Ackley's post got my attention with the discharge pressures he was talking about (and how easily you can reach max), particularly since he is a double E. I tend to press the envelop a touch to reach the node with the highest muzzle velocity (though this may not be the most accurate node), and his discussion about reaching max pressures by playing with loads is a concern, particularly since my bolt lever hasn't been rising so easily after about four shots through my Lapua 6.5x47 cases. So I am interested in hearing from you all about how to use a strain gauge, how to use the resulting data, and where to get one to get a better, more reliable handle on the pressures. Geez, another piece of data.
How do you use a strain gauge, and where do you get reliable one?
- Thread starter Cassidy
- Start date
All good questions.
Unfortunately, for us reloaders, there is no real competition to the current offerings. Thus, the cost to acquire accurate pressure reading is going to be relatively high [read $500].
I only say this, because I have worked in my profession with strain gauges, analog devices such as pressure, vibration, temperature, current, and flow devices; both input and output variations.
I specialize in PLC [Programmable Logic Controllers]. Industrialized microprocessor based systems with specialized software [Ladder Logic with some complex math functions] which allows users to control a litany of processes = read factory automation and process control.
Therein lies the problem, capturing the data is all well and good, how to interpret the data, manage the data, analyze the data, and finally output the data [read graphs, operator interfaces, GUI's, screens,etc] is where the "fun" and cost begins.
I know and understand that strain gauges typically output at the millivolt level; an amplifier is required to "step it up" to a more manageable 4-20mA level [there are converters available].
Without getting to technical, a given load in your chamber will stress the barrel a certain amount. The strain gauge will "deflect" a certain amount via the electrical bridge network, this certain amount will equate to a pressure level over time.
As someone noted in the other thread, challenges are presented in calibrating the strain gauge to the barrel [steel] etc.
The bottom line, I have reloaded for half of my adult life [that is 25 years]. Pressure and the ability to read it correctly seems to be our last bastion in reloading. Sure, the basics have not changed in my lifetime.
I always assumed as everyone else...sticky bolt, flattened primers, case head expansion by 0.001" was always a sign.
However, as an engineer I always questioned, by " how much?".....
So, unless I can really justify spending $500 on the system, I suppose I will "trust" what has worked up to this point in my reloading career so to speak.
Unfortunately, for us reloaders, there is no real competition to the current offerings. Thus, the cost to acquire accurate pressure reading is going to be relatively high [read $500].
I only say this, because I have worked in my profession with strain gauges, analog devices such as pressure, vibration, temperature, current, and flow devices; both input and output variations.
I specialize in PLC [Programmable Logic Controllers]. Industrialized microprocessor based systems with specialized software [Ladder Logic with some complex math functions] which allows users to control a litany of processes = read factory automation and process control.
Therein lies the problem, capturing the data is all well and good, how to interpret the data, manage the data, analyze the data, and finally output the data [read graphs, operator interfaces, GUI's, screens,etc] is where the "fun" and cost begins.
I know and understand that strain gauges typically output at the millivolt level; an amplifier is required to "step it up" to a more manageable 4-20mA level [there are converters available].
Without getting to technical, a given load in your chamber will stress the barrel a certain amount. The strain gauge will "deflect" a certain amount via the electrical bridge network, this certain amount will equate to a pressure level over time.
As someone noted in the other thread, challenges are presented in calibrating the strain gauge to the barrel [steel] etc.
The bottom line, I have reloaded for half of my adult life [that is 25 years]. Pressure and the ability to read it correctly seems to be our last bastion in reloading. Sure, the basics have not changed in my lifetime.
I always assumed as everyone else...sticky bolt, flattened primers, case head expansion by 0.001" was always a sign.
However, as an engineer I always questioned, by " how much?".....
So, unless I can really justify spending $500 on the system, I suppose I will "trust" what has worked up to this point in my reloading career so to speak.
Let's suppose I could put togather my own system
Strain gauge outputs a millivolt signal to an amplifier/conveter which output a 4-20mA signal to my Analog Input Module on my PLC.
I have the software on my work laptop computer to actually make MEANINGFUL conclusions about the data. AND actually show meaningful graphs of the pressure trace.
4mA would represent a ZERO pressure level
20mA would represent the Full Scale = 60,000 PSI
let's say, 0 - 60,000 PSI
Could I get a strain gauge, amplifier/converter for less than $500. Probably so, I will have to do some more homework!
At this point, it would be a hobby project, the guys that have put together a fully functional system for $500 seems reasonable.
