Anyone "roll their own " strain gauge?

[Cal]

I have a pair of AR6 PCP's and could hardly help but notice that the revolver breech would be an easy access to fit a strain gauge.  But,  it would need to custom in order to fit.

I haven't ever needed to build my own,  'just glued on commercial devices.

finest wire in house at present could be pulled for any of the light duty instrument wire cluttering up the shop.

Supplied by a dry cell through a resistor, (use the VOM) it would be easy enough to view the response on the o-scope,  No capture though, it's an old Tek465. Might be able to use the movie function on the DSC.  Or talk my son into bringing home the high speed camera from his workplace.

Any tech advice? 

Trouble with solder connections between the gauge wire and the connection leads as an uncontrolled thermo junction?

arrangement
Wrapped "load cell" to multiply hoop stress distortion as opposed to linear stain wire arrangement.

Ideas welcome!  The data could go a long way towards the Internal Ballistics data calculation work posted on another thread.

TIA

Cal

[QVTom]

Strain gauge are resistive elements that will change resistances by a minute amount when strained.  Generally a wheatstone bridge is used generate a measurable voltage from the minute resistance change.  Search omega.com for application notes on strain devices and wheatstone bridges.

Next.  You are assuming something on your AR6 is yielding enough to measure with  a strain gauge.  Most likely the pressure vessel is design not to yield at normal operating pressures.

Is pressure what you are trying to measure?

Hope this helps.

Tom

[dukemeister]

Commercial strain gauges come in all shapes and sizes. There is bound to be one type that you could bond to your breach. But you will need a conditioning circuit to get measurable readings, a dry cell and a VOM wouldn't cut it. Strain gauges are sensitive enough to record a few microstrains with proper conditioning.
Check out micro-measurements: http://www.vishaypg.com/micro-measurements/stress-analysis-strain-gages/

[Cal]

Good considerations all

Tom,  The pressure at the breech just behind the pellet at the time of firing is the measurement desired.

There will be no attempt to  mount any detector to any existing structure.  If you  familiarize yourself with  the AR6 design, you will see that the present revolving cylinder magazine location provides a useful location to mount anything with a tube.  The "custom" component mentioned in the OP.

Calibration of any device is as simple as plugging the muzzle and firing to equalize to chamber pressure.  Then bleed down and record the strain gauge resistance as the pressure gauge falls.   The plot of the Change in pressure with time, and the peak pressure should be recordable on the o-scope.

Regarding the circuits required,   If the amplitude of the change is great enough,  less sensitive electronics are required.  Hence the question of the original post. Hey,  I just remembered,  I have a couple of the "Dataq" analog signal conditioning modules.  If the PC can handle the sample rate and event timing...should be good!

Commercial strain assemblies,  I am well aware of.  It's more enjoyable to do it yourself. They do offer useful benchmarks such as nominal at rest resistance values.
This link seems to cover my questions about the gauge it's self,  now to get insight into the air gun specifics ?  Trial by fire ;-)
http://www.onegentleman.biz/Hardware%20Design/Strain%20Gages/Electronic%20Strain%20gages.html

Regards and thank you both!

[QVTom]

I'm still not sure what you are trying to accomplish or measure.  Replacing the cylinder with a custom part as location to grab some data?   A properly setup and balanced wheatstone bridge will give plenty of output range and be sure to consider effects of temperature. Think of  your PC sound card as a free daq system.  Google for lots of apps and free software.


Alternatively you may want to check into piezo pressure transducers.  Lots of bandwidth and conditioned output.

Good luck
\
Tom

[Cal]

Tom,  I have made a recent edit,  but let me respond to your follow up.

With the recent posting of an internal ballistics calculation spreadsheet effort by Lloyd and rsterne, I thought it might be useful to provide one set of data points of breech pressure with respect to time for one PCP.  The AR6 offers an "open and removable" breech section perfectly suited.

Making up a strain gauge  equipped sensor block to  replace the 6 pellet revolving magazine will be a simple matter, I may do it tomorrow.  It can double as a "single shot tray", as well.  Thinking simply,  a straight tube, the length of the magazine (18mm) and about 12 mm diameter does it. Some "fixing arrangement will come in handy,   and I might just drill a prismatic block of alu.

I was just sketching a possible iteration of the test block that would be fitted with a quartz crystal micro balance. (I have lots of them)  but that would require either a large window, adding volume to the transfer port,  or a communication tube intersecting the transfer port that may suffer from being some few mm's from the TP connected by a small diameter drilling, or from venturi effect.  Neither would be good during a high speed event.

Temperature swings over the time it takes to calibrate and then re-pump to fill pressure should be small.  (In my lab) and for the accuracy of the present pressure gauges,  likely not an issue.

