16ga.com Forum Index
Author Message
<  16ga. Ammunition & Reloading  ~  Strain gauge or a Pressure Trace II system
MSM2019
PostPosted: Tue Jan 04, 2022 5:24 pm  Reply with quote



Joined: 04 Mar 2019
Posts: 1819
Location: Central ND

Brewster11,

For the Pressure Trace II it is part of the system.

From what I have been told, but I don't know this for sure, there are programs for Windows and you hook it up with a USB cable.....how that works is part of what I am looking for.

There are some professional labs, mostly centerfire, that are using the strain gauges instead of the piezoelectric systems, but I am sure they have programmers and IT guys that can do it in their sleep.

_________________
Mark...You are entitled to your own opinion. You aren't entitled to your own facts.
Back to top
View user's profile Send private message
MaximumSmoke
PostPosted: Thu Jan 06, 2022 12:44 pm  Reply with quote
Member
Member


Joined: 01 Dec 2005
Posts: 1550
Location: Minnesota and Florida

Brewster11 --

I'm a mechanical engineer with a master's degree specializing in mathematical and experimental stress and vibration analysis. I have had a lot of first hand experience with strain gauge work over the last 50 years. As Mark has said, the Pressure Trace II system does all the basically significant things for the user. That's what "makes" their product. The following shows what has to happen with resistant strain gauges, which, in their first commercial form of metal resistance element bonded to the item of strain interest (and hence stress, the most common and significant factor in material failure) were offered in the late 1930's:

As has been already said, today's strain gauge is most often a photo-etched grid (made like a tiny printed circuit board) of some alloy on a backing that facilitates handling and bonding of the grid to the area of interest. They come in all sizes, shapes, combinations, etc. depending on what measurement is desired and where.

The principle is simple -- if you stretch a metallic conductor within certain limits, it's resistance goes up fairly linearly. Typical garden variety strain gauge grid alloys have a characteristic change of resistance with strain (think of strain as the proportional amount of "stretch") of 2 ohms per ohm across the grid for every unit of strain. Strain "units" are unit-less -- they are simply an indication of proportion of "stretch" of a given length, therefore, inches per inch, or mm/mm, or furlongs per furlong, etc. So that proportion constant characteristic mentioned above is a sensitivity to strain (stretch) called "Gauge Factor", usually expressed as ohms per ohm per inch per inch -- (ohms/ohm)/(in/in). This comes from the ratio of the change in resistance of the strain gauge. That is: resistance change as a proportion of the original resistance of the strain gage, caused by the proportion of strain to the original length of the strain gauge.

To put strain in perspective, it is necessary to look at the modulus of elasticity of the strained material. For steels, such as used in gun barrels, the elastic modulus is about 30 million psi -- that unit really is (psi/(inch/inch)). i.e. I takes 30 million pounds to compress a 1 inch cube of steel one inch. This is simplistic. In reality the little cube of steel would never be crushed to zero height and would expand laterally and get stiffer, but this 30 million psi/inch/inch is linear over a range that is practical for the stresses that would fail steel. For most metals, different alloy content and strength have no practical influence on elastic modulus.

Knowing the strain on a piece of steel it is then easy to obtain a simple uniaxial version or component of stress by multiplying the strain by the elastic modulus -- i.e. Using E to denote the elastic modulus:

E (psi/(inch per inch)) X Strain (inch per inch) gives Stress (psi).

So now how sensitive is a strain gauge? Just what is the order of magnitude or amount of resistance change we have to measure? Let's say we're trying to keep the stress in the steel (gun barrel) to 30,000 psi (That's not the peak firing pressure, you shotgun guys, its the stress on the steel.) That means the strain will be that stress divided by "E". "E" is 30 million psi. So that gives 0.001 inch per inch -- one thousandth inch/inch. From the typical Gauge Factor of 2 ohms/ohm per inch/inch that means 2 thousandths of the original resistance of the strain gauge must be precisely measured. Strain gauges come in various nominal resistances, and a typical one is 350 ohms. Two thousandths of that is 0.7 ohms. We'll have to measure that very precisely. You'll never even see that on your home ohm meter. If we want 1 percent accuracy, we'll need to reliably detect 0.007 ohms -- 7 thousandths of an ohm -- that represents one percent of 30,000 psi, or 300 psi. How can this be done?

