Capture Peak Strain Gage Fluctuation For Shotgun Chamber Pressure Measurement

[Silver_Is_Money]

Would a Digital Oscilloscope be capable of capturing and storing an image of the brief fluctuation in the amplified DC voltage witnessed across a  Wheatstone Bridge for the case of a Quarter Strain Gage glued above mid-chamber of a Shotgun?  Or would some other instrument be better suited to the capture of this image, or at the very least the capture of the brief peak of the strain induced voltage increase witnessed upon firing the Shotgun?

[Stray Electron]

  I can't give you a straight answer to that but I would say that it's going to depend on the speed of the scope. They do make digitial scopes that are VERY fast, but FOR A PRICE.  I also know that they do make systems that do what you want so it is possible but you would need a very fast censor and a fast scope.  Just as, a rough estimate if you assume a muzzle velocity of 1000 fps and an 18" barrel and an initial velocity of 0 fps, the entire firing process would only take 3/1000 of a second. (That's assuming uniform acceleration but I think that the initial acceleration would be much higher.) And of course, the pressure rise would be even faster and you would want to sample that single pulse MANY times to get an accurate representation so you would need a very fast scope to accurately show the wave shape and not just the period.

   But IIRC Brownells in Utah used to sell some kind of system to measure chamber pressure. I don't remember what kind of output device it had but it included piezo-electric sensors that were epoxied to the barrel.

[tautech]

Welcome to the forum.

Certainly a DSO is a good tool for this task.
You would use a Single trigger setting at a trigger level and timebase setting to be sure to capture the complete pressure cycle.
Once captured export a BIN or CSV and import it into Excel to graph the pressure cycle.

[TurboTom]

Virtually, any current digital scope would be fast enough for this job. Pressure buildup in the chamber will be in the low millisecond, maybe several hundreds microseconds range. Any entry-level DSO today will record a sample every nanosecond (1GSa/s) or faster.

The more problematic item in the signal processing chain is the strain gauge amplifier since the typical output voltage of the strain gauge is very low. Hence, often lock-in amplifiers with heavy low-pass filtering are applied.

Since the measurement itself will be very short and since the output signal of the strain gauge is ratiometric to the excitation voltage, you may consider pulsing the excitation voltage to a much higher value than the strain gauge would be able to withstand continuously, thus boosting the output level by that factor, and using a faster, low noise operational amplifier for signal amplification. Tests would be required to verify that this approach doesn't affect accuracy due to strain gauge heating.

For triggering the measurement chain, you could use either an optical interrupter type sensor to detect the cock approaching the firing pin (still many milliseconds to go...) or an accelerometer system that senses the vibration when the cock hits the firing pin (a few milliseconds before the shot will break).

...just some food for thought...

[voltsandjolts]

@OP How do you plan to calibrate this measurement system, to get psi, or whatever?

[TurboTom]

@OP How do you plan to calibrate this measurement system, to get psi, or whatever?

Pump in some high pressure hydraulic fluid to do a static calibration? Even hand powered hydraulic pumps will easily reach 800 Bar / 12,000PSI.

[Silver_Is_Money]

I'm not as concerned with absolute pressure as with relative pressure.  Reference loads are available, or factory loads can be used whereby to baseline a safe voltage rise max for relative comparison with reloads.

I'm fairly certain that the resistance change in the stretched strain gage will be linear to the voltage rise across the Wheatstone bridge, and the amplified voltage.  And that both of these are linear to pressure.  So it seems that a highly gun and strain gauge and amplifier specific Multiplicative factor would turn voltage rise into pressure rise, if this last step is even necessary.

[Silver_Is_Money]

If the Oscilloscopes (ahem) 'trigger' was set to just above the nominally zero voltage line of the balanced Wheatstone Bridge, would that 'trigger' the capture of the spike in voltage as post firing chamber pressure begins to stretch the strain gage?  And is there a capture feature which can be stored and played back whereby to measure the peak voltage captured by the Oscilloscope, or better yet, observe the entire event which will end when the payload exits the barrel?

[nctnico]

If the Oscilloscopes (ahem) 'trigger' was set to just above the nominally zero voltage line of the balanced Wheatstone Bridge, would that 'trigger' the capture of the spike in voltage as post firing chamber pressure begins to stretch the strain gage?  And is there a capture feature which can be stored and played back whereby to measure the peak voltage captured by the Oscilloscope, or better yet, observe the entire event which will end when the payload exits the barrel?

