Hacking the CP-2

[WhatUPSbox?]

Thank you for the clarification on the VRT characteristics. I realize that instrumentation grade accels are the standard for this kind of measurement but I was poking at other methods to see if they may be useful, especially for directly measuring velocity or displacement.

I was looking at what the powder burner folks are doing on the barrel harmonics front. There are several pages on this site http://www.varmintal.com/aeste.htm that combine FEM analysis with empirical point of impact results. Granted this is for high velocity rounds, but their heavier barrels keep the frequencies in the general range of interest. The FEM results also discuss the muzzle slope and vertical velocity contributions to POI spread. Not sure how accurate the pressure analysis is as a forcing function but the characteristics of the barrel FEM should align with handbook results. Interesting reading.

For the 2240 with 7" barrel and barrel band that I've been playing with, the affect of barrel harmonics on accuracy is primarily a thought exercise.

Fun stuff

[George Schmermund]

Thanks for posting the link. I've seen it before and it does have some interesting drama in it. The real foible it demonstrates is that the results are obtained when the analysis is confined to just 1 degree of freedom. Most (if not all) of what is presented represents only the vertical motion of the barrel. This makes great graphs and animations, but I highly doubt that they represent more than what can be seen when you only have half of the information. I see this as the trap of spread sheets. The pressures and temperatures achieved with PB guns is a whole other can of worms compared to airguns. I'll stick with my own measurements in that arena.

Keep in mind (pun?) that thought experiments are a close relative of spread sheets. They're good as trailheads, but they can't be reified until that first step onto the path is actually taken. Of course they're a lot easier to tote around compared to experimental hardware.

Your 2240 experiments are still valuable exercises even if you conclude that there's not much going on that needs correction in terms of barrel vibration. When you get your inevitable CP-2 with the 18" barrel you'll have all of the needed hardware ready to go!

[George Schmermund]

The testing is back to the pressure and time of release of the pellets. I'm finally getting comfortable with the settings and displays on the DSO. This will improve the quality of what is reported as the testing proceeds.

One of the things that can be reported on is the durability of bamboo skewers over thin wooden dowels. They're about the same diameter and fit nicely down the barrel for doing the pellet release experiments, but the wooden ones just about explode on the first shot. The bamboo ones hold up very well for up to a half dozen shots. Their mode of failure is typically splitting, but they stay mostly in one piece. This got me to thinking about the fibrous structural differences between the 2 materials. I was going to haul down on of the microtomes from the attic and do some cross and longitudinal sections that could be stained and then microscopically inspected to ascertain whether bamboo was a monocot or dicot. After a couple of more beers I decided that I might be able to save myself a week's work by just looking it up on the web. Bamboo is a monocot. This explains a lot and may be good news for Stan when he makes his grass tree (monocot) probes.

[WhatUPSbox?]

OK, Since George requested monocot probes, here we go. (I'm glad George introduced the monocot label, with all the legalization going on in CA, I was afraid people would get the wrong idea from a "grass tree" probe)

Image 1 shows one of the early probes with the Al flag attached. The weight ranged from 3.6 to 4.6 gn. I also switched to Hobby wadcutters at about 12gn in the 2240. Some of the probes were straighter than others, all were dry to minimize weight.

Image 2 shows the probe in the barrel, wire across the muzzle (I subsequently cleaned up the broken-wire system), and the laser light trap set at the outside  edge of the probe flag.

The hammer probe and the tube/valve mic are in place and used as before

Image 3 shows an overall trace. As before, yellow (1) is the mic on the tube, dark blue (4) is the hammer probe, light blue (2) is the broken-wire across the muzzle, and the pink (3) is the trap at the muzzle. The hammer probe triggers the trace ~when the hammer hits the valve.

Image 4 shows the timing of the valve. The mic and the hammer probe detect hammer impact, independently, within about .05 ms fairly consistently. The hammer probe shows the valve closing after 1.86 ms (this ranged from about 1.6 to about 1.9 ms)

Image 5 shows the pellet timing. The pellet/probe begin to move at .45 ms and exits the muzzle at 2.59 ms for a 2.14 ms transit time. There is a little bit of ambiguity on the exit time since the probe could break the wire as well but the wire and the light trap are within .13 ms of each other and are generally consistent with the pellet only shots

Some observations: Both the hammer probe and microphone appear to capture the hammer strike event pretty well. The pellet probe was consistent at .45 +/-.05 ms and should remain valid for other shots where the probe is not used. I think doing sequential shots with different length pellet probes could map the acceleration of the pellet. Best timing tool for the muzzle exit is the wire but it is probably best to establish exit time in separate shots, without the probe to avoid the chance of the probe breaking the wire (or tripping the light trap). My original question was whether you can get useful information on the 2240 event timing using just hobby detectors. These results are good enough to answer my curiosity, and other than the DSO hit, the wallet was unharmed in the process.

