect on the pressure estimation as well, causing smaller strains for a given pressure. Here again, using the closed ended solution should provide a conservative (if not as accurate) estimation of pressure.

It's rather tricky, trying to read chamber pressure.

ODCMP has Mann Accuracy barrels in 7.62 for sale. http://www.odcmp.com/Rifles/misc.htm

quote:

Originally posted by KenOehler:
... In reality, the M43 is a research instrument and not simply a reloading accessory. ...

Nice quotable thought that even I can agree with.

Dr. Oehler, What would you speculate the actual "Accuracy in PSI" is on a non-Calibrated, fudge factor derived and guessed at Chamber Dimension, Home Strain Gauge System(HSGS) for the non- Test Laboratory user would be?

quote:

Dr. Oehler, What would you speculate the actual "Accuracy in PSI" is on a non-Calibrated, fudge factor derived and guessed at Chamber Dimension, Home Strain Gauge System(HSGS) for the non- Test Laboratory user would be?

Hot Core,

You pose a difficult question. From your description of the system there is absolutely no way I would speculate on the accuracy.

If you will define what measurement of chamber pressure you consider to be the "truth" and specify its proven "Accuracy in PSI" then I'll be able to reasonably discuss the accuracy of the Model 43. It's just hard to discuss accuracy if there is no agreement on "truth". The discussion will have no relationship to HSGS systems in general; it will pertain only to the Model 43 used in accordance with instructions provided. That's where I have my experience and a few scars.

KenO

quote:

Originally posted by asdf:
I was looking at equations for strain in a pressure vessel, and I see there is quite a difference (about 15%) in the relationship between pressure and strain depending on whether one looks at the barrel's chamber as a closed ended or an open ended pressure vessel. It seems to me the .458 Win will look rather like an open ended vessel, and a .25 WSM will look more like a closed ended vessel.

Does the Model 43 compensate for this difference by comparing caliber to chamber I.D. and O.D.? From what I can tell, another brand of software isn't set up to compensate for this, but using the closed ended solution gives one a conservative estimate for pressure in the straight walled cases, so I imagine that is what they are using.

In both cases, recoil will have a small effect on the pressure estimation as well, causing smaller strains for a given pressure. Here again, using the closed ended solution should provide a conservative (if not as accurate) estimation of pressure.

We use the open end "long cylinder" model corrected for the effects of a typical cartridge wall. The model might not be appropriate to "short and fat" cartridges; this is highly dependent on action type (barrel shank) and location of the strain gage. It is also dependent on the length of the cylindrical section in comparison to the diameter. For some cartridges the only appropriate action is a top-break Contender, Encore, or other special action allowing the gage to be mounted over the body of the case.

We tried to address these concerns in an appendix of the M43 instruction book where we placed limitations on the strain gage mounting locations and attempted to define those applications for which we consider the M43 to be appropriate. (Ex. If you put a PPC case in a Remington 788, the neck hardly extends beyond the receiver and there is no place to put the strain gage. How do you measure the pressure of a .25ACP inside a bull barrel?)

We also worried about the effects of placing the gage adjacent to the receiver where the barrel diameter steps down to the shank. Empirically we found little difference between readings of a gage mounted near the shank or mounted closer to the shoulder on a relatively long cartridge.

You are apparently better versed in the geometry and finite element analysis than we are. We just implemented the simplest approximation and by subsequent testing have found it to be better than we had any right to expect.

I really doubt that the effect of recoil is significant in the pressure measurement. Just intuition and back-of-envelope scratching.

The real problem we have encountered in our testing is just what is the true pressure of a shot to which we must compare our reading. Pressure measurement education begins when you install multiple pressure measurement sensors on a barrel and then compare the results on each shot. What caused the difference and which one is correct?

KenO

quote:

subsequent testing have found it to be better than we had any right to expect

... which is what really counts, in the end. Many thanks for the insights, KenOehler.