Any DSO will do that. Just make sure to get one which has at least 10 millione (Mega) points of memory. That should give you a record with enough detail.

[Kleinstein]

With the right trigger the scope should capture the signal from just before firing to well after firing. For this is helps to get a reasonable modern scope with enough memory, so more like 1 Million samples at least. Usually one has the option to get data from before and after the trigger.

For the later time there will likely be not just the strain part, but also a thermal effect.

I am not so sure it would be a good idea to pulse the bridge excitation - it may work, but it may also make things more complicated. I would first do the math and see if one can get away with a constant relatively high excitation. If noise really gets an issue, the pulsed excitation could be worth it.

One would definitely not use modulation and a lockin as it often used for slow changing signals.  The amplifier would be more a low noise DC amplifier. Modern amplifier can often be low enough in noise so that the thermal noise if the bridge itself is a major part of the total noise. 

[nctnico]

I'm not sure what the defacto standard is for measuring pressures in guns but my gut feeling says a piezo transducer could be suitable as well or even a better choice. Such transducers are also used to detect & measure knocking (premature ignition of the mixture) in car engines. A piezo transducer (a simple disc shaped one) can be stuck onto the gun and you won't need any amplification at all to capture the waveform with an oscilloscope.

[Silver_Is_Money]

I'm not sure what the defacto standard is for measuring pressures in guns but my gut feeling says a piezo transducer could be suitable as well or even a better choice. Such transducers are also used to detect & measure knocking (premature ignition of the mixture) in car engines. A piezo transducer (a simple disc shaped one) can be stuck onto the gun and you won't need any amplification at all to capture the waveform with an oscilloscope.

That should work, but it sounds overly expensive.

[Silver_Is_Money]

What DSO brands and models might be up to the task without completely breaking the bank?  I was looking at mid-lower end Silgent, Hantek, OWON, and Rigol.

[Silver_Is_Money]

It appears that peak strain occurs at roughly 0.3 ms (+/- 1 ms) after firing a shotgun.

[babysitter]

Scope bandwith is way more than necessary.
High excitation, high amplification could be necessary.
Placement of sensor or several sensors might be critical for you depending on what you want to know, remember there might be several things travelling back and forth and in different directions (direct and reflected shockfront in gas column inside, backblow acceleration on barrel, shockfront on ambient gas, superimposing themselves in front of each other)

Pressures don't want to be measured.

BR
Hendrik

[tautech]

I'm not sure what the defacto standard is for measuring pressures in guns ……

Test guns have pressure ports and historically used an anvil to crush a metal pellet.
Shotguns used lead for a LUP measurement
Rifles used copper for CUP

Both are now outdated now PSI sensors are available and PSI data sets are still evolving/growing for the large range of different firearms available.

[bdunham7]

If the Oscilloscopes (ahem) 'trigger' was set to just above the nominally zero voltage line of the balanced Wheatstone Bridge, would that 'trigger' the capture of the spike in voltage as post firing chamber pressure begins to stretch the strain gage?  And is there a capture feature which can be stored and played back whereby to measure the peak voltage captured by the Oscilloscope, or better yet, observe the entire event which will end when the payload exits the barrel?

Any reasonable modern DSO will do all of that.  They will record both before and after the trigger, so you could use the slug leaving the gun to break a foil to trigger the scope as well--this might have the advantage of showing you the exact point in time the slug clears the barrel (if you care).  Or, you could just have a manual trigger button that you press about the same time as the shotgun trigger.  It will just take some setting up and if you are new to oscilloscopes, triggering is an art that takes a bit to learn.  As far as which one, both Rigol and Siglent have inexpensive ($400-500) models with 10M+ memory points and low noise inputs.  I would avoid the other cheap brands and ask here specifically about a particular model you intend to buy for this just to make sure it doesn't come up short.

[tautech]

Then one wonders why the OP is wanting to undertake chamber pressure measurements when much info is available from the multitude of reloading manuals and propellant manufacturers websites. 

What reloading experience has SIM ?

[Silver_Is_Money]

I'm 68 years old and I have been reloading metallic cartridges (rifle and pistol) and shotshells since the age of 16.   

