Measuring Chamber Pressure -- Strain Gauges

[Jeff82]

I think there are commercially avialable strain gauges or other sensing equipment that can the measure CUP/PSI curve of a cartridge. Does anyone know about these?

[Doc Highwall]

Here is what you are looking for.
http://shootingsoftware.com/

[junkpile]

I don't think thats what the OP is after. Looks like he wants to measure actual chamber pressure. Computer programs deal with theoretics. I'd like to be able to measure actual pressure, as well.

[felix]

What you are asking for is a direct measurement of the amount of gas expansion totally limited by a fully enclosed system. Very unrealistic request as a hobbyist. ... felix

[Larry Gibson]

I know quite a bit about the Oehler M43 as I have and use one for numerous cartridges. I'm also some what familiar with the RSI system. They do measure the psi via a strain guage attached to commercial barrels. C.U.P. is a completely different system requiring a Universal receiver and special test barrels. There is no direct correlation between psi as obtained by piezo-transducers or strain gauges and C.U.P. pressure measurements.

Larry Gibson

[Doc Highwall]

The RSI does use a strain gage. The strain gage is glued to the barrel and connects to the sending unit that is connected to your laptop by a blue tooth. I have one and like it very much.

[junkpile]

So how do you get the information from the strain gauge converted to useful information? I'm looking at CUP. What does the strain gauge show me? And how would I use it?

[Doc Highwall]

Go to post #2 and click on the link. Save it under your favorites as there is a lot of reading to do that explains better than I can with links to other sites as well.

[Whiterabbit]

so, this is in the pistol forum. A local guy here does pressure testing for his X-frame. He uses a Ruger #1 that he bought for no other purpose that I can see other than to put the strain gauge on it and measure pressure. That'll keep the gauge off your pistols.

Gave him some of my BFR loads to test, but he said they wouldnt chamber

[Larry Gibson]

I use Contender Pistol barrels for pressure testing; .32 S&WL, 32 H&R, 38 SPL, 357 magnum, 44 SPL, 44 Magnum, 45 ACP, 45 American, 45 Shcofield and 45 Colt. I also use Contender Rifle barrels for Pressure testing 22 Hornet, 223 Remington and 30-30. Most other rifle cartridges can be tested in regular actions. The reason for use of the Contender pistol barrels (+ the 22 Hornet and 223 Rem) is I place the strain gauge over the chamber in the same spot that is SAAMI specified for a peizo-transducer.

Larry Gibson

[Whiterabbit]

is the whole kit expensive? require VERY specific strain gauges? You've got me thinking. I'd love to get a 45 colt handi rifle barrel, ream to 460 S&W so I can put whatever throat I darn well feel like on it, and slap a strain gauge on it.

[Larry Gibson]

I use the Oehler M43 PBL and yes it could be considered very expensive. A computer is also required with both the Oehler and the RSI systems. It also uses very specific strain gauges which must be applied correctly. If not applied correctly the will come off or give erroneous readings. If you are just interested in one cartridge you would be far better off (spend a lot less money) to set up the rifle (new barrel and rechambered) and send it along with the test loads (ten shot test strings of each load) to someone with a system to measure it for you. You'd have to pay shipping (both ways) and for the strain gauge along with whatever that person might charge for the testing. It would be far cheaper that way.

In testing bear in mind that the psi measured in the closed breach rifle with short throat will be a bit higher than in a revolver with the long cylinder throats and barrel/cylinder gap. That is good in a way as if the test loads are at SAAMI Map for the cartridge then it will be a bit less and safe in the revolver.

Larry Gibson

[Whiterabbit]

my buddy probably has the oehler setup, I see him dragging around a laptop when he has the #1 at the range, and has an Oehler chronograph.

So if I twisted his arm and got him to help me out with his equipment, I could just show up with a handi rifle with barrel with strain gauge? If so, may I ask are the strain gauges easily available and cheap? (<$10 a pop for just the gauge in my book is cheap, before the wire pigtail and termination). Are they available from sources other than Oehler?

[Doc Highwall]

Here is my RSI set up with my Remington 40X.

[Larry Gibson]

my buddy probably has the oehler setup, I see him dragging around a laptop when he has the #1 at the range, and has an Oehler chronograph.