I'm no finite element whiz, and even were I so, there's too many different geometries to make one or two such analyses meaningful. As for recoil, my envelope back showed between 1 and 2 % error, but I'll try another envelope in the morning. Regardless, such errors can be obscured by not knowing the true values for the properties of the steel. Use of the open ended estimate is a surprise, but compensation has apparently been made elsewhere in the calculations to get the numbers to match to test--and matching to test results is the important part. Again, my thanks.

quote:

Originally posted by KenOehler:
...Hot Core,

You pose a difficult question. From your description of the system there is absolutely no way I would speculate on the accuracy. ...

Dr. Oehler, Thank you for your insightful and obviously wise answer.
---

I see the typical user of a Strain Gauge System at Home(not a Lab, therefore the HSGS) as having little understanding of the complexity involved with the Set-Up or the Calibration.

Just getting the double-radius and double-tapered Chamber dimension without the use of a long probe on a CMM is simply guessing. Even then, the number must be an average because of the area covered by the Strain Gauge. Plus common minor variation from lot to lot in the actual Expansion Ratio of the barrel steels. So, I can understand why a Fudge Factor is appropriate.

I've seen people argue that it is possible to Calibrate a HSGS without the use of actual SAAMI Calibration Ammo. Some seem to think that simple repetitive firing of Factory Ammo will allow them to some how magically Calibrate the HSGS by use of an "unknown Pressure" and result in a "known Pressure".

I appreciate your candid and insightful responses, but I need no additional input.

As I have mentioned in the past:
A HSGS = Reloaders Pyrite(aka Fool's Gold)

There's no old envelope at hand tonight, but I did find a piece of scrap paper. I get a bit over 1% error in indicated pressure due to recoil for my imaginary .458 Win. As I said in my earlier post, it is a "small effect," but in trying to figure out how I'd use a strain gauge myself, I worry about all the many small effects adding up to something significant. I, after all, don't have alternative pressure sensors to compare my results to.

The weakness of the strain gauge is that it needs to be compared to a reference pressure in some way to establish the "fudge" factor. That is likely to be different for each installation.

The weakness of the direct measurement technique is that the pressure barrel should represent the manufacturing technique and tolerances of the rifle in which the ammo is to be used eventually.

In the old days, the Universal Bond receiver was the standard test instrument. It was originally a copper crusher.

In the late 80s, Bo Clerke built a receiver that takes barrels that are fitted with piezo electric pressure sensors that are ported into the bore. Seems to me that the maker of the sensors is/was EG&G. They generate a current signal directly proportional and traceable to PSIG. The signal has to be processed into a voltage which is then read with an Analog to Digital convertor connected to a computer. Data rates need to be pretty high as the projectiles residence time in the bore is something on the order of 3.5 milliseconds or so.

An adapter can be built to allow the receiver to take cylindrical barrels having a diameter less that the 2" units he specified. They have to have enough meat to be drilled and tapped for the sensor.

I think I acquired mine in about 1988 or so. I have a 50 khz 16 bit convertor connected to a computer for data acquisition. The receiver would not be hard to build. The well into which the sensor is threaded is more complex than most holes. I can probably locate a diagram of the hole and specs for the transducer. Can post pics of the receiver if anyone wants to see them. May take a few days.

quote:

Originally posted by Hot Core:

...
I appreciate your candid and insightful responses, but I need no additional input.

As I have mentioned in the past:
A HSGS = Reloaders Pyrite(aka Fool's Gold)

It's apparent that we've agreed to disagree, but I'm curious regarding a couple of points.

1. Is your opinion based on your actual use or observation of the Model 43?

2. If your regard a HSGS as Pyrite, which laboratory method do you regard as 24 carat?

KenO

Hi Dr. Oehler,

Mr. H. Core is our resident strain technology gad fly. Showing him the physics or math is like throwing electrons at a negatively charged plate; you will never connect.

I am quite happy with strain technology for my own use.

The situation I face is that I am going to start loading and selling commercially over the next several months. Do you know if any of the small to medium sized ammunition companies use strain technology to verify SAAMI/CIP pressure measurements?

Most of the calibers I will load for aren't that common in the US. That might make it difficult for me to order up some SAAMI spec ammo to calibrate/validate my strain set-up for that barrel. Likewise I probably won't be able to send ten cartridges of 8x68(S) or 9,3x70M to HP White to calibrate by the back door method.

