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Old February 4, 2018, 06:53 PM   #1
stagpanther
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Pressure Trace

Anyone here actually used it--and what are your thoughts? Are there any practical ways for attaching the senors to an AR?
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Old February 5, 2018, 08:40 PM   #2
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Well, I took the plunge today after a long and interesting discussion with Jim and ordered one--should be very interesting learning about gun fizzics--never my strong point.
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Old February 5, 2018, 09:29 PM   #3
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I look forward to your results.

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Old February 5, 2018, 09:42 PM   #4
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Biggest challenge right off the bat will probably be figuring out where the sensors can be attached on an AR. Learned some remarkable things about ammo, SAAMI compliance etc that was a bit of an eye-opener for me. I'm about as far away from being an engineer as anyone can be--so I'm guessing it's going to be a looooong learning curve for me.
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Old February 6, 2018, 08:12 PM   #5
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I suspect you'll have to pull the hand guard off to place the little sensor. Maybe you could put a little pigtail on it and fish it out through and then replace the hand guard?
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Old February 6, 2018, 08:38 PM   #6
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The real problem is that best results are recorded when the sensor is close to parallel to the bore--and ideally in the area around mid case body. In most short AR 15 cartridges that will be under the receiver collar and behind the barrel nut. Even with AR10 platforms--which is initially what I'm getting it to test--it looks like I might be able to position the sensor over the case neck since the chamber cut does seem to clear the barrel nut. The way I understand it--accuracy is enhanced by having the sensor over a relatively large area of continuous metal thickness. Another issue is stainless steel--apparently the formula and hence expansion characteristics of stainless varies widely--(and at least half my barrels are stainless) so I'll need to get a known baseline pressure cartridge to extrapolate adjustments. Going to be an interesting learning experience--with I'm sure lots of goof-ups along the way, because....
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Old February 8, 2018, 04:46 AM   #7
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This is waayyyy outside my wheel house.

But good for you for trying it.

I hope you know that I, and I suspect a bunch of others, are living vicariously through your experience and really want you to keep posting on how this works out.

You should probably figure out a way to charge us for the experience.
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Old February 8, 2018, 05:17 AM   #8
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Whoa !!! Slow down there, Dale!!

I'm as interested as the next guy. Till it hits my wallet.. :

Others i talk with on another forum use the Pressure Trace.
It is a very valuable tool, especially when loading wildcats.

I'm still hoping one day to own one.
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Old February 8, 2018, 07:51 AM   #9
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I'll try my best--but don't get your hopes up too much--I would characterize my level expertise as "average" at best--so I'm sure there's going to be a lot of trial and error, plus a whole lot of fizzics learning which I'm not especially good at. Took me a bit of time to come up to speed with labradar--but have since found it to be a great tool well worth the money (it only has a couple of weak spots IMO--linear comps that send the muzzle blast straight forward--and the borderline between high velocity pistol bullets and low velocity rifle bullets). Also--winter conditions here make outside testing difficult--I'm guessing that very low temperatures might throw the results a bit--I've seen some pretty significant changes in bullet velocities once the temperatures approach 0 degrees. Those are my excuses and I'm sticking to them. But yeah--I'm really excited and curious to see "real world measurements" instead of guestimates. I've been told to prepare myself for some interesting revelations on the nature of ammo manufacturing.
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Old February 8, 2018, 10:02 AM   #10
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I am following this with interest. I did notice the $..$
They sell the pressure trace, software, and chronpgraph, either piecemeal, or in bundles.
Did you get a "package"? Or, just the pressure trace?
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Old February 8, 2018, 10:24 AM   #11
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I already have a labradar--which they say can interface with the pressure trace--so I got the "intermediate complete package with everything excluding their chrono for around $800. I also have QL and don't necessarily need their external ballistics package--but I figure I'd try it out out of curiosity. So, you can get everything that you really need for just the chamber pressure analysis for a couple of hundred smackers less
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Old February 8, 2018, 11:31 AM   #12
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Thanks. I will be reading this!
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Old February 8, 2018, 12:02 PM   #13
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Quote:
especially when loading wildcats.
Yeah--there's only so much prediction software can do
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Old February 8, 2018, 12:04 PM   #14
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Let us know how the results go. Also while I am sure they will provide in depth information on strain gauge placement and how to surface prep is extremely important before placing the gauge on the surface.

Ron
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Old February 8, 2018, 02:31 PM   #15
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Quote:
surface prep is extremely important before placing the gauge on the surface.
read about that already--have to keep it under pressure. with no bubbles while it sets--that should be fun.
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Old February 8, 2018, 02:35 PM   #16
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The barrel which I intend on using this on is a custom 1-off production which might be a couple of months or more in the making--so I'll see about taking apart one of my other ARs to learn on.
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Old February 8, 2018, 03:59 PM   #17
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I've owned the pre-Bluetooth pressure trace for a number of years. It is valuable. I think the wireless feature of the new version will simplify data gathering as mine has to go through an RS-232 to USB converter now, as well as having the wires run about.

The NATO EPVAT test method measures pressure just past the case mouth. The main drawback for using a strain gauge that far forward is the standard barrels have a shoulder contour there that is not cylindrical to glue to. You need the gage to lie flat on the steel with a thin glue line and with the long stripes of the gage foil paralles to a circumference line (perpendicular to the bore axis). The built-in gage equation solves for cylindrical steel. So if you want to use an AR rather than a bolt gun or an Encore, as I got for quick test barrel changing, a heavy barrel with cylindrical contour is better. You can get one in chrome-moly steel and do all your testing in it and avoid the whole steel modulus issue. I've measured .45 Auto rounds in a gaged Encore heavy barrel I had cut custom chambered for the purpose. It detects pressure through that thick thing, so it will be fine with the AR heavy barrels.