It would take me at least 8 - 10 hours to invest the time to "engineer" and program a solution.
Most engineering houses get between $100 - $150 per hour in the Midwest. So, really, $500 for a completely running system really isn't all that bad!
Strain gauge outputs a millivolt signal to an amplifier/conveter which output a 4-20mA signal to my Analog Input Module on my PLC.
I have the software on my work laptop computer to actually make MEANINGFUL conclusions about the data. AND actually show meaningful graphs of the pressure trace.
4mA would represent a ZERO pressure level
20mA would represent the Full Scale = 60,000 PSI
let's say, 0 - 60,000 PSI
Could I get a strain gauge, amplifier/converter for less than $500. Probably so, I will have to do some more homework!
At this point, it would be a hobby project, the guys that have put together a fully functional system for $500 seems reasonable.
It would take me at least 8 - 10 hours to invest the time to "engineer" and program a solution.
Most engineering houses get between $100 - $150 per hour in the Midwest. So, really, $500 for a completely running system really isn't all that bad!
CatShooter
260Ackley said:
The above fragmanets are the sum of it.
Even the industry (Olin-Winchester, et al) cannot measure pressure accurately.
We can come close, but the differences in steel, the the differences in gauges, plus the effect of hysteresis (strain tapes over steel are bad, but copper plugs were worse) means that we cannot get accurate measurements... we can only come close - maybe +/- 10%.
Some years ago, SAAMI made up a batch of test loads - all the same lot of brass, same lot of primers, lot of powder, etc... and sent some to every manufacturer and test lab in the country, for a pressure test of the cartridges. The returned answers ran from ~42,000 Kpsia, to ~61,000 Kpsia (or thereabouts)
But is it really necessary for any reason, other to satisfy the curiosity of the anal compulsive, and the Mensa intellectual, that we have pressure measurements, other than relative/comparative?
What difference does it make if our loads are actually 48,000 Kpsia, or 53,000 Kpsia, or 63,000 Kpsia? None to us. Many benchrest and match (and varmint) shooters are running loads well up into the mid 60's, and the only effect is... well, not much at all!
The only effect of pressure (assuming decent design mechanical), is the level of pressure that our system begins to fail, that that "point of failure" is the case.
But the case is not uniform, so the failure point varies between lots, and more so, between makers.
So it makes absolute sense to use the signs of the failure point, to be our guidance in determining pressure limits, and not an abstract number, who's lineage and parentage, is more than somewhat suspect.
First off, calibration to a specific pressure is going to be a nightmare...probably a rouh estimate at best. But for measuring data such as time and pressure/time waveforms, I believe the pressure trace system would be awesome.
Then you could do batch samples for repeatability, etc. you could "see" what the waveform snapshot of a good accurate group looked like. Primary and secondary pressure spikes etc.
My primary job is in Vibration analysis and I am working in how to implement that to testing my rifle. I even have a ultra slow motion camera but have just not had the time to set it up.
Then you could do batch samples for repeatability, etc. you could "see" what the waveform snapshot of a good accurate group looked like. Primary and secondary pressure spikes etc.
My primary job is in Vibration analysis and I am working in how to implement that to testing my rifle. I even have a ultra slow motion camera but have just not had the time to set it up.
260Ackley said:
Considering that people will drop that amount for a new barrel, custom stock, reloading press, twice that for a sartorius scale, etc, etc.
A "Pressure Trace system" can not only be useful, it can also be a major safety tool for those that like to "fly too close to the sun".
BoydAllen
Gold $$ Contributor
I think that it is that "appetite to fly too close to the sun" that is the problem. A fellow that I used to know caused me to coin the phrase "chronograph idiot". Ignoring the fact that some barrels are significantly faster or slower than the average, he had the attitude that if someone published a particular velocity for a given caliber and bullet weight, that he should be able to match or even exceed that. Beyond that, there seems to be some danger that is created because we use a number system that is base 10, with the result that certain velocity levels have more psychological impact than others, for instance being over three or or four thousand FPS seem to be disproportionately exciting than say fifty FPS slower, and if a load is close to these magic numbers, can cause us to want to tip the can just a little more, to meet or exceed these numbers. This is similar to the significance we give to certain ages, turning 50 etc.