The math to internally calibrate is pretty straightforward (thank you Mr. Powley); the trick there is convincing others to accept those figures.

At the moment I'm planning on using one of you model 83 systems with the IR "screens" and eventually an acoustic target array.

If the industry is going to strain gauges I will do so gladly. Otherwise I'll keep saving and also looking for a used receiver.

Many thanks for your words here, and many more thanks for your products. Your company recently rebuilt my old model 33 that was used hard for a couple decades - no charge. Unbelievable service in this day and age, and the reason I will get my big boy set-up from you.

lawndart
John Noak
johnnoak@cableone.net

Lawndart, your comment that "math to internally calibrate is pretty straightforward (thank you Mr. Powley)" caught my attention. I'm curious to what math you're referring and how you hope to use it. I'm familiar with only a small portion of his work, but I'm always eager to learn more.

quote:

Originally posted by KenOehler:
...It's apparent that we've agreed to disagree, but I'm curious regarding a couple of points.

Dr. Oehler, Actually I think we are closer to agreement than most people would expect. I completely agree that the M43 is an excellent "Test Instrument"(your words) when Set-Up properly, Calibrated properly and used in a Laboratory environment with rigid documentation guidelines. I would be very surprised if we are in disagreement about that.

quote:

1. Is your opinion based on your actual use or observation of the Model 43?

I have never owned or had personal access to a M43. But for many years I used your, as well as others, chronographs. I do have a lot of experience with Strain Gauges and realize their use is not as straight forward as some folks would lead the Beginners/Rookies to believe.

quote:

2. If your regard a HSGS as Pyrite, which laboratory method do you regard as 24 carat?

From what I've seen and experienced first-hand, there is no "single" Pressure Detection Method or System that is "24 carat" or perfect. All have nuances and quirks which have the potential to "mis-lead the Beginners". I recommend the use of every Pressure Detection Method available to Reloaders during Load Development.

When people get the mistaken impression that a simple chronograph tells them what the Pressure is, it concerns me. There are people who still believe that it is OK to just keep dumping in Powder until some arbitrary, randomly-selected velocity from one of their Manuals is reached. From my experience, that may be SAFE, but there is also the potential for it to be UNSAFE. Perhaps the rifle will hold up fine, or perhaps the effects of Cumulative Metal Fatigue will appear.
---

The very first person I had any communication with concerning a M43 was a guy that used to post on the long gone "Shooters" Board who posted as OKShooter. I was as interested in the M43 as I suppose anyone outside of your company could be. I saw(and still see) a great potential for it. I suppose I clogged that Board with questions for stanley about his M43 and was quite happy with his responses until he began posting Loads which he claimed were 100,000psi. stanley would always post it as 100,000psi(M43) to give it an air of credibility.

Fortunately my technical background and Strain Gauge experience allowed me to ask stanley questions which led me to believe there were a couple of problems with users understanding the accuracy of the data provided by a M43 "outside of a Lab environment", who do not have a Product Test background.(Any other HSGS as well.) And of course the questions eventually led to the real issues, correct and accurate measurement of the Chamber where the Strain Gauge will be attached, actual attachment of the Strain Gauge, the lack of an overall System Calibration with a "Known Pressure" and the necessary Fudge Factor(which I understand).

When I see someone post that they got 58,946psi using 63gr of XXX Powder with a 165gr Bullet, it is an indication they "think" they know the Pressure, but have been mislead into the level of accuracy. If everyones HSGSes truncated the last three digits and included a WARNING on the LCD that the reading has a Potential Variance of Plus or Minus X.5Kpsi, the Rookies would then be clear about the potential for inaccuracy. But I also understand that would negatively impact sales.

Over the years I've seen only two HSGS buyers who understand why the proper System Calibration is important. One actually called Hornady and asked for specific Pressure information on a "Lot" of ammunition he was using. He told us here at AR, that the Quality folks at Hornady actually gave him the Data from their Laboratory Testing, which both surprised me and made a lasting good impression about the excellent people Mr. Joyce Hornady left behind. The other person is a rather unique character. I know he has mentioned "shooting his truck" on occasion, he can doctor folks(perhaps Mules too), represent them in court, is a competent wordsmith, is an excellent source of info on semi-auto firearms and I know him as JCN. But he posts as the highly esteemed "Lawndart".
---

An improperly Set-Up, Non-Calibrated HSGS is Full-of-Beans. But they are excellent "Test Instruments" for the Industry when the Set-Up is done correctly, the Strain Gauges are applied properly and the entire System is Calibrated with SAAMI Calibration Ammunition(which has a Known Pressure and allows for a proper Pressure Adjustment or Fudge Factor.)