Fortunately certain circuit arrangements, most commonly the Wheatstone Bridge, can be used for these "micro" measurements. I presume you will thank me for not going into the details of the Wheatstone Bridge and it's cousins and variations. Almost all force and pressure measurement devices today use this strain gauge technology. While one could certainly rig up his own devices, this makes it understood how fortunate we are to have enterprising folks like RSI do all the hard work for us.

All of the above is just the "static" part of strain measurement. Another large part of the measurements important to an explosive or impulse/impact event such as the firing of a gun, are the dynamic responses of the test item and the readout/recording devices. How accurately can they recognize, represent and record a very brief transient event. Again, RSI has apparently done that for us.

Cheers!
Tony
Back to top
View user's profile Send private message
MaximumSmoke
PostPosted: Thu Jan 06, 2022 12:50 pm  Reply with quote
Member
Member


Joined: 01 Dec 2005
Posts: 1550
Location: Minnesota and Florida

Mark, as you say:
Quote:
As far as making a "pressure gun". That is tough, and probably not necessary.


How true. Sometimes things that are fun and educational or even useful, are tough and unnecessary. For sure, whatever I wind up with will not be a SAAMI qualified pressure gun, but rather a tool for my own enjoyment.

Cheers!
Tony
Back to top
View user's profile Send private message
MSM2019
PostPosted: Thu Jan 06, 2022 1:45 pm  Reply with quote



Joined: 04 Mar 2019
Posts: 1819
Location: Central ND

Tony,

That is why I am looking at strain gauges and RSI, as I don't need a pressure gun or a pressure barrel for what I am interested in.

As a side note, when I was a kid I wanted to be a mechanical engineer but my math skills ran out somewhere between algebra 1 and algebra 2. However that doesn't mean I am any less interested in certain subjects regarding engineering. I remember working with the production engineers at Pratt & Whitney, they offered a lot of interesting knowledge that really helped along the way. Kinda always envied the guys that understood the math and knew how to put it to good use.

Be well.


Last edited by MSM2019 on Thu Jan 06, 2022 1:46 pm; edited 1 time in total

_________________
Mark...You are entitled to your own opinion. You aren't entitled to your own facts.
Back to top
View user's profile Send private message
Brewster11
PostPosted: Thu Jan 06, 2022 1:46 pm  Reply with quote



Joined: 08 Feb 2009
Posts: 1301
Location: Western WA

Tony
Thank you for your very clear remarks, much appreciated. I spoke yesterday with an engineer who relayed very similar comments. RSI has indeed done a nice job with their Pressure Trace II system at an eminently reasonable price. To offer that level of precision and completeness to the layman is quite an accomplishment.

B.
Back to top
View user's profile Send private message
Brewster11
PostPosted: Thu Jan 06, 2022 2:54 pm  Reply with quote



Joined: 08 Feb 2009
Posts: 1301
Location: Western WA

Tony
One device offered to me by the engineer was a NI 9236 strain gauge module. Evidently it is used in the lab to collect data from the strain gauge and feed it into the computer.

Thanks
Bruce
Back to top
View user's profile Send private message
MSM2019
PostPosted: Thu Jan 06, 2022 3:28 pm  Reply with quote



Joined: 04 Mar 2019
Posts: 1819
Location: Central ND

The Pressure Trace system is complete. Adhere the strain gauge to the barrel, download their program onto a computer with a Windows OS and you are ready.

What I am trying to find out is if someone has found a manufacturer of strain gauges that has software that would work.

This would mean more work consolidating the outcome, but even less expensive.

I am not interested in making this into a research project or developing a business. I just want to play around with some loads that I have always wondered about.

I have the area for a patterning board which is all setup. I have the computer that I can dedicate just for testing. I have a decent chronograph, just need something to test chamber pressure with.

