PressureTrace Users- How to calibrate?

[Fred_C_Dobbs]

So I have a chrono and I have QuickLoad. I also have access to a Pressure Trace and would like to use it to confirm and correlate my pressure reads and my QuickLoad data. But what I'm interested in is absolute values, not relative. My understanding is that extracting that degree of accuracy from Pressure Trace requires that I calibrate it to a known reference pressure in each different barrel.

Because of its consistency in manufacture, and because it's such a well-documented round, I figure FGMM is a no-brainer for my .308, but how about the pure hunting calibers? There's no match ammunition for them and I can find no manufacturer's pressure data for them.

So what home brew have you used to calibrate Pressure Trace? Is the powder manufacturer's published load data close enough to be an effective calibration tool?

 

[Unsichtbar]

Re: PressureTrace Users- How to calibrate?

www.frfrogspad.com/PTTIPS.htm

One of the most knowlagable people I can think of with the preasure trace system and direct experiance is Fr. Frog, his email is at the bottom of web page linked above, I would be suprized if he did not have the information you are looking for to calibrate.

 

[Fred_C_Dobbs]

Re: PressureTrace Users- How to calibrate?

No kidding? I've got most of Fr. Frog's website leached but I'd never noticed he's linked to shootingsoftware.com.

Thanks!

 

[Unsichtbar]

Re: PressureTrace Users- How to calibrate?

Glad to help, I would be interested in what you fine out.

 

[Fred_C_Dobbs]

Re: PressureTrace Users- How to calibrate?

Wow, Fr. Frog answered my E-mail already. The portion relevant to this thread follows:

"In using PT what most folks do is to try and locate the strain gauge at about the same place as the piezo unit would be, carefully measure all the parameters (a chamber cast is the most accurate), plug in the numbers, and then fire some factory ammo. You can then plug in a correction factor in PT to make the actual reading close to the SAAMI spec. This is close enough for "gummint" work.

"I recently did some development work for a heavy bullet MK262 equivalent 5.56 ammo which is spec'd at 57,800 psi (piezo) measured at the case mouth under gummint spec in a NATO chamber. With the gauge in about the same position pressures using GI MK 262 ran about 53-55 K on PT and I just dialed in another 3K and started load development. As long as there are no flattened primers all will be well. What I have done is to do initial load development with Federal primers to match the GI velocity and then switched to military primers (CCI #34), dropped back on the charge a little and then worked back up to velocity, and indicated pressure with the harder/hotter mil primer."

Not sure I understand the implications of what he's saying, but then I've not yet familiarized myself with the use of the PressureTrace. I suppose I should cast my chambers, then play around with the device.

 

[The Mechanic]

Re: PressureTrace Users- How to calibrate?

Don't you have to scrape down to bare metal to mount the strain gauge? Hate to mess up the finish on a good rifle.

 

[Fred_C_Dobbs]

Re: PressureTrace Users- How to calibrate?

No permanent modification to your guns is required.
Strain gage kits include special glue, cleaning swabs, teflon strips, instructions and gage(s) already soldered to a connection receptacle and plastic carrier. Unlike other systems PressureTrace does not require a ground connection to the firearm so there is no need to ruin the barrel's finish. Gages can be removed with acetone leaving no indication the system was ever installed.

 

[moosetracker]

Re: PressureTrace Users- How to calibrate?

I have a Pressure Trace and have not been too successful in getting it to work properly. I guess I'll have to do a chamber cast to get exact dimensions.

Also the Bluetooth function stops working in freezing conditions, so I guess I'll have to wait for the Spring to experiment further.

 

[Clark]

Re: PressureTrace Users- How to calibrate?

I can epoxy bond to the barrel, a CEA-O6-250UW-350 strain gauge.
I can solder together and bias up a Wheatstone bridge and amplify with AD8554 op amps [I think that Oehler used a 1NA128 amplifier which is not as good] in an instrumentation amplifier configuration.
That amplified output could go into battery powered storage oscilloscope for used at remote ranges.
I can do some math with Roark's book on stress vs stain on open ended tubes and Young's modulus for steel.
http://www.amazon.com/Roarks-Formulas-Stress-Strain-Warren/dp/007072542X

But when I tried to quantify the errors traceable to NIST [National institute of standards and technology] I ran into some problems.
1) The miss fit between the rifle barrel tube shape and the formula and the accuracy of the position of bonding, will render the measurements too inaccurate for absolute measurements without calibration.
2) One could calibrate his system with calibration ammo or factory ammo. Ammo shoots at different pressures in different guns, how do I quantify that?

While pondering this, I realized that absolute pressure does not matter to the individual handloader.

It matters a lot, if a guy wants to sell ammo for guns sight unseen.

But for my own strong rifles, all that matters is long brass life. If I could know the pressure with good accuracy, it would be a red herring, a distraction from the reality of the brass.

 

[The Mechanic]

Re: PressureTrace Users- How to calibrate?