Generally, experienced loaders know what the signs of excessive pressure are, yet we may ignore them, pushing the case that we have beyond its reasonable limit, instead of accepting that limit and going to a larger case if it cannot safely reach our velocity goal. Ultimately, being able to have pressure readings will only be useful if we do what they tell us should be done, where to stop, although there are other useful things that are only related to this primary function, like having more than one pressure peak, or how consistent the pressure curve is. For example when Jerry Stiller was testing the striker assembly for the prototype of the Viper action, an Oehler 43 was used to check the consistency of pressure rise for various firing pin spring weights. Were it not for the expense, I would love to play one of the units that I posted a link to, but I would have trouble justifying the expenditure if it was only used for the few experiments that I might do. Perhaps the answer would be for a group of friends to invest in one that could be shared.
Generally, experienced loaders know what the signs of excessive pressure are, yet we may ignore them, pushing the case that we have beyond its reasonable limit, instead of accepting that limit and going to a larger case if it cannot safely reach our velocity goal. Ultimately, being able to have pressure readings will only be useful if we do what they tell us should be done, where to stop, although there are other useful things that are only related to this primary function, like having more than one pressure peak, or how consistent the pressure curve is. For example when Jerry Stiller was testing the striker assembly for the prototype of the Viper action, an Oehler 43 was used to check the consistency of pressure rise for various firing pin spring weights. Were it not for the expense, I would love to play one of the units that I posted a link to, but I would have trouble justifying the expenditure if it was only used for the few experiments that I might do. Perhaps the answer would be for a group of friends to invest in one that could be shared.
Seems like a useful tool, so thanks for the link!
I can certainly see the problem of calibration but outside of that and absolute pressure measurement, it seems the ability to measure and visualize the pressure curve would be extremely useful. For example, it would allow one to compare different primers or different batches of primers, the effect of a void i.e. less than a full case with specific powders. How a pressure curve looks like with different seating depths, neck tension and of course how it looks relative to other rounds if one has a flyer….
Will probably look into getting one – as Wayne knows, its kind of in my blood to measure! LOL! Now you've done it....
I can certainly see the problem of calibration but outside of that and absolute pressure measurement, it seems the ability to measure and visualize the pressure curve would be extremely useful. For example, it would allow one to compare different primers or different batches of primers, the effect of a void i.e. less than a full case with specific powders. How a pressure curve looks like with different seating depths, neck tension and of course how it looks relative to other rounds if one has a flyer….
Will probably look into getting one – as Wayne knows, its kind of in my blood to measure! LOL! Now you've done it....
Boyd has touched on a good use for the system...
I don't use mine to judge max pressure. The system is more of a comparitor. You can play with the variables and see how they effect the pressure rise, peak and the shape of the curve. Inputting the velocity from a chronograph will show you the bullet exit timing so that it can be compaired to an OBT chart. Now if this information is worth the cost of the system to you, then go for it!
I did buy mine a few years ago so it was a little less expensive, mine isn't the new and improved wireless version.
I don't use mine to judge max pressure. The system is more of a comparitor. You can play with the variables and see how they effect the pressure rise, peak and the shape of the curve. Inputting the velocity from a chronograph will show you the bullet exit timing so that it can be compaired to an OBT chart. Now if this information is worth the cost of the system to you, then go for it!
I did buy mine a few years ago so it was a little less expensive, mine isn't the new and improved wireless version.
bigedp51
BoydAllen said:
And this is one of the reasons why reloading manuals vary so much, they are using actual firearms and strain gauges. You then need a cartridge that was fired in a pressure test barrel to calibrate the strain gauge with a known pressure.
Then you add the variations in firearms and your better off letting the brass tell you when to stop.
Read the link below, the SAAMI interchangeability warning for the .223/5.56 did not come out until 1979 with the adoption of the M855 round.
5.56 vs .223 – What You Know May Be Wrong
http://www.luckygunner.com/labs/5-56-vs-223/
I keep looking at the "Piezo Transducer Max Recorded Pressure chart" in this posting and wonder why the green readings are so low and how they could be used as a base line pressure against military loadings.
As a side note, during weapons testing on Army Helicopters we placed strain gauges on the air frames of several aircraft. These aircraft were tested in a hover and at given air speeds and none of them had the same "zero" calibration reading.
The stress on the air frame was an average for where to add reinforcement due to stress. And old fashion direct pressure gauges in the cockpit worked better than electronic gauges because aircraft voltage varied with rotor speed.
I say your brass will tell you a better story on your firearm.
bigedp51
260Ackley said:
Quickload defaults to a H2O capacity lower than normal and needs actual H2O readings to be more accurate.
When loading with mixed brass in the AR you load with the default setting of Quickload for H2O capacity or use the cases with the least capacity. You are only shooting imaginary villains made out of paper at 100 yards or less.
