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All Springer/NP/PCP Air Gun Discussion General > Engineering- Research & Development

Trying to add pressure sensors to LDC testing.

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WhatUPSbox?:
Over in the 3D printing gate and elsewhere there have been some pretty intricate LDC designs made. For my education, I wanted to look at the very basic features to see what effect they have. I previously posted a brief description of my approach https://www.gatewaytoairguns.org/GTA/index.php?topic=212304.msg156510848#msg156510848 Basically an LDC made from segments that can be mixed/matched and some audio instrumentation. I've added some ports for pressure sensors to see if adding that data helps in figuring out what drives what.

Warning, this is heading off into the technical weeds and may not come out with anything useful.

The first image shows the basic building blocks of the test LDC. There is a mount to the barrel that has a laser gate used for timing and a pressure port right at the exit of the barrel to try to get the pressure behind the pellet. There is a short section that is a little larger than the pellet so the gas can't expand too much right after the pellet skirt exits the muzzle. Once you enter the LDC there are options for segments with cones or just hollow volumes. Some have pressure ports. At the other end is a cap which can have either a radial or an axial pressure port.

The hardware is shown in the second photo. It is mounted to a 2240 with a 1377 barrel. The 2240 has a power adjuster but is otherwise stock.

The third image shows an example of a test shot. The pellet is exiting the muzzle at time = 0 when the laser gate trips. In this case the pressure sensors were placed in the first segment and radially in the cap. They basically provide a time trace of the static pressure at those locations.

Note the pellet spends a little under 1 milli-sec in the LDC (starting at 0)

In the fourth image, one of the pressure sensors is moved from the first segment to the location right at the muzzle.

Not sure if this deep in the weeds is of interest or if any conclusions will come of it but it keeps my brain busy. Doing the pressure measurements with a hobby budget is tricky (for example I could not use my 4 channel DSO because of noise floor), so I have a bunch more checkout to do and the curves in the images are meant to be examples. Hope to combine this with some of the audio response measurements in the other thread.
 

rkr:
That's one part of silencer design, dropping the pressure peak at the end cap exit which usually means longer duration for the peek. One easy way of doing that is to use bigger hole in the cap. From noise perspective it's only part of the story, you also need to look at the noise made by the pressure blast hitting the walls of your silencer.

WhatUPSbox?:
Yes, part of the motivation for looking at the pressure was that in my microphone only testing the noise spike was starting before pellet exit from the LDC. The hope was to start separating those two effects.

WobblyHand:

--- Quote from: WhatUPSbox? on March 03, 2024, 11:11:15 AM ---Yes, part of the motivation for looking at the pressure was that in my microphone only testing the noise spike was starting before pellet exit from the LDC. The hope was to start separating those two effects.

--- End quote ---
Stan, you are doing a nice job at this!  Fascinating.  I hesitate to say this, but could be interesting to apply an accelerometer or similar sensor to detect vibration of the LDC cylindrical body, sort of a ping detector.  I'd bet it really rings.  To reduce the cylinder ring, I'd imagine one would need to increase mass or stiffness, by construction or material choice, or significantly increase damping.  TPU might do this to some extent.  That's why I was printing TPU LDC's.  A lead loaded rubber material would probably work as well.

Had a crazy idea for an LDC material, but no one seems to make the material, which is tungsten loaded TPU.  I did search for quite a while.  Even asked Prusa if they knew of any sources.  They didn't, and told me they wouldn't be manufacturing tungsten loaded TPU either.  I'm sure someone would or could do it, but it would be custom.  Would increase cylinder damping a lot.  I have found tungsten loaded PETG, but it's expensive, and stiff.  The density is about 4gm/cc vs the ordinary density of 1.27 gm/cc for PETG.

WhatUPSbox?:

--- Quote from: WobblyHand on March 03, 2024, 02:34:14 PM ---I hesitate to say this, but could be interesting to apply an accelerometer or similar sensor to detect vibration of the LDC cylindrical body, sort of a ping detector.  I'd bet it really rings. 

--- End quote ---
Yes, the hope is to integrate some of the other sensors into the set-up. If you look at the segment with the port for the pressure probe, there is a flat spot on the opposite side for either an accel or at least a contact mike. One thing I need to check is how sensitive the pressure sensors are to vibration. On one of the test shots I clipped the front cap and the sensor had a response.

The tricky part for me has been the data acquisition side. The pressure sensors have a sensitivity of about 3mV/psia. Unfortunately my 4 channel Rigol DSO only goes down to 10mV/div and is noisy at that setting. I don't want to spend the money (even surplus) for a lab instrumentation amplifier. I tried the AD620 based modules on Amazon but they had their own problems. I may try an authentic AD620 at some point. For now, I have a humble 2 channel hobby DSO that you see in the picture. It does have some useful features. It is battery powered and relatively low noise with a 5mV/div sensitivity and claims 12 bit. It also accepts an external trigger that I can trip with the laser gate. It will store and export 4 pairs of waveforms (1024 points). That's where the Excel plots came from.
As I add the mic/accel channels, those can go to the Rigol and share the trigger. I just have to make sure I avoid the EMI issues. I'm not an EE so this moves more slowly.

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