I also have a wildcat 6.5mm rifle chambered to my own design, so for that one there is no looking at manuals. 

[Stray Electron]

I'm 68 years old and I have been reloading metallic cartridges (rifle and pistol) and shotshells since the age of 16.   

I also have a wildcat 6.5mm rifle chambered to my own design, so for that one there is no looking at manuals.

   Cool! What cartridge case and what bullet weight?  A gunsmithing friend of mine designed a wildcat .257 cartridge based on the .300 Win Mag case. That thing was a ripper!

   If you haven't done it already, I suggest posting your question on some of the forums that deal with reloading such as Castboolits.com and TheHighRoad.org and see if anyone there has actual experience with what you're trying to do. You can also post on AR15.com, it's a larger forum but the noise level there is significantly higher.

  In many modern digital scopes you can set them to start recording before the trigger event and continue until after the event so that the trigger event appears right in the middle of the recording. Or you could set the scope to trigger on the muzzle flash and have it save all of the readings prior to the trigger.  Initially it might sound like both are impossible but what the scope really does is just to take readings continuously and puts them into it's memory until it fills and then it starts loading the memory at the the beginning again. It just keeps doing that until it is triggered and it then stops the recording at the selected point while the readings prior to that point are still in memory.  Think it it like driving down the highway and video taping non-stop until you get to a certain mile marker and then discarding everything prior to say one minute prior to reaching the marker and everything more than one minute after the marker. With digital electronics you're basically only limited by the rate that you want to take readings, the time that you want to record and your memory size. One note though, acquisition memory and internal memory used for computing are two different things and most manufacturers advertise the computing memory size since it is always much larger and much cheaper.  If you want to take say 1 million readings in real time, you need to be sure that your scope has enough acquisition memory for that.

  2nd I think that finding a sensor that is capable of taking readings as fast and as with as much resolution as you desire will be the hard part of this project.  IMO you need to find a sensor that will do what you want and find our how fast it can take readings and then select a scope that will keep up with the sensor.

[tautech]

I'm 68 years old and I have been reloading metallic cartridges (rifle and pistol) and shotshells since the age of 16.   

I also have a wildcat 6.5mm rifle chambered to my own design, so for that one there is no looking at manuals.

Similar experience of decades of reloading shot shell and rifle loads.

Still not sure what your intentions are logging shot shell pressure curves.
There are a wide range of shot shell propellant burn rates available and loading data for each.

Rifle wildcats are no longer the challenge they once were with SW like Quickload now available where you can tailor loads to bore, projectile weight, cartridge volume and propellant.
Over on Accurate Shooter forum you frequently see someone asking another member to run a Quickload simulation on some combination of components you wouldn’t find in reloading manuals.

[Silver_Is_Money]

I call my wildcat the 6.5mm LWS.  It begins life as a 270 brass, gets necked down and blown out, and has a water capacity (for Winchester brand brass) of 74 grains on the nose (vs. ~67 grains for a 270).  I mostly have shot 140 grain Hornady flat base spire points.

[tautech]

Similar to the decades old 6.5/06 then.

[David Hess]

I have done what you suggest but it was not with firearms.

Any digital storage oscilloscope can do what you need.  You may need several tries to get the gain of the amplifier and trigger level adjusted properly.  Back when I did it, I used an analog storage oscilloscope although DSOs were available.

Besides mechanical issues, like delamination of the strain gauge because the peak strain is too high, the largest difficulty will be with the strain gauge amplifier because normally they are limited to lower bandwidths for performance and noise reasons.  This should not present a problem now because there are many modern fast low noise precision parts.

[Silver_Is_Money]

I have done what you suggest but it was not with firearms.

Any digital storage oscilloscope can do what you need.  You may need several tries to get the gain of the amplifier and trigger level adjusted properly.  Back when I did it, I used an analog storage oscilloscope although DSOs were available.

Besides mechanical issues, like delamination of the strain gauge because the peak strain is too high, the largest difficulty will be with the strain gauge amplifier because normally they are limited to lower bandwidths for performance and noise reasons.  This should not present a problem now because there are many modern fast low noise precision parts.

Is there a reasonably priced amplifier that you can recommend please?