I could use either straight ohmic resistance, or the delta of some applied voltage  (my choice).  The bridge may not be required.

Getting 100 plus ohms of wire resistance in a compact package  might be.  I'm wondering where I can get some shellac insulated fine wire locally,  Not likely in this rural area .  Newark,    here I come ;-)

[Cal]

inadvertant double post

[lloyd-ss]

Cal,
good project, and I hope it works.  After you get a suitable output from whatever sensor you end up, how will you record the output?  The entire event will be less than 10 msec?  Response time will make or break the experiment.
I used a circuit similar to this to make a pressure sensing circuit but ran it thru an A-D convertor to process the output. 
http://www.nakka-rocketry.net/strainlc.html#Circuit
Recording the output could be very useful.
Lloyd-ss

[lloyd-ss]

I'm still not sure what you are trying to accomplish or measure.  Replacing the cylinder with a custom part as location to grab some data?   A properly setup and balanced wheatstone bridge will give plenty of output range and be sure to consider effects of temperature. Think of  your PC sound card as a free daq system.  Google for lots of apps and free software.


Alternatively you may want to check into piezo pressure transducers.  Lots of bandwidth and conditioned output.

Good luck
\
Tom

Tom,
I have a pressure transducer (tiny die about .25 x .25x .09) that I run thru an instrument amp to get output from 0 to 5Vdc, then thru and A-D convertor.  Can you shed a little more light on collecting data while the signal is analog?
Thanks,
Lloyd-ss

[QVTom]

Analog CRT scope and film camera -  LOL.  Seriously, you are on the right track. Pay close attention to offset in you op-amp circuit, most single sided devices perform  poorly near the rails (including rail to rail types), Sample at least 2X the max frequency you intend to capture.  PC sound cards are great if you don't have a nice DSO.  Another is the PICO line of usb scopes that use a PC and are very affordable.

Tom

I think the audio input is +/- 400mv

[Cal]

Recording?

Analog scope, using a Digital Still Camera in "movie" mode relying on the persistence of the phosphor screen to allow image capture.

Best sweep time from this scope goes down to .05 micro seconds.  so with limited input complications, the event should be captured.

I used the PC sound capture to record the firing event of a CO2 rifle some years back,  The range of the mic, coupled with response time, led to a significant "smoothing" of meaningful data while the trace was still complicated by full amplitude swings.  Perhaps if a higher quality mic were at hand.

I'm not at all interested in purchasing some "other" untried hardware.  IF I can't do it with a scope and camera,  well,  tough.  There are lots of things to sway my attention.

With the nice weather before winter comes,  I've been busy putting up a small building.  The quiet concentration of shop work will need to wait.

Thanks for all the comments.... keep 'em coming!

regards

Cal

[jimmie lee]

Lordy, I won't even pretend to know what you guys are talking about...

[QVTom]

I used the PC sound capture to record the firing event of a CO2 rifle some years back,  The range of the mic, coupled with response time, led to a significant "smoothing" of meaningful data while the trace was still complicated by full amplitude swings.  Perhaps if a higher quality mic were at hand.

I'm not at all interested in purchasing some "other" untried hardware.  IF I can't do it with a scope and camera,  well,  tough.  There are lots of things to sway my attention.

With the nice weather before winter comes,  I've been busy putting up a small building.  The quiet concentration of shop work will need to wait.

Thanks for all the comments.... keep 'em coming!

regards

Cal, as you stated, I wouldn't use a mic as the 3db roll off is probably somewhere around 20kHz or much sooner on a consumer unit.  The mic input of a decent sound card should sample well above 44kHz to give the required 44kHz audio data (CD data rate).  Assuming the event you are capturing is a projectile @ 900fps, traveling 24" that is ~2.2 ms, then considering the mechanical valve time constants, acceleration of pellet, etc., I'm guessing the actual time is 4X that amount or 8-9ms.  With the 44kHz sample data you would get ~400 data points for the complete firing cycle.

The 44kHz is not Ideal or even considered fast but will definitely give clear insight to what is happening.  Enough said about the PC and sound card

Tom

[Cal]

Tom

Thanks so for your insightful comments!

The actual event timing is only that required for the high pressure air to be released by the valve opening until the pellet starts to move. divide by 10 ;-]
 It will be difficult to stop the recording there however.   The additional information will be of interest to many.

I was working with the scope and a signal generator last evening.  There is considerable distortion of the wave form at the 10s of Mhz range.

I did not have opportunity to work through the guilty party.  Scope, generator or BNC cable?  But the distortion is not so significant that useful amplitude observations could not be made.

I will try to run the signal source through a PAR Lock- in amp to see if that  offers any improvements or flexibility

Cheers

Cal