[George Schmermund]

Your measurements are looking really good. The laser/flag signals can't get much cleaner than what you're showing now. It might be of interest as an exercise to try some digital filtering on the mike channel. There seem to be at least 4 signals on it.

How well are your probes holding up as projectiles?

[George Schmermund]

So as not to get left behind on these measurements I did a quick test with a Crosman 14.3 gr hollow point. This current arrangement for doing pellet release time and pressure is working well. The yellow channel is the total T/P curve for the shot and the aqua channel is the probe/pellet contact time. The cursors show the release time to be 390 µS. The pressure at that time is 232 psi. The measurement panel gives the peak pressure to be 504 psi.

It will be interesting to do an average of a few shots with these CPHP pellets and then see how it compares to other pellet averages.

[WhatUPSbox?]

At 4 gn each, even the mighty monocot material does not survive impact. The range is short enough that the pellet may help in the destruction. Definitely want to have a scatter shield around the path.

Yes, I definitely want to do a dive into the filtering and waveform math capabilities of the DSO. The slower features on the right side of the first trace image are around 400 hz. I see that with a vertical tap on the barrel. Not sure if one can ID any of the Khz stuff.

[WhatUPSbox?]

George,
I really like that pressure curve and pellet release data. There are a lot of pellet sizing and bolt tip designs that this will shed some light on.

It is probably just a coincidence but the pellet release times being similar for the two guns is intriguing.

[George Schmermund]

Stan, I like your probe alliteration. Even if it's a 1 shot probe, from the image's I've seem of your source you should have access to plenty of spares.

Now that we're into Probe Wars 2 I'll offer up a new contender. Though not the bantamweight that your putting into the ring, I'm confident that my guy can become a worthy opponent after he gets a proper workout.

He's a relic of a bygone era when wire wrapping circuit boards together was King. Who ever thought that he could make a comeback?

To start with he stands 9"tall and weighs in at a flabby 5.6872 gr. I'm going to put him through a tough workout regime that will force him into a better shape.

First I'm going to put him on an improvised Inquisition rack (a vise and pair of pliers) and see how much he can take. He should come out taller then when he went in. Once he's cut back to 9" again he will have lost some weight and have been hardened by the exercise. This should keep him standing upright against an inevitable pellet haymaker.

One of his sterling virtues is that he's silver plated and has a very tough hide. I'd add that he's got a wiry build, but I don't want to give too much away before we get into the ring.

The photos show a 14.3 gr CPHP and my guy before conditioning.

[WhatUPSbox?]

The smallest solid wire I had was .018" core and with the insulation it started to get chunky. I also wasn't sure that the leads would look like. I considered using the magnet wire on the monocot probe but it seemed like too much work per shot, so I stayed with the opto-mechanical approach.....and it had a laser

I do have scale envy though...very nice

[George Schmermund]

Now, this is a monocot probe. I pulled this bamboo skewer out of the trap after it had already been used several times for pellet timing shots. It speared either a CPHP or CP domed pellet that I had been shooting. I think I'll build my next house out of bamboo.

[George Schmermund]

Most of the accelerometers used in the type of vibration measurements we've been discussing come in 2 flavors if they are piezoelectric.  Typically the crystal elements will be quartz or PZT. The two main construction methods are to either compress or shear stress the crystal by using an attached seismic mass. The mass is attached to the crystal and is free of the base and housing. The crystal can then produce a signal that is proportional to the acceleration of the seismic mass. This is a simplistic description, but useful for designing a small DIY device.

That being said, I decided to explore the possibilities of a start from scratch compression style device. What I've come up with is about as simple as it gets as far as raw materials go. So far things are not much different than Stan's microphone devices. The main ingredients include a pellet tin lid, some single and double backed tape, and 2 pieces of thin shim stock. Also included is a double duty fridge door mini magnet that was pilfered from the kitchen. The double duty part is by using the magnet as both the compressor and the seismic mass.

As can be seen in the photos the crystal is sandwiched between the magnet and the lid. Also seen is another disc like the one used in this experiment before a 1/4" piece was punched out of the center of the first one. The DSO image is the device's output from a tap on the lid from a monocot probe. This is a proof of concept device. The next one will dispense with the lid as a building platform.

This test opens the door to more and better DIY designs that are easy to accomplish and cost a total of about 50 cents each.

[George Schmermund]

Stan - Thanks for taking note that these experiments could have a valid use for anyone interested in the finer details of how some airguns work. It's still early in the game, but the results are getting better and easier to arrive at. Your involvement in your own experiments is further indication that this can be an interesting and meaningful endeavor. The clever approaches to instrumentation you've come up with may push someone else over the edge. This is after all an R&D area for posting.

I see the measurement results that coincide with 2 different guns and methods of generating data as confirming that if we both use the same caliber pellets and push them with similar power plants the release times and pressures should be similar. What happens by the time the pellet gets to the muzzle may have a larger variance.