Good Hunting and clean 1-shot Kills.

quote:

Lawndart, your comment that "math to internally calibrate is pretty straightforward (thank you Mr. Powley)" caught my attention. I'm curious to what math you're referring and how you hope to use it. I'm familiar with only a small portion of his work, but I'm always eager to learn more.

Hi asdf,

My brain is mush right now (too many patients with pneumonia today). I have PM'ed you a copy of the original technical memorandum. I'll attempt to talk through the concept and math here:

We express the pressures in a rifle as pounds per square inch (force per unit area). That is equal to mass times accereration divided by unit area (square inches).

Here mass equals the mass of the bullet plus 1/2 the powder charge (i.e. bullet weight plus 1/2 of the powder weight divided by g.

Multiply the area under the time pressure curve by the area of the bore. Divide that result by the mass. That gives a calculated velocity of the projectile as it exits the muzzle.

Compare the calculated velocity to the measured velocity. The ratio of observed muzzle velocity to the calculated velocity provides a correction constant. Multiply the pressures given by the machine at any point on the curve by the correction constant. That will give you the true pressure at that point.

The good news is that we don't have to construct a bunch of pressure time integrations manually these days.

I'm sure the above paragraphs are as clear as mud. If I get some time in the next few weeks I'll make a power point presentation that goes through the physics and math step by step. Big IF this time of year.

Does anyone know if there is a compilation of all of Mr. Powley's technical memorandas? That would be a great read.

quote:

Originally posted by Hot Core:
Dr. Oehler, Actually I think we are closer to agreement than most people would expect. ...

Hot Core,

Many thanks for the response. I'm looking forward to continuing the discussion, but grandsons are coming to our house for spring break, and they get top priority for a few days. They are at the airgun stage, and might even get to handle a .22 if they can keep their heads in the game.

Obviously pressure measurements is a subject I don't take lightly and will try to return a considered response.

KenO

quote:

Originally posted by KenOehler:
Short of a M43, we feel that the typical reloader is best served by limiting velocities to factory levels, limiting charges to book maximums, and not seeing any of the classic pressure signs.

Excellent quote.

Hi lawndart

Have a look at this link http://www.deva-institut.de/ if you got problems with the German language let me know I will try to translate it they a basically government approved and international accepted institution.
The work they do is excellent and should also cover liability risk you could face. Cost seams to reasonable http://www.deva-institut.de/Gebuehren.htm

They got the pressure barrels you need see http://www.deva-institut.de/l_messl.htm

Hope this helps and good luck with your plans

quote:

Originally posted by KenOehler:
...Many thanks for the response. I'm looking forward to continuing the discussion, but grandsons are coming to our house for spring break, and they get top priority for a few days. They are at the airgun stage, and might even get to handle a .22 if they can keep their heads in the game.

Dr. Oehler, Certainly agree with the Grandsons being a high Priority. I always look forward to being around good children.

quote:

Obviously pressure measurements is a subject I don't take lightly and will try to return a considered response.

Previous discussions on HSGSes at AR have typically degraded into personal attacks. It is a great honor to be able to discuss this with you. If you see something in my thoughts that you know to be incorrect, I would appreciate your insight.

Good Hunting and clean 1-shot Kills.

Thanks for the reply, lawndart. Your explanation is clear enough, and the article sent in the PM adds what I should have guessed, namely that one begins with a strain gauge trace of the pressure and then corrects the "height" of this curve to make its integral correspond to the measured velocity.