Incidentally, you can look up the modulus of elasticity for your type of steel on Matweb.com, and ratio it with that for 4130 and 4140 steel (they are the same 29,700 kpsi) commonly used in barrels. Divide your steel's modulus of elasticity by that number and use the result as a multiplier for the gage factor to create a correction.
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Old February 8, 2018, 04:31 PM   #18
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Quote:
read about that already--have to keep it under pressure. with no bubbles while it sets--that should be fun.
On that note I applied pressure using a piece of surgical tubing. I don'y know what they give you. Same stuff that works well for stock repairs.

Ron
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Old February 8, 2018, 04:37 PM   #19
stagpanther
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Quote:

I've owned the pre-Bluetooth pressure trace for a number of years. It is valuable. I think the wireless feature of the new version will simplify data gathering as mine has to go through an RS-232 to USB converter now, as well as having the wires run about.

The NATO EPVAT test method measures pressure just past the case mouth. The main drawback for using a strain gauge that far forward is the standard barrels have a shoulder contour there that is not cylindrical to glue to. You need the gage to lie flat on the steel with a thin glue line and with the long stripes of the gage foil paralles to a circumference line (perpendicular to the bore axis). The built-in gage equation solves for cylindrical steel. So if you want to use an AR rather than a bolt gun or an Encore, as I got for quick test barrel changing, a . You can get it in chrome-moly steel and do all your testing in it and avoid the whole steel modulus issue. I've measured .45 Auto rounds in a gaged Encore heavy barrel I had cut custom chambered for the purpose. It detects pressure through that thick thing, so it will be fine with the AR heavy barrels.

Incidentally, you can look up the modulus of elasticity for your type of steel on Matweb.com, and ratio it with that for 4130 and 4140 steel (they are the same 29,700 kpsi) commonly used in barrels. Divide your steel's modulus of elasticity by that number and use the result as a multiplier for the gage factor to create a correction.
Great info--thanks for that--I know I can move the sensor further out from the case body--but as I do so the total mass of adjacent consistency declines--and the change in diameter from that of the chamber must be mathematically compensated for--or that's my rudimentary understanding from my initial conversation with Jim at RSI. He told me about matweb but said that stainless is so inconsistent in formulation it's hard to come up with a reliable baseline. Anyway I hope you won't mind dropping in and giving me pointers as I go down the path; )

My first blush survey of my barrels is that there seems to be no rhyme or reason to where the taper is, meaning it seems to be more a function of the particular manufacturer's barrel cutting scheme--so the chamber cut could be unpredictable in relation to it--I/m hoping the AR10 builds give me enough to work with in front of the barrel nut
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Last edited by stagpanther; February 8, 2018 at 04:44 PM.
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Old February 9, 2018, 12:52 PM   #20
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I think maybe he means there are enough different stainless alloys used that you can't make a generalization about it. But if you know the alloy and it is within tolerances, you should be able to get a usable number. No question the chrome moly barrels are more consistent, though. It's because iron is a much larger percentage of the total alloy in them so that small errors in alloying have a lower net effect on the modulus property.

A lot of AR barrel contours are somewhat arbitrary, as you've found. If you have access to a lathe, you can change one. Force a 3/16" wood dowel into a case flash hole and with the bolt and carrier and handguard removed, set the case all the way into the chamber. Look through the ejection port at it as straight from the side as you can, and put a pencil mark on the dowel where it is flush with the front edge of the port. Then withdraw the dowel and case and line the mark up with the outside of the ejection port with the case forward so you can then see exactly where the case mouth is on the outside of the barrel and mark that. This will give you an idea where the gage can go and where you can safely remove metal to form a cylinder.

You can also gage a straight taper, but the formula for allowing for wall thickness in the software won't work as well. To do that, you would want to use something like a reference load to achieve calibration.

Applying gages with a superglue you really just need to start at one end of the gage and roll it on with your thumb through some release paper and hold it there until the glue has set. I was taught that by an engineer who did aircraft landing gear stress analysis with strain gauges for the military. It works fine.
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Old February 9, 2018, 01:57 PM   #21
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Thanks for that very useful information nick. When you say "carefully form a cylinder of metal" do you mean machine a level area on the taper cut so that the strain gage can be applied there and lies level (parallel) to the bore?
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Old February 9, 2018, 02:41 PM   #22
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Yes. If you look up Hatcher's experiments bursting guns, you will see he could not burst a Springfield '03 barrel in .30-06 until he got the steel about 1/16" thick over the chamber and bore. You can almost always take a little metal off a modern contour without a pressure consequence. Accuracy consequences are another matter, however. I would not expect Hatchers thinned barrel to have been a tack driver, even before he made it thin enough to pop. Steel is pretty strong stuff.
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Old February 9, 2018, 03:05 PM   #23
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Everything just arrived today from RSI! I just opened up the box and now my whole house smells like an ashtray full of cigar butts.
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Old February 9, 2018, 04:39 PM   #24
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Heh, heh, heh! Isn't it funny how tobacco smells are all becoming strange to us now? I'll have to break out a cigar. Been a year or so.
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Old February 9, 2018, 05:08 PM   #25
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I never was a smoker--the cigarettes and odd joint or two I tried in high school to look cool never caught my fancy--thank God.
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