_________________
Mark...You are entitled to your own opinion. You aren't entitled to your own facts.
Back to top
View user's profile Send private message
Brewster11
PostPosted: Thu Jan 06, 2022 4:05 pm  Reply with quote



Joined: 08 Feb 2009
Posts: 1301
Location: Western WA

Mark
I think what Tony (and my engineer friend) is telling us is there is not a ready to go strain gauge that you plug directly into a computer for pressure testing. An additional device, whether it is a Wheatstone Bridge circuit or a NI 9236 module, is needed to collect signals from the strain gauge and convert it into useable computer data.

Tony, did I get that right?

Thanks
B.
Back to top
View user's profile Send private message
MSM2019
PostPosted: Thu Jan 06, 2022 4:18 pm  Reply with quote



Joined: 04 Mar 2019
Posts: 1819
Location: Central ND

Brewster11,

I know that is correct. But they have to be out there. I won't be able to use it anytime soon, so I have the time to research the options.

_________________
Mark...You are entitled to your own opinion. You aren't entitled to your own facts.
Back to top
View user's profile Send private message
Brewster11
PostPosted: Thu Jan 06, 2022 5:30 pm  Reply with quote



Joined: 08 Feb 2009
Posts: 1301
Location: Western WA

Mark good luck, let us know what you turn up. Sounds like a neat winter project. I will be loading up some 7mm-08 for the boys, thanks to a generous member here.
B.
Back to top
View user's profile Send private message
MaximumSmoke
PostPosted: Thu Jan 06, 2022 5:54 pm  Reply with quote
Member
Member


Joined: 01 Dec 2005
Posts: 1550
Location: Minnesota and Florida

Very good, Brewster11. The NI 9236 module seems to be a good one. It is a lot more modern than I am. I checked out it's specs here: https://www.ni.com/pdf/manuals/375336a_02.pdf It can acquire up to 8 "quarter bridge" strain gauge channels. "Quarter bridge" relates to the configuration of the aforementioned Wheatstone bridge circuit that uses a strain gauge element in only one of the 4 legs or "sides" of the Wheatstone bridge. The NI 9236 can sample each of these 8 channels at 10,000 times per second, which, by the Nyquist criteria, means it can characterize signals with frequencies up to 5000 Hz (5000 cycles per second).

A 5000 Hz sine component has a period of 1/5000 sec, or .2 millisecond, and it's rise time from zero to peak is one fourth of that, or about .05 millisecond, so probably such a sampling rate will work to find the peak of a shotgun or rifle firing event. I'm not a digital whiz -- I'm old school from the analog days, so if anyone out there sees a mistake in my thinking, please correct me. With digital sampling, i.e. discrete samplings over time, we need to get enough samples to be able to define the peak of the pressure "rise", which is what we're after. If I recall correctly, the peak pressure in a shotgun or rifle firing happens in the neighborhood of 1 millisecond after ignition -- that's one one thousandth of a second. Mark, MSM2019, please let us know if that's about right.

I have squirreled away some old analog strain gauge electronics, hoping someday to use them. Analog equipment also has frequency response limitations which affect accurate capture of a dynamic event. I do have an older RSI system, so I'll likely start with that, but it can only acquire one channel, and I'd really like to capture about 8 channels, starting at 1" from the breech, and then every inch or so out to at least 9 inches. That's what Sherman Bell did. If I have to do that one strain gauge at a time it will take a lot of ammo. The problem is I'd like to have pressure vs. distance down the barrel, and a single strain gauge only gives me pressure vs. time. That's a useful, but different thing. Until the projectile passes a station along the barrel, the barrel at that station effectively is not stressed. If one doesn't know the location and the stress from a shot, he's only guessing about how thick the barrel should be.

I know, I know . . . time and experimentation has long ago made it possible for gunmakers to design barrel profiles that are "safe". Nonetheless, I still want to look at stress at a location down the barrel. Sherman Bell wanted the same in his black vs. smokeless experiments, because of the idea that for a given set of exterior ballistics, black powder loads had lower peak pressures than smokeless, but higher pressures down the barrel -- it's the area under the pressure vs. distance or time that defines the energy imparted to the projectile, so if you reduce the pressure somewhere, it has to be increased somewhere else in time in order for the "pressure curve" to have the same area, and thus to impart the same energy to the same projectile. Yes, I know that is simplistic, as some energy goes to friction (it all goes to heat), and the ejecta is not only the shot or bullet at some velocity, but also powder gases and their velocities upon exiting the muzzle after the projectile, but the idea is still quite sound. The theory behind that is that the black powder, being about 3 times the volume of the smokeless in an equivalent load, takes longer to burn. His tests showed that this idea is basically true, but the differences in the pressure vs. distance between black and smokeless powders for loads of equivalent exterior ballistics was basically negligible.

