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?