quote:

Originally posted by tnekkcc:

My " spider sense " tells me this one has had a bit much ...


Hotcore,

I agree that velocity measurement is not perfect, lot to lot variations etc. , as you mentioned.

When velocity measurement is combined with the other visual signs provided by the case , and any other observations made ( bolt lift , ETC. ) It seems to get close enough for general purpose guesstimation.
I admit that this method still involves a certain amount of guessing , that's why I refer to it as " spider sense ".

Thanks for sharing your measurement technique, that is a good way to measure a round object with a mic.


Travis F.

quote:

and no absolute way to calibrate (comparing to SAAMI ammo is a "relative" ...

SAAMI does specify ways to calibrate their preferred sensor set ups. The barrel is (with an empty brass case in place) pressurized hydraulically and a calibration curve made logging the sensor reading vs. the true pressure. Various labs offer hydraulic pressure sensors plenty accurate enough to 80 ksi and beyond. The only "if" here is does the piezo somehow fail to track the brief pressure spikes in a gun? I imagine SAAMI did enough tests decades back to give them confidence the crystals to indeed respond quickly enough.

The SAAMI ref. ammo is not used to calibrate the pressure sensors! It is used to qualify the pressure barrel as producing pressures and velocities comparable to existing SAAMI spec. test barrels.

A copy of the SAAMI specs isn't terribly expsensive (about $40), and you can get the ordering info from their web site. I believe their pressure spec method is more rigorous than CIP's simple ratios, but I think the CIP approach adequate.

Hey Travis, Then I do believe we are now in complete agreement. I understand what you are saying about the "spider sense".

When you do have a few spare moments, try measuring the Pressure Ring on a few cases. Then see how well it repeats.

And if you have a chance, post the results or PM me.

Really nice talking to you. Good hunting and clean 1-shot kills.

quote:

Originally posted by asdf:
SAAMI does specify ways to calibrate their preferred sensor set ups. The barrel is (with an empty brass case in place) pressurized hydraulically and a calibration curve made logging the sensor reading vs. the true pressure.

I would be curious to learn how they hydraulically pressurize a barrel to 80,000 psi. Vaughn (Rifle Accuracy Facts) did it to 15,000 psi but his setup was fairly complicated and somewhat risky, and 15,000 psi is not adequate to calibrate for a cartridge operating at 60,000 psi, and he had no way to calibrate his 15,000 psi hydraulic pressure gage. You could send the hydraulic gage out to a lab, but what if UPS drops the box en route to you?

I assume this hydraulic calibration takes place when the barrel is manufactured, and maybe once a year thereafter, if even that. The problem is, the calibration can drift, solder connections may fail, humans push the wrong button, etc.. If we are talking about a piezo sensor, there will be other electrical devices measuring the signal, and those other devices are also subject to calibration drift, connection problems, and operator error. It seems to me that it would be very desirable to calibrate or "qualify" the pressure barrel every day that you use it.

Which brings us back to the SAAMI ammo, and how reliable is it really, considering some of the wild data that has been published by companies supposedly following SAAMI procedures?

quote:

Various labs offer hydraulic pressure sensors plenty accurate enough to 80 ksi and beyond.

Yes, but how are they calibrated?

When I was in school we were taught to calibrate pressure gages with a "dead weight" machine that used a test weight sitting on a hydraulic cylinder to create a pressure that could be easily calculated. You can buy test weights that are tracable to an official standard, so that's about as direct and absolute and reliable as you are going to get.

Can the dead weight method be applied to 60,000 psi ?????? That would be a heck of a lot of dead weight, but I suppose it is possible. Does Hodgdon calibrate their pressure barrel all the way back to a dead weight every day that they use the barrel? It seems unlikely.

I don't calibrate my voltmeter every time I use it, but if its readings don't look right, I check it against another voltmeter. I don't calibrate my speedometer every time I drive my truck, but if it doesn't seem right, I can test it using the mile markers on the interstate and a watch. But if I were going to publish a reloading manual or manufacture ammunition for sale, knowing that I might
get sued if my data was faulty, you are darned right I'd calibrate and verify and qualify and check and recheck, especially if something didn't seem right.

And yet Hodgdon has put out some pretty wild data for the 357 mag. Somebody at Hodgdon wasn't on the ball. But that's another story.

My point is, day to day load testing procedures, even by the big companies, don't seem to be super reliable or super accurate, because of the inherent difficulty of measuring barrel pressure and the lack of a simple, reliable, and direct way to calibrate.

asdf, thanks for the insight. The SAAMI specs have been on my wish list for some time, but there's a lot of other stuff ahead of it. If SAAMI were really and truly interested in promoting safe shooting, they'd put the specs on the web.

quote:

Originally posted by popenmann:

quote:

Originally posted by asdf:
SAAMI does specify ways to calibrate their preferred sensor set ups. The barrel is (with an empty brass case in place) pressurized hydraulically and a calibration curve made logging the sensor reading vs. the true pressure.

Yes, but how are they calibrated?