[nctnico]

It says here that using a piezo test is the best way:
https://en.wikipedia.org/wiki/Chamber_pressure

It there a flat surface on the gun? If yes, you can stick a piezo disc like this:

on the outside. These come in many different diameters and will produce a signal that you can measure using an osciloscope without needing amplification.

[TurboTom]

The piezo you suggest will detect acceleration or dynamic deformation (warping) of the piezo's brass disc (in this example that is), but there's no way you can analyze an elastic deformation of another construction component in a quantitatively accurate manner.

The strain gauge approach is much more accurate and reproducable since the strain gauge is inherently calibrated and adds much less weight / inertia to the test setup. Moreover, accurately bonding the strain gauge to the exterior of the chamber is way less troublesome.

The piezo may do as a trigger element for pulsing the strain gauge's bridge excitation voltage if that's necessary, but I'ld not recommend it as the measurement transducer itself.

[Silver_Is_Money]

It says here that using a piezo test is the best way:
https://en.wikipedia.org/wiki/Chamber_pressure

It there a flat surface on the gun? If yes, you can stick a piezo disc like this:

on the outside. These come in many different diameters and will produce a signal that you can measure using an osciloscope without needing amplification.

Nothing flat on a cylindrical barrel.

[David Hess]

I have done what you suggest but it was not with firearms.

Any digital storage oscilloscope can do what you need.  You may need several tries to get the gain of the amplifier and trigger level adjusted properly.  Back when I did it, I used an analog storage oscilloscope although DSOs were available.

Besides mechanical issues, like delamination of the strain gauge because the peak strain is too high, the largest difficulty will be with the strain gauge amplifier because normally they are limited to lower bandwidths for performance and noise reasons.  This should not present a problem now because there are many modern fast low noise precision parts.

Is there a reasonably priced amplifier that you can recommend please?

Do you mean a reasonably priced integrated circuit amplifier?  In the past I used the LT1007 or LT1028 and built my own amplifier.

I never found a suitable amplifier that was already built.  Most are intended for lower frequency applications and have limited bandwidth to control noise, but I did not look very hard.

[David Hess]

The piezo you suggest will detect acceleration or dynamic deformation (warping) of the piezo's brass disc (in this example that is), but there's no way you can analyze an elastic deformation of another construction component in a quantitatively accurate manner.

The strain gauge approach is much more accurate and reproducable since the strain gauge is inherently calibrated and adds much less weight / inertia to the test setup. Moreover, accurately bonding the strain gauge to the exterior of the chamber is way less troublesome.

The piezo may do as a trigger element for pulsing the strain gauge's bridge excitation voltage if that's necessary, but I'ld not recommend it as the measurement transducer itself.

I thought he was referring to piezoelectric strain gauges which have greater sensitivity than metal foil strain gauges.  I never used piezoelectric ones myself.

[Silver_Is_Money]

Similar to the decades old 6.5/06 then.

Yes, but with roughly 10% more internal capacity.

[voltsandjolts]

The term 'piezo' transducer is a little vague, it could mean either piezoelectric or piezoresistive. Piezoelectric are the more traditional sensor for fast dynamic pressure measurements, combustion engine, explosive detonations etc. Kistler and PCB Piezotronics are the better known dynamic sensor manufacturers. Also well known for taking lots of your money :-)

https://www.pcb.com/Contentstore/mktgcontent/LinkedDocuments/Aerospace/AD-BallisticPressureGuide_LowRes.pdf
https://en.wikipedia.org/wiki/Small_arms_ammunition_pressure_testing

[Silver_Is_Money]

For the goal of utilizing an Oscilloscope to measure 'relative' shotgun chamber pressure as strain gage voltage output variation (linear to pressure) across a Wheatstone Bridge, would it be best to utilize DC or AC?

[babysitter]

Recommend DC, bridge current as high as reasonable without temperature/drift swamping your signal too much.
Use a instrument amplifier IC as a simple preamp. You might want to dive into https://www.analog.com/media/en/technical-documentation/application-notes/an43f.pdf in case you want to go deeper in the rabbit hole.

