Trying to add pressure sensors to LDC testing.

[OldCrow]

The spec for a somewhat newer version of the same model shows a resonant frequency (f_n) of 500 Khz. The endevco doc shows a response curve with 4% amplification at f_n/5. So, I would say 100Khz and a rise time consistent with that (2.5 micro-sec according to their doc). In the data I was capturing with the handheld DSO a useful shot cycle duration was 10-20 milli-sec and I was getting 1024 samples. Of course one can capture a shorter duration but you start loosing context and the DSO runs out of sampling rate. With amplification, I may be able to go back to the 4 channel Rigol with a much higher sampling rate. So I don't expect the sensor or the amplifier to be the limiting factor. 10-20 Khz would give me plenty of info. The pellet is moving at about 6000 inches/sec.

You are doing brilliant work here.  Thank you.

You're showing maximum PSI in the range of 25 to 30?   or have I mis-read or skimmed over something?

[WhatUPSbox?]

The curves I had in the first post have some issues with cross talk between the channels, as was discussed in the subsequent posts. For the moment, I added some instrumentation amplifiers I had on hand. They are a bit noisy but at least decouple the channels. I'm still working through some of the options Bruce suggested. Here is a more recent plot from the checkout process with the amplifiers. Note the pressure is in absolute terms so the pressure rise would be less 14.7 psia. The Barrel pressure is in a short, artificial extension of the bore (plus ~1mm diameter) to get a feel of what the residual pressure might be at the end of the shot cycle. It is not the LDC pressure. These are just initial checkout tests, a lot more needs to be done before observations are meaningful. 

[WhatUPSbox?]

Bruce, attached is an updated schematic using modules I had on hand. I think I got rid of the coupling between the three signals. Now I'm looking at improving on those AD620 modules from Amazon (noise). Also, since I'm not tied to earth ground (battery powered) I'm thinking the cable shield on the transducer wires may not want to be attached to the module ground. Long ago I shared an office with an EMI/EMC guru. Unfortunately not enough was absorbed on my end.   

[WobblyHand]

@WhatUPSbox Stan, I wasn't paying attention to the EMI guy either.   

I'm reluctant to tie the shields together.  The best thing would be to have isolated supplies, but that's not what you have.  So some experimentation is in order to determine what works best.  Try shielding at one end vs the other, but not both ends.  Then swap ends.  But even if you get it right, from what I can tell, you seem to have non differential non isolated probes on your scope, and that means you are making a common ground at the scope.  Honestly, from here, can't say what to do. 

If I were to do a clean sheet, I'd have isolated supplies and independently hook up the transducers.  Then have the amplified signals go through separate isolators to the scope.  This would keep all the grounds separate, except right at the scope.  But I could see (if I was you and had the setup in your diagram) trying various permutations of grounding the shields to see if any of them were better.

[WhatUPSbox?]

My understanding was that the instrumentation amp (AD620) converted the each of the differential signals into a single ended output and that a common ground and non isolated probes on the output side of the amp was OK. Actually, much of the noise is generated inside the AD620 module (voltage converter that generates the -V). I'm looking at some of the other amplifier options.

[WobblyHand]

My understanding was that the instrumentation amp (AD620) converted the each of the differential signals into a single ended output and that a common ground and non isolated probes on the output side of the amp was OK. Actually, much of the noise is generated inside the AD620 module (voltage converter that generates the -V). I'm looking at some of the other amplifier options.

It's usually ok, but sometimes it isn't 

Is this too noisy for your use?  What would be good enough?

[WhatUPSbox?]

Is this too noisy for your use?  What would be good enough?

It is usable. you can see it in the trace I posted. I'll try more gain on the low pressure readings to see if it is on the output side.

[WhatUPSbox?]

I've done some more work on the test instrumentation (This is heading into the data weeds. Proceed at your own risk ). I'm still using the Amazon sourced amplifier modules but have done some calibration to give me more confidence in the output.  (Bruce I did get some of the AD8422 chips and will be checking them out)
The first image shows the overall setup. The LDC segment that has the pressure port can be moved to any of the three positions and I can also put the pressure sensor into the cap. I keep one pressure sensor at the muzzle to act as a monitor of sort on shot repeatability. In the photo the pressure sensors can be seen at the muzzle and in the outer segment.
The second image show the basic pressure time trace capture for any given shot. In this case, the muzzle and the outer segment are measured. Time =0 is when the pellet trips the laser gate as it exits the muzzle. The pellet spends a little less than a milli-sec in the LDC and you can see the pressure rise in the outer segment is delayed by about that time.
The third and fourth images show repeatability. Since I will be depending on the shot cycle being relatively repeatable so I can compare the pressure traces in different segments, I overlayed the results from 4 shots for both the muzzle and the outer segment. For my purposes, those are reasonably repeatable.
The final image shows such a multi shot comparison for different locations of the pressure sensor. There is a trace for each segment, the cap, as well as at the muzzle. Fortunately it shows some expected trends. As you move from the muzzle, through the segments to the cap, the pressure rise is delayed, roughly matching the progress of the pellet. The pressure peaks become lower as well. There are lots of things that still need to be understood, such as the cause of the ripple on the traces, etc.
I hope to make the instrumentation package convenient enough to take it outdoors where a more meaningful sound measurement could be added.

[WobblyHand]

Great work Stan!  Is it reasonable to assume that the variation is related to the valve timing, which one normally qualifies by extreme spread, or velocity standard deviation?  Or are there other mechanisms at play?

Looks like a cool playground, what did you use for the pressure sensors again?  They're bridge sensors, as I recall.  Are they expensive?