When I was in school we were taught to calibrate pressure gages with a "dead weight" machine that used a test weight sitting on a hydraulic cylinder to create a pressure that could be easily calculated. You can buy test weights that are tracable to an official standard, so that's about as direct and absolute and reliable as you are going to get.

Can the dead weight method be applied to 60,000 psi ?????? That would be a heck of a lot of dead weight, but I suppose it is possible.

When you were in school, did they also teach any math? Yes, on a 1sq in piston you would need 60,000lb, however on a 0.1sq in piston you only need 6,000lb. Reducing the area under test is very common in materials testing, "dog bone" and .505 test pieces being the most common examples.
For annual calibration, the sensor (typicaly a load cell) and readout device are sent to a ANSI (formaly NBS) certified lab (primary standard) and put up aganst a dead weight machine. This combination is than certified as a "secondary" standard and can be used to certify the accuracy of testing machines. In the case of our test barrel, the secondary standard would be used to calabrate the "barrel sensor". Daily checks are done by placing a "shunt cal" resister across the signal leads and checking the display for repeatability of the # that the resister showed when everything was freshly certified.
Note: Some specs/names may have changed in the 15yrs since I built and used a "certified" load testing device (I used to calibrate tensil/compression machines)

popenmann, yes, dead weight testers are available to 80 ksi. As Tailgunner pointed out, you just use a smaller piston. I don't, though, believe a SAAMI test station uses a deadweight tester for calibration. Tailgunner has outlined a more practical approach, perhaps the very one used.

The SAAMI doc's don't give details on the calibration rig (other than the name of one manufacturer), but I imagine the muzzle is threaded on the outside to take a cap through which the oil flows. Again, the breech is plugged by a cartridge brass of the same type to be used in firing. The brass lies between the gases and the conformal transducer, so it is part of the calibration. Such calibration must be done each time the lot of brass changes, of course.

Reference ammo is used for day to day checks on drifts in the measurement system, both the pressure sensor and the chrono. (Note, SAAMI requires the use of accurate chrono readings with all firings.)

Ref. ammo. can be made in house. SAAMI ref. ammo. only is used to see if your new barrel is producing about the same ksi and fps as other SAAMI barrels at the other testing stations. In this way, you can get a quick check that your new barrel is correctly cut inside.

The homemade ref. ammo. is used to check for drifts. You build up identical ammo from the same lots of brass, bullet, primer, and powder and then fire several to see they are fairly consistent. Later, you can fire one of these samples to see if the ksi and fps readings are the same. If not, something has changed; exactly what cannot be determined by the ref. ammo. firings. Instead, you must recalibrate your pressure and velocity sensors to find where the error lies. If those cal. ok, then try SAAMI ref. ammo. to see if your barrel has slipped out of qualification, due to, say, a worn throat.

Yes, all labs do make mistakes eventually. Pressure transients of this magnitude are definitely a challenge. It is always best to look for test data from multiple labs. If one lab reports fps seemingly too good to be true, it probably is.

When I was in High School they told me that changing the piston diameter changed the force. The reason it worked was that the pressure remained the same.
Good Luck!

quote:

Originally posted by hawkins:
When I was in High School they told me that changing the piston diameter changed the force. The reason it worked was that the pressure remained the same.
Good Luck!

Same thing, different direction.

Force = area x pressure. This is the most commonly used one. IE with this pressure and piston how much can I lift?

Pressure = force/area. This is the formulia used when your trying to create/calculate a pressure. IE: if I put this weight on this piston, how much pressure will it generate/will I need?

Area = force/pressure. This is the formulia you use when sizing a piston to provide a given force with a known pressure. IE: You have a 3000psi pump, and need to lift 50ton, what size piston do you need?

Exactly, but how do you reduce the area of the test specimen when it is an existing chamber.
Vaughn was limited by the gague available to him. He relied on the linearity of the strain gage. When you measure a round hotter than your reference you are doing the same thing.
Vaughn's results didn't rely on absolute accuracy anyway, mostly they relied on repeatability.
good luck!

With a strain gauge mounted to the OD of the receiver (or a peizo rig), you pressurize the barrel. Pressurize to what ever pressure you feel will be 110-125% of chamber max. Using your calibrated (NIST tracable) load cell reading x the hydrolic force multiplier ratio (remember why we were discussing how to calculate pressure using a force and piston area?) you calibrate your barrel mounted SG rig.
Standard practice would be to calibrate it at 2 places (Low/High, say 50,000 and 75,000 psi) on the curve, but it can be done at as many places as desired/is practicle along the curve (say at 5 places, every 10,000 psi, from 40,000 to 80,000psi). BTW, the internal construction af a fluid pressure transduce is a SG mounted to a bridge (flexing member). To make a higher pressure T-ducer, they simply increase the section of the bridge. Substute the OD of the barrel for the bridge, and you have a working pressure system.
The hardest part is not the SG or calabration, but getting a high enough sample rate in the insturment, followed by the resalution of the A-D converter circuit.
You don't need to know anything about the chamber area, size or shape, because all you need to know is that this barrel expands x at P1 and Y at P2 (etc if your using more points)and as long as we are below the yeild point of the barrel steel, the change will be linier.