So if I twisted his arm and got him to help me out with his equipment, I could just show up with a handi rifle with barrel with strain gauge? If so, may I ask are the strain gauges easily available and cheap? (<$10 a pop for just the gauge in my book is cheap, before the wire pigtail and termination). Are they available from sources other than Oehler?

Not sure about the RSI but with the Oehler the strain gauges are permanently attached to the barrel by a very strong industrial super glue. The bluing at the spot must be removed and it is a process to clean and attach the gauge correctly. Frankly it is not something I would do "at the range". There are also measurements that have to be accurately taken of the chamber and barrel thickness over the location of the strain gauge. Quite a bit of data regarding the load, the firearm and the conditions also have to be input into the program. I get new strain gauges direct from the manufacturer. The last ones I bought were about $15 apiece but I expect they are considerably more now like everything else. The gages also come with calibration data that must also be entered into the program.

All in all such testing is not something you just run out and do. It takes considerable prep time at home to double check the test guns, to enter as much data into the test sheets as you can and make sure you've got everything. At the range it takes me 45 to 70 minutes to set up the equipment, longer if I'm putting screens at 100 yards also for TOF/BC measurements. I then run a "reference ammunition" test on the set up to ensure all is set up correctly. Consistency of set up and data input is essential for repetitive results that are comparative.

Here is my set up of the Oehler M43 at the range. Only have muzzle screens set up. M43 "box" is under laptop. You can see the wire running from the M70 target rifle to the M43. Big blk case carries everything except the screens and tripods. Lots of accouterments and I obviously have the bursting radius of a 4 deuce.......

My project this next winter is to build a indoor test range in my RV garage. That will give permanent set up for pressure and velocity testing with a more controlled "climate". I can then test selected loads for accuracy out to 300 yards at the local range. I can also easily set up to measure the TOF/BC at the local range with all the other testing (pressure, velocity and accuracy) already completed.

Larry Gibson

[Doc Highwall]

Larry, the RSI system is the same as your Oehler system. I have to do the same with the strain gages and everything else. I can say one thing I am sure you will agree with, that using our pressure set ups gives us a lot more confidence in what our loads are actually doing.

[Whiterabbit]

Not sure about the RSI but with the Oehler the strain gauges are permanently attached to the barrel by a very strong industrial super glue. The bluing at the spot must be removed and it is a process to clean and attach the gauge correctly. Frankly it is not something I would do "at the range". There are also measurements that have to be accurately taken of the chamber and barrel thickness over the location of the strain gauge. Quite a bit of data regarding the load, the firearm and the conditions also have to be input into the program. I get new strain gauges direct from the manufacturer. The last ones I bought were about $15 apiece but I expect they are considerably more now like everything else. The gages also come with calibration data that must also be entered into the program.

Hi Larry,

I could have been more clear. I've used strain gauges before for mechanical tests and understand what they are, how they work, and how to apply them (to flat test metal samples). That it would be different on a gun barrel is a given, but I did not mean to imply id show up with a virgin gun and we'd bang something out. I was asking more if it was feasible to independently buy the strain gauge and install on the barrel in my shop on my own time. So we could meet at the range and just plug in and go.

No speed data needed, just pressure.

Makes sense that significant measurement data would be required with respect to the barrel

[Doc Highwall]

Whiterabbit, you have to measure the outside of the barrel where you are going to attach the strain gage, then input what the wall thickness of the barrel is at that point, and measure the case wall thickness at the same point where the strain gage is in relation to where it is located on the barrel. Once this is done these measurements are kept in the program for that gun. The wall thickness of the cases are part of the equation of how the strain gage works and will have to be rechecked if you change lots of brass.

[Whiterabbit]

I could measure each case to be used frankly, for how much testing would be done. Are those the only three measurements that need to be taken of the hardware? (I did catch that the straingauge has a cal file too) Is accuracy to the nearest .001" acceptable? Or maybe put another way, is .001" measurement accuracy sufficient to measure pressure accurate to the nearest 1000 psi? if not, how about the nearest 2000 psi? Nearest 5000 psi?

[Doc Highwall]

Whiterabbit, measuring to the nearest .001" with a tool that has a resolution of .0001" is good enough. When I measured my cases for my 308 Winchester I took the average of 15-20 cases and used that for the input. I used a dial indicator that has a resolution of .0001" and set it up in a RCBS CASE gage that is made for measuring things like this.

The instructions that come with the RSI system are pretty clear on what to do.