You seem to know about some instrumentation, are there any good audio accelerometers around?  The one's I've seen have a pretty low high frequency cut off.  Any extend out to 22KHz?  Guess they'd have to be tiny.

[WhatUPSbox?]

If by variation you mean the somewhat lower blue curve in the second and third chart, yeah probably valve timing, though since it is CO2, it could be my taking two shots too closely together, or since it is not as prominent in the peak, it could be the flow through the LDC was a little different that time.

The pressure sensors are the Endevco 8530. They are industrial/scientific = very expensive. I bought some surplus (probably decades old) and made a calibration rig to establish the sensitivity of each one.

I have more curiosity than knowledge on the instrumentation and am happy when I keep the blue smoke in the wires. There is a lot of trial and error knowledge on how airguns work, as well as a few spreadsheet or basic principles analyses. I've been curious about whether, at the hobby level, one can measure any of the key parameters. Several years ago there were several fun testing threads that George Schmermund started. George had a seemingly endless variety of industrial instrumentation. I tried to approach it from the hobby side. They are long threads but can be fun reading on a rainy day if your interest is in that direction.
https://www.gatewaytoairguns.org/GTA/index.php?topic=137566.0
https://www.gatewaytoairguns.org/GTA/index.php?topic=176445.0
For accelerometers, there are several different ways to go. The industrial/scientific is the gold standard. Expensive, requires conditioning electronics and expensive cabling. I went the middle road using the Analog Devices mems based chips. The ADXL 1000 series provides high bandwidth in a single axis.  The ADXL 300 series provides multi-axis but  at a much lower badwidth. If one doesn't care about amplitude fidelity but just wants to detect timing events (hammer strike, etc) then hard mounting a small mic works surprisingly well. I did a comparison of the ADXL1005 and The ADXL 377 here. https://www.gatewaytoairguns.org/GTA/index.php?topic=182332.msg156142141
All fun stuff if you enjoy playing in the tech details with no guarantee of applicable results.

[WobblyHand]

Ok, valve timing.  Makes sense.
Thanks for the info on the pressure sensor and the accelerometers.  Appreciate that.

I have read some of George's posts.  I'll go back and reread them.

My interest in the accelerometer would be to use one (or multiple axes) to measure the movement of the can.  Guess that's the ping.  Thinking that making the can stiffer would raise the resonant frequency above hearing range.  But it's hard to judge progress, without instrumentation.  The tube/enclosure can resonate in various ways, so it might be interesting to characterize it.

Also, making slow steps towards getting CFD run on my computer.  That's been painful, as there's very little support getting it to work on a Mac.  I managed to install OpenFOAM on the Mac, and Paraview.  I'm missing some of the solvers right now, as well as a meshing tool, and trying to figure out how to get there from here.  These not so simple structures have interesting interactions with the air packet that sure aren't intuitive to me.  If I ever get CFD to run, (that's not a given,) that's what I am going to look at.  Guess the first thing I'd look at is a simple hollow can with an aperture or two, and get more complex after that.  It would be nice to visualize what was going on.  Maybe I'd even learn something.

[WhatUPSbox?]

I've posted a link to this video in the past. For me, it provides some initial insight as to what may be going on in the can and the geometry involved. Unfortunately I have not found a hobby budget solution for high speed cameras.
   

[WhatUPSbox?]

With that video as reference, I did a quick comparison with a hollow tube (same segments no cones). I think my geometry should roughly envelope what is seen in the video. Measurements are at the cap exit and the muzzle exit as before. Interestingly, the spike at the cap appears before the pellet gets there. Interesting data but not enough to draw conclusions.

[WobblyHand]

Hi Stan, interesting stuff.  I think what you are seeing is the high velocity air pressure pulse escaping from the barrel.  The molecules have an average velocity, but it is not uniform, it's probably gaussian distributed.  So the higher velocity molecules are faster than the pellet and escape first.  In the video that was the first smoke ring (I think).  Cones tend to slow things down, but the high velocity air goes around the pellet and straight out the hole.  As you can see, except for the spike, the shape is smeared out and then likely is exponentially decaying.

When I plotted the audio power of a shot in dB's, I saw an obvious straight line decay, which converted to linear, is an exponential decay.  These moderators are complicated things, despite being passive objects.

[WhatUPSbox?]

I think the molecular mean free path is too short for molecular motion to show up. I wonder if the smoke ring is caused by initial leakage past the pellet before the skirt seals.

[Back_Roads]

Possibly the column of air in the barrel ahead of the pellet, getting rapidly compressed Or kicked in the rear so to speak.

[WobblyHand]

Possibly the column of air in the barrel ahead of the pellet, getting rapidly compressed Or kicked in the rear so to speak.

There definitely is a cylinder of air being hustled along and ejected ahead of the pellet even getting to the muzzle.  And some high pressure air flowing around the pellet just after the pellet clears the muzzle.  After that, hmm, I have difficulty conceptualizing the chaotic flow.

[WhatUPSbox?]

A while back I had pulled some frames from that video and overlayed a rough scale in pellet diameters. That bottom image would roughly fit inside the segments I'm using in my setup. Good seed for flow theorizing. 

[WobblyHand]

Is this series of photos straight out of a barrel, or is there some device on the end?  Doesn't look like the end of a barrel.

Very interesting to see the almost packetized clumps of air, that are about a pellet diameter long.  Like some sort of resonance in the air column was set up. 

[WhatUPSbox?]

They are just frame grabs from the video I linked. The only info is the frame rate (15K). Looking at the diameter, I would guess that it is a bare barrel.
The packets in the jet stream almost look like shock diamonds
https://en.wikipedia.org/wiki/Shock_diamond