A long time ago when you would measure lead with a tool that stuck a pointed needle into the lead and you would measure the depth with a microscope, I always wondered if you could take a piece of brass with a dent in the side (thought of this from the dent my HK91 made on the side of the brass) and put a piece of shot #7 1/2 or #8 on the side, fire it and measure the deformation of the shot. Never really gave it a try at the time as I was limited on time and had no way of verification of results.

 

[Fred_C_Dobbs]

Re: PressureTrace Users- How to calibrate?

Do any of you PT owners/users get what he's talking about when Fr. Frog says, "...plug in a correction factor in PT to make the actual reading close to the SAAMI spec..."?

From that statement, I take it there's a calculator function in PT that can interpolate from your data (including firing chamber dimensions and chrono numbers) and the manufacturer's MV claims for that factory cartridge and guesstimate the actual chamber pressure. Is that the case?

If that's not it, what do you make of that statement?

 

[kombayotch]

Re: PressureTrace Users- How to calibrate?

I calibrate my PT system using the following:

*The Lyman manual indicates pressure for it loads in PSI. Use this data to get you in the ballpark if it isn't a wildcat.

*Fire some factory ammo if it isn't a wildcat, these should fall within a certain pressure range, usually 56-58,000 psi in 308 Win., for example. Also puts you in the ballpark.

*Increase charge weight until you get pressure indicators on the cases and primers. This gives an indication of where the pressure is.

*Compare the results of several loads (different powders) to Quickload simulations. QL can be way off with some powders, which is why I check several.

*Fine tune the calibration by running an OCW test on several loads, using different powders, and aligning the OBT diamonds with the OCW nodes. If you read Chris Long's OBT paper, he derived the theory to explain Dan Newberry's OCW, and he actually wrote the OBT algorithms for PT. OCW nodes and OBT nodes are one in the same. This is the best method for fine-tuning the calibration. Once it has been done, I can usually align a load to an OBT diamond and it will be dead on an OCW node when I run an OCW test.

With the system matching up the OBT nodes to OCW nodes and giving me reasonable values for the pressure indicators on the cases, I consider the system calibrated.

In the end, you will never know that your calibration is exactly right. However, it doesn't have to be accurate to the psi. The usefulness of the system is that it allows you to know where you are relative to know safe loads. And, that really helps a lot during load development.

 

[kombayotch]

Re: PressureTrace Users- How to calibrate?

P.S. in cold weather use lithium batteries, not alkaline batteries. Lithium batteries function down to lower temperatures.

Here's and example OCW test:
http://www.snipershide.com/forum/ubbthreads.php?ubb=showflat&Number=1087404#Post1087602

Note the repeatability of it, even with different cases...

 

[Fred_C_Dobbs]

Re: PressureTrace Users- How to calibrate?

Excellent, kombayotch. And you're right, it's not so much the numbers I'm concerned with, it's determining as precisely as possible how hot I can go before I'm flirtin' with disaster.

I'm well-acquainted with OBT. In fact, I swear by it. I've had E-mail discussions with Chris Long about certain aspects of its use, mostly surrounding the effects of muzzle brakes and suppressors.

I wrote in the OP that I "have access" to a Pressure Trace. What I meant by that was that an acquaintance has one he's willing to loan to me (I have to buy my own sensors, of course). Since I intend being a good guest, I naturally need to do my business with it reasonably promptly and give it back to him. But it sounds like, best case, making use of the PT can be pretty time-consuming. That doesn't put me off because I'm willing to invest the time, but my concern -- and this stems from the fact that I haven't yet used one so I'm completely unfamiliar with its operation -- is what happens to my data when I give it back? Is there some method for storing my data off the device so if I borrow his a second time, or buy one of my own, that I can reuse the old data and won't have to reinvent the wheel?

 

[kombayotch]

Re: PressureTrace Users- How to calibrate?

You can save your data easily enough by hitting a print screen and pasting it into an image file. Just open all the info and calibration windows before doing this. The PT files can also be opened in Excel for additional processing.

What I wrote sounds more complicated than what it is. Calibration takes little more time than running the required OCW tests. I had PT calibrated for 6mm Crusader with an hour and a half of testing, which included OCW tests with the 115 DTAC and Retumbo, H1000 and RL-25.

 

[Clark]

Re: PressureTrace Users- How to calibrate?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Fred_C_Dobbs</div><div class="ubbcode-body">..it's not so much the numbers I'm concerned with, it's determining as precisely as possible how hot I can go before I'm flirtin' with disaster.
..</div></div>

What disaster?
You have to throw a case away because the primer pocket is loose?

 

[Jumper]

Re: PressureTrace Users- How to calibrate?

I've done some work with piezo sensors, enough to follow this thread anyway. To calibrate the pressure trace measurement one could load up a batch of rounds using a recipe that is as consistent as posible; i.e. low extreme velocity spread. Fire a set of these rounds through the barrel you wish to calibrate and record the values obtained by Pressure Trace. Take the second half of the sample rounds and sned them off to Hodgdon or Hornady (can't remember which company right now) and have them tested for pressure with their test barrels. Correlate your results with Hodgdon/Hornady's and you should have a factor to find absolute pressure. Just a thought...