One problem I see is that it ignores the work of engraving the bullet and dragging it along the bore, work that is supplied by pressure beyond that required to accelerate the bullet and charge. QuickLOAD gets some pretty decent fps estimations even though it pretty much ignores such work, so perhaps it is minor. You can contact QL's author by e-mail, and I suspect he'd give you some estimates of these effects, based on his research. His contact information comes with the software. Steve Faber, who marketed an early strain gauge meter for shooters, has written a simulator more detailed than QL, and it included a bullet engraving model, so he's done some reasearch into this as well. His web site is here.

QL's author also cautions the accuracy of his simulations is hampered by not knowing how much pressure is expended in accelerating the partly burned charge out of the case. Differences in straight walled vs. bottleneck cases are thought to exist and be significant. (Reading the manual to QuickLOAD is a good introduction to the math of internal ballistics, I feel.) Powley's assumption the average charge velocity is 1/2 that of the bullet's may not be valid in the early stages of the burn, especially with straight walled cases.

Another problem is one Ken Oehler has pointed out on another forum, namely, with a pressure trace, one cannot readily detect when the bullet exits the muzzle. This would make it difficult to decide how much of the trace to integrate over. You might be able to add some external sensor to detect the bullet passing the muzzle, but this can be tricky as well. Tying the chronographs first sky screen timing mark to the pressure trace would do the trick, but this signal is not usually available. RSI's software tries to estimate bullet exit, but I don't know how accurate their calculations are.

You might try sweet-talking good Mr. Oehler into giving you some traces of strain gauge readings vs calibrated piezo readings and see if the technique Powley outlines works in practice. Now, Powley wrote this up in the 1990's, so he might have had a chance to investigate this. Still, I would be cautious.

As for your Universal Receiver, as KenO. has more or less hinted, the T/C Encore makes a pretty cheap receiver to mount a barrel suitable for strain gauging any reasonable sporting cartridge. The trick is getting the bore and chamber cut to SAAMI specs, but I bet one of the aftermarket T/C barrel suppliers, such as Bullberry or SSK, would be willing to use whatever barrel and reamer you supply.

I believe all of Homer Powley's papers are in the possession of his friend Ken Howell. You can get his contact details from his user profile (he goes by his full name) at the forums of www.24hourCampfire.com, and I'll PM you a copy of these details. He's been off-line for a few weeks, reported to be a problem with his PC. He hopes to compile these into a book, but he has a pretty full plate. I hope to visit Howell and get copies of a few of them for publication on my web site. Powley published a lot of useful information over the years.

Karl

quote:

Originally posted by asdf:

Another problem is one Ken Oehler has pointed out on another forum, namely, with a pressure trace, one cannot readily detect when the bullet exits the muzzle. This would make it difficult to decide how much of the trace to integrate over. You might be able to add some external sensor to detect the bullet passing the muzzle, but this can be tricky as well. Tying the chronographs first sky screen timing mark to the pressure trace would do the trick, but this signal is not usually available. RSI's software tries to estimate bullet exit, but I don't know how accurate their calculations are.

I'd like to spend more time on this thread, but as I told Hot Core, my grandsons just arrived to spend the week. Got to keep priorities straight.

It just so happens that the Model 43 puts a tic mark on the pressure-time curve at the point of bullet exit. The time of this mark is determined by the distance from muzzle to first screen, velocity as measured, and the time the bullet arrived at first chronograph screen. The M43 actually computes the area under the curve from fire to muzzle mark and displays the result in "power factor" units. Knowing muzzle velocity and bullet weight, we know momentum of bullet; the area under the curve is the input "impulse" and rocket scientists seem to be fond of equating impulse and momentum. Fifteen years ago I thought that this might be a good way of calibrating the pressure curve. It turns out that there are still too many unknowns in terms of friction, etc.. As I recall, the ratio of output momentum to input impulse typically ranged from mid 60% for typical bottleneck cartridges to mid 80% for the old straight-walled loads. That appeared reasonable at the time. It is a good gut check, but I wouldn't dignify it by calling it a calibration.

I'mm looking forward to continuing a discussion with Hot Core, but hope to spend more time on it than I have right now.

KenO

For the benefit of others reading this thread, I really should have outlined the method Powley wrote about, which was written up in an issue of Handloader magazine. While in my earlier post I referred to it as "Powley's", Powley gives credit for it to a fellow at Remington, back in the '50s.