Enough for now . . . maybe forever Laughing

Cheers!
Tony
Back to top
View user's profile Send private message
MSM2019
PostPosted: Fri Jan 07, 2022 6:37 am  Reply with quote



Joined: 04 Mar 2019
Posts: 1819
Location: Central ND

OK Tony, you opened the door.

I have seen enough pressure curves and none of them concern me. Why? Because you can't change them. The powder does what it is designed to do and unless you are going to formulate a new powder you really can't change that curve. That is my POV, that does not mean I am correct, because I would be way over my head if I said I understand anything more than the very basics of pressure curves and especially the "under" the curve stuff. WTF does that mean? Laughing Laughing Laughing

So someday when you feel like dumbing down the conversation, would you be able to explain the pressure curve and why you would want to measure barrel pressures an 1" at a time?

_________________
Mark...You are entitled to your own opinion. You aren't entitled to your own facts.
Back to top
View user's profile Send private message
MaximumSmoke
PostPosted: Fri Jan 07, 2022 10:45 pm  Reply with quote
Member
Member


Joined: 01 Dec 2005
Posts: 1550
Location: Minnesota and Florida

Mark --

I had my reply all written out this morning and punched the wrong button and deleted it. I tried to quickly re-do it, but the honey-do list emerged and now I've finally gotten back to this reply. That's my story; honest. . . . And the dog ate my homework . . .

Well here goes:

Pressure curve -- You already know that's the graph of pressure vs. time, and pressure is not constant. The curves have a characteristic shape -- a quick rise to the peak pressure we all hope we are measuring, and then a more gradual fall-off as the shot/projectile moves down the barrel increasing the volume into which the powder gases expand and eventually burn out, and then finally going to ambient atmospheric pressure when the projectile exits the muzzle. Assuming the area of the bore is constant (a good assumption), the pressure times that area at any time during the event equals the force that accelerates the projectile. Assuming the mass of the projectile is constant (a reasonable assumption), area under the force vs. time curve is proportional to the muzzle velocity. Again, knowing the mass of the projectile, and the velocity at the muzzle, one can calculate muzzle energy of the projectile:

E = 1/2 mv^2

E is kinetic energy
m is the mass of the projectile
v^2 is the square of its velocity

For example, if the units of m (mass) are (pound-sec^2)/ft, and velocity is in ft/sec, the resulting energy calculated has units of ft-lb.

Pressure vs. distance down the barrel -- This is useful to be able to properly size barrel wall thickness. Since this curve has similar rise and fall characteristics to the pressure vs. time curve, one can see it allows barrels to be designed with wall thickness tapering down, which they typically do, especially in the first 9 or 10 inches or so. And thank goodness for that. Otherwise a shotgun would be as unwieldy as a fencepost. I'm interested in pressure vs. distance because I want to get all the performance I can from a barrel that tapers down to a wall thickness of 40 thou at 9 inches. It's a damascus barrel, too.

By the way, the area under the pressure vs. distance curve times the bore area of the gun directly equals muzzle energy -- the sum of force times distance. If the forces are in units of pounds, and the distances over which the forces act are in units of ft, then the energy expended, or work done in raising the velocity of the projectile to the muzzle velocity, is in units of ft-lbs.

Can't change a pressure curve? -- You don't really believe that, do you, Mark? Of course you can. Yes, the nature of the projectile moving off down the barrel after the powder ignites does result in pressure vs. time or distance curves having a typical basic shape, but we know that we can use slower burning powders (powders of less brisance) to reduce peak pressures and have the same exterior ballistics. Same exterior ballistics (same projectile at the same velocity) means the area under the pressure curve has to be the same. So if you pull down the peak, somewhere along the rest of the curve pressures have to be higher. Since the peak pressure occurs in a very short pulse, and the fall-off of pressure occurs over a much much longer time, the pressure increase over that long time doesn't have to be much to make up the "area" that was lost in the lower peak pressure pulse. If one is not looking, he might not notice this area of increase, but it is there. Pre-occupation with pressure peaks, which are much more significant to gun barrel strength is justified, and a trade-off of a lower peak for a very small increase downstream is usually a very good one, at least up to the point when the peak pressure is so low it affects shot-to-shot consistency. I'm sure you know this very well from first hand experience, Mark.