It may be just you and me in the lab, but there seem to be a lot of people looking in through the windows! 

[WhatUPSbox?]

Yes, a lot of the fun has been in breaking down the airgun timeline into steps that can be measured individually by relatively simple instruments. Other than aiguns not being politically correct these days, these measurements would make a great STEM project for someone interested. The DSO is convenient, but with a little cleverness I think most of the timing measurements could be captured with a sound card, someone mentioned a logic analyzer, or a microprocessor board like Arduino or Raspberry Pi.

I think the pressure, pellet release timing, and final muzzle velocity is an interesting relationship. Bolt tip design, pellet placement, and pellet skirt sizing all play in this and there are plenty of ideas and explanations to consider. For example, I still don't understand why the pressure rise in your pellet probe cases is significantly slower than the ones without the probe.....calls for more of those thought experiments

Fun stuff as always

[George Schmermund]

The measurements that I've been doing up until now are pretty much seat of the pants stuff to test out the hardware. Now I need to test a series of pellets from the same tin and see if there is a set of predictable curves that will somewhat define the lot. If that's done for different pellets there might be some interesting characteristics that can ascribed to them when they're all shot from the same barrel under the same conditions. 1 point doesn't define a trend, so we'll need lots of points.

[WhatUPSbox?]

That's a great plan. The 232 psi on a .22 pellet should be a little under 9 pounds of force on the pellet, maybe a little less if the bolt tip is masking some of the pellet. I tried a crude version of the pellet push test using a postal scale and for the spare 2240 barrel, stock with no leade clean up the initial force was in the 3.5 to 4.5 lbs for the cphp and wadcutter respectively (note, I didn't check how far the stock bolt pushes the pellet into this skirt sizing so the actual firing force may be less). I would expect this value to increase when the skirt has outward pressure on it. I don't know if you did any pellet push test after you cleaned up the breech side of the barrel. It will be interesting what you get from different pellets, also if you pre-size a pellet using a spare barrel. This also ties into the pellet measurement exercise from a few months back...I thought that would be useful at some point.

The other question is what is the optimum pressure for pellet release and pellet drag in the barrel.

I'm probably 1-2 months from being able to do pressure measurements so....Go George!

[George Schmermund]

I've been reading some posts in other threads that are claiming that the pressures and times of pellet release in PCP guns is something very different than what Stan and I are measuring. I'm becoming fairly confident that the numbers we're arriving at while doing these experiments are real. This also would include notions about the real world of simple harmonic motion (SHM) as it relates to barrel harmonics.

...

I'm editing this post at this point to just say that upon reflection it seems best if I just surrender to the fact that there are two camps reporting on how some airguns may actually work. Some posters are offering the results of instrumented test data and others are offering spreadsheet calculation data.

This is not to say that no calculations were done by the first group nor that there were no measurements made by the second group. The differences between the two camps can be viewed as a choice in the foundations on which they chose to build their understanding of how some airguns work .

For my part I differ with the idea that for a 1" long transfer port system, it takes about 5 uSec (0.000005 sec.) for the air to get to the pellet, and maybe only twice that for the pressure behind the pellet to reach nearly full valve (reservoir or plenum) pressure. Nor do I subscribe to the idea that we can consider the pressure rise at the pellet to be basically complete and instantaneous. I'm putting nothing in quotes here so that I don't set myself up for a claim that I'm taking anything out of context. I'd just like to say show me the measured P/T curves.

The same goes for single axis SHM reports. It requires at least 2 transducers to actually know what the barrel is doing during the shot cycle in my world. POA/POI information will always contain unknown and ostensibly uncontrollable variables.

Therefore, I'm going to take the 'separate but equal' view on these matters. If I embrace a "suspension of disbelief" approach to all of this, community harmony can prevail.

I'm here to learn and share.

[WhatUPSbox?]

George,
If you don't mind, post a couple of links to the PCP threads. I would not be surprised if there was a broad range in times and pressures for pellet motion. When I first got my CP-1, the lack of a leade sized the pellet to where it would fall out the barrel if I pointed it down. I think the pellet fit is an additional tuning parameter.

[George Schmermund]

Stan - I've edited post #256 to try and avoid any conflicts if can. I don't own a PCP airgun yet, so maybe I'm just being naive about the differences of how they work.  Take what you will from the edited post above.

[WhatUPSbox?]

George, I don't know about others, but I don't see comparisons between analysis and test as conflict. In my past life these were always intertwined and mutually beneficial. Complex phenomena like the airgun cycle need to be broken down into steps that can be addressed sometimes with analysis and sometimes with tests. When the results differ, that's when the best learning takes place. Both approaches have an obligation to discuss and estimate the uncertainties in their approach.

After all, this is a hobby....here to learn