You try to convert the pressure-time trace into an acceleration-time trace. Multiply the pressure by the cross sectional area of the bore to get the force on bullet. Now divide this force by the mass being accelerated. For the mass, use that of the bullet plus one half the charge weight (ie., assume the average velocity of the charge is one half that of the bullet's). Since F=ma, dividing F by m gives a, or acceleration. Integrate acceleration (find the area under the curve), and you get a "muzzle velocity." The ratio of this velocity to that chronographed was taken to be the calibration error of the pressure sensor.

There are several errors in this chain of thought, some of which I mentioned in my earlier post. KenOehler many years ago, then, found out the errors are too great to ignore.

freischuetz,

Thank you for those links. There is no getting around the need for me to learn to speak read and write German in the next few years. I do have some help in that arena. There is even a book that translates the specific words and terms used in the ballistics, firearms and ammunition arenas.

asdf,

Thanks for cleaning up the turgid prose I inflicted on everyone a few post up in this thread. I suppose the bullet-barrel friction and other issues are dealt with empirically by measuring the actual velocity. We spend a lot of time chasing the angels around the head of this pin. Thankfully the natural and normal several hundred PSI difference between consecutive shots greatly overshadows our assumptions and estimations. If I ever have to substitute teach a physics or calculus class, I will give this example as a homework assignment.

Dr. Oehler,

My thanks for the privately conveyed information. I read that three times last night. In the quantum world we chase Schrodinger's cat around. In the Newtonian world of ballistics we are chasing the Oehlerian constant that will unify the three methods of pressure measurement: strain, gas transducer and conformal transducer. That search is your burden in this vale of tears .

If anyone out there has a set of blue prints for a universal receiver, please let me know.

Here is my poor mans pressure testing rig:

A Remington Sendero in 270 Win gave its life for this venture.


The bolt is faced off. Empty cases have to be poked out from the front. The upside is that I can test cartridges up to the 470 NE in size.


The shank portion of the barrel is turned down to a uniform 1.200" in diameter. It is easy to glue on strain gauges in situ.

Best use for a 700 that I have ever seen. Nice tool.

Yep,

I tried using it in the garden, but my tomato plants all died .

JC

If you grind a small groove in the breech just opposite the gas vent hole, you can reach through with a tiny screw driver and pluck out the case. Sometimes my "muzzle loading" the barrel can freak out the range officer if he doesn't know what I am doing. Also a good secondary pressure test. If the tiny screw driver doesn't get it out, the pressure is too high for the case, no matter what the computer says. I've ran into that several times with Bell/Mast brass taken to the limit.

quote:

Originally posted by Hot Core:
I completely agree that the M43 is an excellent "Test Instrument"(your words) when Set-Up properly, Calibrated properly and used in a Laboratory environment with rigid documentation guidelines. I would be very surprised if we are in disagreement about that.the entire System is Calibrated with SAAM

The only thing I'll quibble about is your distinction between "home" and "laboratory".
For the Home Strain Gage System (HSGS=pyrite), you place much significance on contrasting a home system to a laboratory system. My observations do not justify that conclusion. To me, the most important consideration is the competence of the operator. I’ve seen excellent research conducted on a picnic table under a shade tree, or off the pick-up tailgate in a back pasture of the ranch. I’ve also visited very expensive professional ballistics labs with expensive professional equipment and a high-paid staff not really competent to test. I’ve seen instances of professional laboratory personnel ignoring the instructions and restrictions provided with the Model 43 and then arrogantly testifying to their incorrect results.

How about we compromise and let the “H†stand for “haphazard†instead of “home�

quote:

I do have a lot of experience with Strain Gauges and realize their use is not as straight forward as some folks would lead the Beginners/Rookies to believe.

There's nothing to disagree with here.

quote:

there is no "single" Pressure Detection Method or System that is "24 carat" or perfect. All have nuances and quirks ...

Again we agree. Some will be surprised when I state my opinion that of the commonly used laboratory pressure measurement methods (conformal transducer, gas transducer with drilled case, case-mouth gas transducer, or strain gage), none can demonstrate accuracy better than two percent and probably none are worse than eight percent. This sounds horrible, but it's probably five times the accuracy I would assign to pressure estimates made without benefit of instrumentation.

quote:

I recommend the use of every Pressure Detection Method available to Reloaders during Load Development.

