Capture Peak Strain Gage Fluctuation For Shotgun Chamber Pressure Measurement

[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.