Your shooting in inches of man and not minutes of an inch. You are not shooting from a bench and you shoot while moving and running between shooting positions.
It boggles the mind they hit the target, so just remember this...
Two in the chest and one in the head always makes your opponent very dead.
Below, mixed range brass, the brass is not trimmed, the primer pockets are not uniformed, the necks are not turned. The ammunition is reloaded and calibrated in inches of man.
With a scoped rifle the rules change and scoring zone at longer distances changes.
Looking over the informative website on PressureTrace (I learned a lot) provided by the first two repliers, I noticed that the software requires selecting the firearm being evaluated (apparently with known baseline pressure data) from a dropdown list, and also part of the software data output was FPS. Does this mean the firearms listed in the PressureTrace dropdown list are only those commercially available firearms with known chamber dimensions (not BR rifles with various parts from various cottage mfrs), and how does a pressure gauge assess FPS (which I assume is muzzle velocity) or is a chronograph integrated into the data collection.
An appealing aspect of this whole strain gauge concept is using these data to model-predict optimal loads (find nodes). However, recognizing much of this science is still theoretical (that's what the website said), I don't know if such model-predicting strongly correlates with smaller POI groups, i.e., I don't know if model-predicting loads is more efficient than the usual load testing approach and allows finding the optimal load sooner without shooting up all those test rounds (which I enjoy anyway).
Also, by using integrated statistical software with software like PressureTrace, though I am not a statistician (but have seen such stats models resolve multiple factors), I wonder if multiple regression statistical analysis modeling can help, with appropriately matrixed set of test rounds, determine optimal settings for the multiple factors involved (bullet selection, seating depth, powder selection, powder load), apart from the elephant in the room, wind.
Another factor which I have wondered about but have not seen much data or discussion on, is barrel length. If barrel length is an independent factor for POI group precision, maybe shaving a few ten thousanths off the muzzle might possibly produce a more accurate rifle if all the other factors are optimized. This idea came to me from information/results/groups size data I heard about using a micro adjustable length muzzle break in Florida - they micro adjusted the muzzle break - effectively the barrel length subjects to harmonics - to get much improved groups. How much of a factor is barrel length considered with these other factors in shooting tight groups.
An appealing aspect of this whole strain gauge concept is using these data to model-predict optimal loads (find nodes). However, recognizing much of this science is still theoretical (that's what the website said), I don't know if such model-predicting strongly correlates with smaller POI groups, i.e., I don't know if model-predicting loads is more efficient than the usual load testing approach and allows finding the optimal load sooner without shooting up all those test rounds (which I enjoy anyway).
Also, by using integrated statistical software with software like PressureTrace, though I am not a statistician (but have seen such stats models resolve multiple factors), I wonder if multiple regression statistical analysis modeling can help, with appropriately matrixed set of test rounds, determine optimal settings for the multiple factors involved (bullet selection, seating depth, powder selection, powder load), apart from the elephant in the room, wind.
Another factor which I have wondered about but have not seen much data or discussion on, is barrel length. If barrel length is an independent factor for POI group precision, maybe shaving a few ten thousanths off the muzzle might possibly produce a more accurate rifle if all the other factors are optimized. This idea came to me from information/results/groups size data I heard about using a micro adjustable length muzzle break in Florida - they micro adjusted the muzzle break - effectively the barrel length subjects to harmonics - to get much improved groups. How much of a factor is barrel length considered with these other factors in shooting tight groups.
BoydAllen
Gold $$ Contributor
I don't think that this tool has seen much use by top benchrest competitors, so it sounds to me like you will need to give the manufacturer a call to see what they have to say, and learn the rest if you buy one, by experimentation. Broadly speaking, tuners "work" but comparable results have been obtained by adjusting loads. One little anecdote...A friend had just built a large magnum that showed exceptional accuracy on its first accuracy. The powder bottle was getting low, so when he started loading for his next test, he grabbed the one behind it, since it had been his practice to arrange like behind like in rows. The result was a chamber swelling load of RL10 that the action survived, but which required that an inch and a half be cut from the back of the barrel. After that, he retested, but the accuracy was mediocre at best. Thinking that something in the barrel's metal had changed, he put the rifle away, until recently, when, for lack of something better to do he built and installed a muzzle brake on it. When he retested the rifle to evaluate the brake, its accuracy was back at fingernail size for three shots. Conclusion, barrel weight, length and distribution can have a marked effect on accuracy.
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