I'm glad you called it Pressure Detection Method and did not call it a Pressure Measurement Method. My opinion is that the typical reloader can probably detect when his loads exceed 75K psi, but can't read rifle pressures under 60K psi (or certainly any of the handgun cases below 40K psi).

quote:

There are people who still believe that it is OK to just keep dumping in Powder until some arbitrary, randomly-selected velocity from one of their Manuals is reached.

Any time you exceed factory velocities you have probably exceeded factory pressures. Just because you are under factory velocity does not mean that you are under factory pressure.

quote:

When I see someone post that they got 58,946psi using 63gr of XXX Powder with a 165gr Bullet, it is an indication they "think" they know the Pressure ...

I agree. The SAAMI convention is to report pressures in hundreds of psi. They (and the Model 43) would report 58,946 psi as simply 589.

quote:

... the entire System is Calibrated with SAAMI Calibration Ammunition(which has a Known Pressure and allows for a proper Pressure Adjustment or Fudge Factor.

Here's where we probably part company. I know of no "SAAMI Calibration Ammunition" which has a know pressure. Such wonderful ammo just doesn't exist. I suspect that you are referring to SAAMI Reference Ammunition. This reference ammo is not intended for pressure calibration, but it's primary application is the determination of the "average" test barrel. Use of reference ammo by SAAMI goes back to the days of copper crushers when the actual calibration of the crushers was the construction of a "tarage table" showing apparent static pressure versus deformed length of samples from that specific lot of copper crushers. The reference ammo is simply a large lot of ordinary ammo that is well behaved and stable. Samples of the ammo are distributed to those SAAMI members who produce ammo of that caliber, and each member is responsible for the calibration of their own transducers (using static hydraulic pressure in a adapter fixture for the transducer and unfired cases from the sample lot of ammo). The ammo is tested at each SAAMI site, the data is analyzed, and the resulting average of the averages becomes the "assessed value" of that particular lot of ammo. In subsequent pressure testing at the individual labs, it is proper procedure to fire a sample of the reference ammo in a barrel to determine the "barrel correction" to be applied to convert the readings from that particular barrel to what might be expected from the mythical "average" SAAMI barrel. This whole procedure establishes two "fudge factors" for the conformal transducer, a slope of the electrical output versus static input pressure and an "offset" pressure related to the pressure contained within the case before it presses against the transducer. Note that each of these two fudge factors are valid only for the particular lot of brass used duing the calibration. The third fudge factor is determined by firing samples of the reference ammo in the test barrel to correct to the "average" barrel. The proper use of the "reference ammo" requires that all instrumentation be "SAAMI standard" and that all SAAMI test procedures be followed.

There is absolutely no guarantee that the reference ammo will generate the "assessed" pressure in any firing. The pressure may vary significantly depending on the barrel in which it is fired. Just having chamber and barrel dimensions within SAAMI specifications does not guarantee pressures equal to the assessed values. Take an extreme example of firing a round of .308 Win in a .270 barrel; do you really expect to see the assessed pressure?

My description is abbreviated and may not be clear; the process is laborious and complex. There is room for much uncertainty and error in the many steps. That's why I expect accuracy in the 2% to 8% range.

Why all the discussion about reference ammo? It is simply that I regard reference ammo as being inadequate for the calibration of any pressure measurement system, strain gage or otherwise. Reference ammo is useful for establishing the relationship between an individual test barrel, made to SAAMI specs, and the "average" SAAMI barrel. That's its intended purpose in life.

How do we properly calibrate a strain gage system? In my opinion, it is adequate to pay attention the the mounting location of the strain gage and make reasonably accurate measurments of the inside and outside diameters at this point. Properties of the individual strain gages (gage factor) are provided by the gage manufacturer with each lot of gages. The modulus of elasticity of modern barrel steel varies little between barrels with a relatively constant 3% difference between stainless and chrome-moly barrels. This difference can be accounted for in the test procedure. So used, I'd opine that the accuracy of the Model 43 probably falls in the middle of the 3% to 8% range. There's room in this band for a few small errors and approximations. It's hard to measure accuracy if nobody knows truth. We've seen such agreement in many tests over the years with special barrels fitted to measure pressure of the same shots with two or more methods on each shot.

quote:

... stanley would always post it as 100,000psi(M43) to give it an air of credibility.