I hope this answers your questions, especially the part about why I would want to have the tools to measure pressures at various stations down the length of a shotgun barrel, caused by the loads I want to use. I certainly understand why this might not be of interest to other folks. That is OK with me, but, by George, we should all be thankful it was and still is of interest to the folks who design and build safe guns and ammo for the unwashed masses, such as we are.

Cheers!
Tony
Back to top
View user's profile Send private message
Brewster11
PostPosted: Sat Jan 08, 2022 9:02 pm  Reply with quote



Joined: 08 Feb 2009
Posts: 1301
Location: Western WA

Tony
Thank you for your erudite discussion of pressure curves, very clear and complete. Your explanation begs the question: How did the classic gunmakers in the late 1800s determine the safe barrel thickness taper when presumably all they had were simple lead crusher methods? They crafted extremely thin barrel walls which suggests they knew the pressure profile down the length of the barrel. Did they install multiple crusher ports along the length of their pressure barrels?

Thanks
B.
Back to top
View user's profile Send private message
MaximumSmoke
PostPosted: Sun Jan 09, 2022 9:13 am  Reply with quote
Member
Member


Joined: 01 Dec 2005
Posts: 1550
Location: Minnesota and Florida

Brewster11,

I really don't know, but I suppose the tapering of wall thickness was at first determined from evolving experience. I have not completely read the books of Sir Gerald Burrard, or Gough Thomas Garwood, but it might be in there somewhere. I doubt someone put a crusher or any pressure measurement device at intervals down the barrel, at least not before the practice of slimming down barrel walls had already begun.

My guess is that early gun barrels were tubes with no taper, and eventually the idea came to some people that since the firing of a gun is a sort of explosion, the pressure must be high at first and then taper down as the container of the pressure expands -- e.g. as a projectile moves down a gun barrel, leaving a larger, softer more compliant gas volume, pressure must decrease. The oldest cannons I can think of were fatter at the breech end.

I suppose someone developed a theory of the physics of a gun firing event and described it mathematically -- math is the language of physics. Since mathematical descriptions of physical phenomena are approximations, and in the absence of hard info on pressures and materials, no doubt experiments were made to test and refine the theory. e.g. How much material of one kind or another was actually sufficient to contain a firing event, extract the most from the propellant and guide the projectile accurately?

I also suppose the development of experimental methods, one of which was to quantify maximum pressures produced by ammunition, marched hand in hand with the desires for greater exterior ballistic capability of guns. Proof tests are the ultimate verifications of gun safety. Because proof test are destructive, a gun designer certainly wants to avoid designing guns and ammo that tread the limits of proof tests. To that end, better ways to measure pressures, material properties, etc. have evolved over time, increasing the body of knowledge that ensures safe design, and enabling a greater understanding of what constitutes "proof" of safety.

I can't recall exactly when wingshooting emerged, maybe in the last 200 years or so, which is relatively recent in the history of firearms. I'm away from my gun books right now, but it might be in Burrard's works. Wingshooting dictated a "wield-able" gun -- one with good distribution of weight and no excess of it. I'm sure this triggered analyses and experimentation with shotgun barrel thicknesses, and probably the results of those experiments and analyses are still with us today. Never underestimate the wisdom, insight and techniques of our forebearers.

My $.02

Cheers!
Tony
Back to top
View user's profile Send private message
Display posts from previous:   
All times are GMT - 7 Hours

View next topic
View previous topic
Page 2 of 3
Goto page Previous  1, 2, 3  Next
16ga.com Forum Index  ~  16ga. Ammunition & Reloading

Post new topic   Reply to topic


 
Jump to:  

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum




Powered by phpBB and NoseBleed v1.09