two strain gages on one gun

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mtngun
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two strain gages on one gun

Postby mtngun » Mon Feb 04, 2008 6:33 pm

from the old forum:

Since the last range session, I mounted a second, new strain gage on the 30-06. It is located in the same place as the other gage, except the old gage is on top of the barrel while the new gage is on the bottom. In theory, they should give the same results, but I was concerned that the original gage may have been damaged by the epoxy fiasco, previously discussed on another thread, so I wanted to check it.

Heres the "validation" load with the old gage, in 30° temperatures. Note that this gage is set up with a -8000 psi calibration offset. Note that the validation load\'s pressure was about 58 ksi last time and now it has dropped to 43 ksi. Note that there is just a hint of a baby secondary spike, which is normal for this load and this gage.

Image ** missing pressure chart, darn it **

Here's the "validation" load with the new gage, with no calibration offset. Note that it shows a major secondary spike.

Image ** missing pressure chart, darn it **

So which gage is correct?

I think the true pressure is around 55 ksi, judging by the low velocities. In warm weather, the velocity is 100 - 150 fps higher and the pressure is probably closer to 60 ksi. Gage number two\s 5000 psi error could easily be explained by the barrel\s considerable taper, which the calculations don't take into account.

Why did gage number one lose 15,000 psi since the last range session? Possibly adhesive deterioration. For now, I can only say that it pays to do a validation" test at the beginning of each session.

Why does gage number two show a strong secondary spike while gage number one shows only a very faint secondary spike? I don't have an answer, but I tend to have more confidence in gage number two since it is brand new.

So here is the classic powder for the 30-06, IMR 4350, giving ugly secondary spikes. This load is MOA if I do my part. The only thing I don't like about the load is that it is very temperature sensitive, barely 2700 fps today but 2850 on a hot summer day. Makes you wonder what the velocity would be during hunting season when temps often drop below zero???

For now I will stick with the new gage. The old gage will be removed and replaced by yet another new gage from a different source. Check back in a couple of weeks and we'll see what gage number three does.

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Re: two strain gages on one gun

Postby mtngun » Mon Feb 04, 2008 6:40 pm

This data was taken with gage #2, as discussed on another thread, and no adjustments were made to the calibration even though the validation load was 5000 psi high. In other words, I believe the pressures shown here may be about 5000 psi higher than the true pressures.

Image

Here's one round left over from last time with IMR 4350 pushing a 180 gr. loverin, as discussed on a previous thread.

Image ** missing pressure chart, darn it **

Here's a new improved 180 gr. loverin pushed by Ramshot Big Game. Big Game is faster burning than 4350 and I bought a pound hoping it would solve the secondary spike problem. Obviously, it didn't. The particulars are 53 gr. Big Game, Fed 210, HTWW, Rooster HVR, 2.975" COL, and the front band engraved the rifling. Five shots spanned 6.4" at 100 yards. Velocity was 2573 fps.

By the way, there is a big discrepancy between what Quickload predicted and what Western Powder Co. data predicted for this powder. Now that we have some hard data, Quickload is vindicated.

Image ** missing image, darn it"

Here's a 180 gr. bore rider pushed by Ramshot Big Game. The particulars are 54.1 gr. Big Game, Fed 210, HTWW, Rooster HVR, 3.133" COL, and the 0.3007" nose did not engrave the bore when seated just deep enough to cover the lube groove. A loverin makes more sense for this long-throated rifle, but I had to try a bore rider just to say I tried it. Velocity averaged 2769 fps.

Image ** missing image, darn it **

How about those secondary spikes? Yes, they are real, and yes, they suck. They go off the scale, so we don't know exactly where they top out, but it looks to be 120 ksi or so, enough to put the gun in harm's way. Since Big Game didn't get rid of the spikes, I'll try an even faster powder next time, maybe Varget. I suspect it would also help to load to SAMMI max pressures, because these powders need pressure to burn correctly. Today I deliberately loaded below max because of the discrepancy between Quickload and Western Powder Co.

The secondary spike needs to be fixed before the poor accuracy can be addressed, since the spike may be part of the accuracy problem.