I'll take the blame for the (M43) portion. Our instructions suggest reporting pressures measured with the Model 43 in this fashion. There are so many methods of measuring and estimating pressure that I usually ignore any reported pressure until I know the method used to measure it. The (M43) flag alerts the reader to any of the "nuances and quirks" associated with the Model 43, but also tells him that the reported pressure is much more than an guess.

Back to your "HSGS" --- I'd regard the final "S" to be critical. The "System" includes not only the strain gage and readout, but also the instructions and procedures. If the instructions, procedures, and limitations are not observed, the results are indeed fool's gold. This is true for many things, not just the HSGS.

KenO

quote:

JC

If you grind a small groove in the breech just opposite the gas vent hole, you can reach through with a tiny screw driver and pluck out the case. Sometimes my "muzzle loading" the barrel can freak out the range officer if he doesn't know what I am doing. Also a good secondary pressure test. If the tiny screw driver doesn't get it out, the pressure is too high for the case, no matter what the computer says. I've ran into that several times with Bell/Mast brass taken to the limit.

Thanks Mike,

I'll do that. I have been using take off barrels just to get a feel for several calibers. I had a bad dream recently about launching a Bore Tech cleaning rod down range . That still beats dreaming about walking into a final exam without having studied all semester. This is thirty years after the class in question; WTF???

PS I was remiss above in not giving credit for the design of the Remington Sendero strain testing assembly to Mike. He explained the design to me two years ago. It has saved me much $$, and has given very repeatable results ever since.

ncompetent

quote:

I’ve seen instances of professional laboratory personnel ignoring the instructions and restrictions provided with the Model 43 and then arrogantly testifying to their incorrect results.

Just a wild guess, but that might be our Famous But Incompetent friends.

The real reason I am determined to build a SAAMI/CIP spec facility is to see if I really can out spend my ex-wife for a few months.

Now that all the strain gage experts are here;
What ever happened to the mystery of the second
pressure pulse that occured after the bullet left the muzzle?
Take care!

Dr. Oehler, I've no problem with "Haphazard" nor "SAAMI Reference Ammo". Just read your post a couple of times and I do have a serious issue - about finding ANYTHING I disagree with. This is bad for my reputation.

So, perhaps I can find SOMETHING we can disagree on if you would give us your thoughts on:

1. How to properly measure the thickness of the Chamber where the Strain Gauge will be applied.

2. How to Calibrate the entire System without access to SAAMI Reference Ammo.

3. How to protect the "Bluing" on a fine firearm where the Strain Gauge will be applied.

4. Do you recommend just winding ALL the Strain Gauge Leads around the Scope and Duct Taping it in place when Hunting?

5. If we consider the "Range" of variance you mentioned, 3%-8% with an average of 5%, if we consider the worst case situation with a HSGS, and if the system indicates 60.0Kpsi, are you saying the actual pressure could vary plus or minus 4.8Kpsi for a "potential Range" of 55.2Kpsi <-> 64.8Kpsi? But for most users who pay close attention to the Set-Up and follow the directions the average variance would typically be(5%) with a Range from 57.0Kpsi <-> 63.0Kpsi?

6. Please verify that there is NO direct conversion from CUP to PSI nor PSI to CUP. Haphazard use of any attempt at this is simply mis-leading to the end user and has the potential to get Beginners/Rookies into Pressure Levels that are best avoided.

7. I also look forward to your thoughts on Hawkins question. Mr. Sisk used to post here a bit, but I've not seen him post since he was repeatedly blowing the a Muzzle Brake off a rifle? Is that tied back to Hawkins question?
---

I will mark the Thread for future reference. No way to explain how good it is to have you on this Board. The Beginners/Rookies that join the site have a wealth of first-hand experience to draw from, and your insight makes it even better.

Welcome Aboard!