To those of you who own one of these, how does one calibrate the system? I am interested in getting one, but if it cannot be calibrated I am skeptical about the accuracy of the "pressure" measurement.

Thanks for the information,
TEG

Hey TEG, I know there are a bunch of M43 users on this Board, so I'll kick this back up to the top for you.

Your concern is legitimate. Unless a person has access to the SAAMI Pressure Cartridges and their Conversion Charts, it does seem to be just a best guess situation using factory ammo.

Another concern is measuring the actual Thickness of the Chamber Wall where the Strain Gauge will be mounted. Unless the person has access to a CMM with a l-o-n-g mounted Ball Tip, it also seems to be another case of - best guess.

Oehler makes absolutely excellent equipment and the M43 is no exception. But, a person has to have access to the things needed to make the results Valid. For companies in the Industry, the M43 is a great tool indeed.

However, knowing the above limitations, I question the usefulness of the M43 by the average Reloader. Since they are having to "guess twice" on key elements for the Strain Gauge information to be accurate, it would be possible to be easily mislead.

Best of luck in getting some good answers that address these concerns.

[ 10-24-2002, 17:51: Message edited by: Hot Core ]

I have used the Oehler 43 on 6 or 7 rifles. For standard calibers,I typically use factory ammo as a baseline. That is more difficult for those calibers that may be used in a wide range of both old and new rifles. For those, factory ammo seems to be loaded to a lower pressure. All of my rifles are either Ruger 77s or Remington 700s, although I do have one Savage in the Lazzeroni Patriot cartridge. For wildcats, I use a combination of other pressure factors and a pressure limit somewhere around 60,000 psi. I have been surprised about the pressure readings from some handloads put together before I had the Oehler 43. They showed no pressure signs (primer cratering or flattening, hard extraction, etc.), but were still over 65,000 psi. I have found the Oehler to be very useful. The Oehler equipment is very good, the service topnotch. The documentation on the software could be better.

Logger,
Is the output of the machine "pressure" then? I admit to never using one. I am just wary of spending all that money on something I have concerns about the accuracy with.

Is there a calibration circuit that you run to verify the system is "reading" correctly?

Also, from you post it sounds as though you are using factory ammo as a reality check. Are you using the SAAMI specification for said factory ammo to establish a pressure corelation to the reading of your Oehler?

I appreciate all the information you can give me.

TEG

I believe the output from the machine is actually varying voltage, which is converted to "pressure" for the readout. As long as you have a baseline to work with (and I think factory ammo would work fine) then that output, no matter what it is, is going to be relative to the factory ammo. Think of it as a much more sophisticated and detailed (smaller measurements) version of measuring case expansion electronically and you'll have a pretty good idea of what's going on. The output is NOT an absolute pressure measurement by any means, but still a very useful tool. - Dan

Dan,

From what I have been told your comments are exactly correct. The strain gage does generate a micro voltage. This is amplified by a conditioning circuit. This amplified output is then "fitted" to a generic calibration curve and a "pressure" is displayed on the machine's read out.

As such, there is NO calibration possible with the Oehler 43. You can only get relative pressure estimates based upon a factory load baseline.
So you shoot a factory loaded cartridge and get a machine displayed pressure of 55000 psi. You then fire your hand load and get 60000 psi. This may or may not actually be 60000 psi. All you know for certain is that your hand load is generating more pressure than the factory load. The above was what I got from Oehler's technical service rep.

I guess to me, anyway, $900 is too much money for this kinda accuracy. I want the real deal for that kind money.

TEG

TEG = Todd E. = SRS = Axel

BULLSHIT ALERT!

The troll is back.

[ 10-25-2002, 20:22: Message edited by: Pecos45 ]

Folks,

To do better than this you have to go to something like a pressure gun with piezo electric pressure sensors.

I have a Clerke, It's sensors are linearly related to pressure from 0 to over 100k psi. The sensors are not inexpensive. The sensors generate a continuous signal.

The drive and acquisition electonics are not trivial and involve a significant little computer. An A to D converter is started and the round fired. Data is taken at 50 kilohertz with 16 bit resolution. The pressure is mapped from the time the projectile exposes the breech sensor until the bore vents to the open air.

The total force applied to accelerating the projectile is related to the total area under the pressure curve.

Dan,

I think the chamber thickness is measured and the reading is based on that measurement.

Mike

Just noticed Hot Core's post.

[ 10-25-2002, 20:40: Message edited by: Mike375 ]