BR
Hendrik

[Kleinstein]

Definitely DC as the relevant signal is only short, like a few ms at most. AC drive is a thing if one cares about long time drift and 1/f noise from the amplifier.
For the amplifier I would consider a normal single ended amplifier and have a floating (e.g. battery) to provide the bridge drive. It is about getting lowest noise and the scope input directly is not really made for such low impedance (e.g. 350 ohm for common strain gages).

[Silver_Is_Money]

Definitely DC as the relevant signal is only short, like a few ms at most. AC drive is a thing if one cares about long time drift and 1/f noise from the amplifier.
For the amplifier I would consider a normal single ended amplifier and have a floating (e.g. battery) to provide the bridge drive. It is about getting lowest noise and the scope input directly is not really made for such low impedance (e.g. 350 ohm for common strain gages).

Would a precision multimeter with peak voltage capture capability be an overall better choice in this application than an Oscilloscope?  And if so, are there any suitable multimeter recommendations?

[bdunham7]

Would a precision multimeter with peak voltage capture capability be an overall better choice in this application than an Oscilloscope?  And if so, are there any suitable multimeter recommendations?

No, both not fast enough and it is going to be a lot handier to see the overall pressure pulse not just a number.

[Silver_Is_Money]

No, both not fast enough and it is going to be a lot handier to see the overall pressure pulse not just a number.

By your statement regarding both not being fast enough, does this imply that neither a good Oscilloscope nor a good multimeter will reliably capture the moment and magnitude of peak pressure as seen by the strain gage?  If so, then my quest is stillborn.

[bdunham7]

By your statement regarding both not being fast enough, does this imply that neither a good Oscilloscope nor a good multimeter will reliably capture the moment and magnitude of peak pressure as seen by the strain gage?  If so, then my quest is stillborn.

No, the 'both' refers to the DMM being both not fast enough and not as handy.  I guess I could have written a bit more clearly.  The scope will be more than fast enough, your only worry would be the response time of the strain gauge.  A fast DMM will register a peak 250µs long, also known as 1/4000 of a second.  An entry level scope will register peaks shorter than 10ns, or 1/100,000,000 of second.

[joeqsmith]

Say you have an 8-bit scope. 2^8 is 256 discrete levels.   Say you have a peak chamber pressure of 60ksi.  60000/256 giving you a resolution of 230PSI.   

Looks like you only need a ms or so of data.  Even a us (0.000001 second) resolution may be good enough.   You may be better off with a data acquisition system that could give you a higher pressure resolution.   You would need to provide details. 

[tautech]

Say you have a peak chamber pressure of 60k psi.

Not for a shotgun. 

[pickle9000]

Interesting thread.

So because the weight and volume of the shot is known could you use a few piezo's glued down the outside of the barrel scope that and figure it out mathematically? Am I wrong in thinking that the springiness of the air is a massive issue?

[Silver_Is_Money]

Say you have a peak chamber pressure of 60k psi.

Not for a shotgun. 

Shotgun pressures are in the 10,000 PSI range. The SAAMI MAP (maximum average pressure) for a 12 gauge is 11,500 PSI.

[Silver_Is_Money]

For now I've settled upon a low noise Tacuna Systems EMBSGB200 Amplifier with the 930 kHz option by which to amplify the rapid voltage spike evolving across the Wheatstone Bridge,  And a Rigol DHO802 Oscilloscope.

[TurboTom]

Looks like a decent choice. It may not be relevant for your current application, but nowadays I would always opt for the four-channel version of an oscilloscope if available. The price difference usually isn't that big and if you only once or twice need more than two channels, it's already worth the spending. Moreover, the resale value (and potential) is much better. Just my two cents... 

[Silver_Is_Money]

Is Oscilloscope "rise time" something to be considered when selecting a DSO for this project?  I've been looking at the new Rigol DHO802 due to its features for the $299 price, but I see that it's rise time is only rated at less than or equal to 5 ns,, whereas better DSO's seem to be rated at 3.5 ns or better.

[Silver_Is_Money]

I'm now wondering how I might make this project fully (and literally) field operable, and with that in mind I've come across the FNIRSI 1013D Digital Tablet Oscilloscope, which runs on an internal battery and claims up to 4 hour battery life (wherein a real-world 2 hour life would be sufficient for my needs).  The greatest drawback I'm seeing (upon initial casual look at the specs) for this tablet oscilloscope is that its DSO storage memory for waveform event recording is only a paltry 240 kbits.  Would this highly limited storage depth be a deal killer for my project application?