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Re: two strain gages on one gun

Postby mtngun » Sun Feb 10, 2008 6:15 pm

3006 pressure trace: 3 gages on one gun

The usual validation load, which was averaging 53655 psi at the previous range session.

Gage #2, (the gage used at the previous session). The barrel had not been cleaned. The first shot had low velocity and a funny trace, but shot #2 was near normal. Apparently, some guns may need several fouling shots before things settle down.
Image

Gage #4. This is an Omega gage, part number SG-3/350-LY11, attached to the bottom of the barrel with JB weld. The traces seem very similar to gage #2, so I'm pretty happy about that. :D
Image

Gage #5, an Omega gage attached 2" from the muzzle with Loctite 401. The 30 ksi pressure agrees fairly well with the 30 ksi spikes seen by gage #4, but does not agree with the 50 ksi spikes seen by gage #2.
Image

A second Omega gage was attached near the breach with Loctite 401, intended to provide a comparison between Loctite 401 and JB weld, however, it would not calibrate at the range. When I got home I isolated the problem to the female connector. The connectors are pretty dainty.

The good news is that the Omega gages seem to work just fine, and the primary pressure was nearly identical for the two gages on the breech. At 10 for $55, the Omegas are more affordable than the RSI gages. The Omegas do not incude a solder pad or a connector. I can live without the solder pad, and for now, I am using RSI connectors. If anyone knows an alternate source for the connectors, please chime in.

The gage on the muzzle seems to confirm that there is a pressure spike near the muzzle, but we still have some questions about the magnitude of the spike. Gage #2 spikes are around 50 ksi, gage #4 around 30 ksi, and gage #5 around 30 ksi.

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Re: two strain gages on one gun

Postby mtngun » Sun Feb 10, 2008 6:24 pm

Here's a picture of the Omega strain gage.

Image;

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Re: two strain gages on one gun

Postby mtngun » Sun Feb 10, 2008 6:53 pm

More muzzle gage data, attempting to verify that the muzzle gage reads about the same magnitude of muzzle pressure as the breech gage.

Here's a Barnes X as seen by breech gage #4. The Barnes X usually has little or no spike, depending on which gage is being used. Gage #4 seems to think that the Barnes has some 30 ksi mini-spikes.
Image

Here's the Barnes X as seen by the muzzle gage. The muzzle gage is only seeing 15 - 25 ksi, and it could just as easily be normal muzzle pressure, not a spike. On the one hand, the muzzle gage is seeing about 15 ksi less than breech gage #4. On the other hand, both gages are seeing modest muzzle pressures and not seeing huge secondary spikes.
Image

Here's a 210 gr. cast loverin, 54.9 IMR4350, 2532 fps (the 210 grainer was a futile attempt to make the spike go away by increasing the bullet weight. It did bring the spike down from the redline -- but still unacceptable). Breech gage #4 thinks the spikes are 62 - 105 ksi, with quite a bit of shot-to-shot variation. The primary pressure is excessive and I couldn't tell you what those funny primary peaks are about.
Image

Some of the cast bullet loads that I shot today failed to "trigger" the muzzle gage, and others triggered but only flat lined, even though those loads usually have a mega-spike. The PT trigger had been set on the recommended level of "5". If you set the trigger level too low, it will sometimes trigger when you close the bolt or even when you dry fire. If you set the trigger level too high, it may not start recording data until it is too late, or even fail to trigger at all. Thinking maybe the muzzle gage was not triggering properly, I changed the trigger level from "5" to "2" for this next trace.

This is the same 210 gr. load, as seen by the muzzle gage. Due to the over-sensitive trigger level, some of the traces started too early, and they all started at different times. That makes the graph look funny, but the important thing is that they all recorded the secondary spike, and all the spikes are 65+ ksi, roughly consistent with breech gage #4..
Image

Conclusion there are noticeable differences in the pressure of the secondary spikes as seen by the various gages, however the general trends are consistent. When the breech gage sees a mega-spike, the muzzle gage sees a mega spike. When the breech gage sees low muzzle pressure, the muzzle gage sees low muzzle pressure. In between, there is some grey area, but no major contradictions. If you care to try a muzzle gage, you may need to lower the trigger level.


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