[bdunham7]

Is Oscilloscope "rise time" something to be considered when selecting a DSO for this project?  I've been looking at the new Rigol DHO802 due to its features for the $299 price, but I see that it's rise time is only rated at less than or equal to 5 ns,, whereas better DSO's seem to be rated at 3.5 ns or better.

Rise time and bandwidth are correlated.  3.5ns is a 100MHz scope, 5ns is a 70MHz version.  I'll skip the math and theory on that.  Nanoseconds aren't going to matter in your application.

The greatest drawback I'm seeing (upon initial casual look at the specs) for this tablet oscilloscope is that its DSO storage memory for waveform event recording is only a paltry 240 kbits.  Would this highly limited storage depth be a deal killer for my project application?

Yes, that limited memory will make your setup more difficult.  It can be done, but you'd have to have a surefire triggering system and I don't mean for the shotgun.  Long acquisition times give you the luxury of capturing your event and then zooming back to it.  FNIRSI is pretty crap from what I've heard elsewhere, you'd probably be better off looking at MicSig tablet scopes if you want battery operation and good portability.  I've no direct experience with either.

[johansen]

https://www.checkline.com/product/TS621HD

40khz should be fast enough.

[Silver_Is_Money]

Yes, that limited memory will make your setup more difficult.  It can be done, but you'd have to have a surefire triggering system and I don't mean for the shotgun.  Long acquisition times give you the luxury of capturing your event and then zooming back to it.  FNIRSI is pretty crap from what I've heard elsewhere, you'd probably be better off looking at MicSig tablet scopes if you want battery operation and good portability.  I've no direct experience with either.

Thanks!  The MicSig tablet DSO's  look great!  The only problem I see is that the lowest priced model I see (in an admittedly quick search) costs about 5 times more than the Frnsi tablet.

[joeqsmith]

Say you have a peak chamber pressure of 60k psi.

Not for a shotgun. 

Shotgun pressures are in the 10,000 PSI range. The SAAMI MAP (maximum average pressure) for a 12 gauge is 11,500 PSI.

I went through the basic math to show you how to determine the pressure resolution.  "Say you have"  or in other words, an example.

[Silver_Is_Money]

What is the best Oscilloscope means for triggering the capture and image recording of a brief rise in an otherwise stable (aside from the expected background noise) DC voltage?  My initial guess is to position the trigger pointer above the signal noise and set it to trigger on rise.

[David Hess]

What is the best Oscilloscope means for triggering the capture and image recording of a brief rise in an otherwise stable (aside from the expected background noise) DC voltage?  My initial guess is to position the trigger pointer above the signal noise and set it to trigger on rise.

Modern DSOs may have special trigger modes which could be used, but level triggering will work fine and should return a result on the first try.

Estimating the peak strain will give an idea of how large the signal will be, so the vertical sensitivity can be set to get a reasonable display.

[csuhi17]

Yes, that limited memory will make your setup more difficult.  It can be done, but you'd have to have a surefire triggering system and I don't mean for the shotgun.  Long acquisition times give you the luxury of capturing your event and then zooming back to it.  FNIRSI is pretty crap from what I've heard elsewhere, you'd probably be better off looking at MicSig tablet scopes if you want battery operation and good portability.  I've no direct experience with either.

Thanks!  The MicSig tablet DSO's  look great!  The only problem I see is that the lowest priced model I see (in an admittedly quick search) costs about 5 times more than the Frnsi tablet.

Forget Fnirsi, it's a waste I say from experience, take a look at the Fnirsi topic open here, You can't believe what he draws, and he has little memory.
Not worth more than $50 in my opinion...
You can use a power bank for Rigol, which you can also use for other things. If memory and bandwidth are not enough for you, you can hack it,

[Silver_Is_Money]

Forget Fnirsi, it's a waste I say from experience, take a look at the Fnirsi topic open here, You can't believe what he draws, and he has little memory.
Not worth more than $50 in my opinion...
You can use a power bank for Rigol, which you can also use for other things. If memory and bandwidth are not enough for you, you can hack it,

Thanks!  The Fnirsi is forgotten.  I've had my eye on the Rigol DHO802, but finding a field usable power bank for it may be interesting.