doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Strain gauges

OK, now I've got it. But I'm a word guy, and you techie types are still spreading confusion with the use of "peak". Pressure only peaks at one point in the barrel. Whatever the pressure is 6" or 12" or 18" down the bore, it ain't a "peak". It's just the pressure at that point. Look at it in reverse: a car accelerating from a stop. If it reaches 25 mph in, say, 3 seconds, that is not the "peak" speed of the car, unless it stops accelerating at that point. That's just its speed, as measured at that point. Peak is not reached until the point at which the car ceases to accelerate. Same for pressure within a shotgun barrel, but in reverse. If you use "peak" for anything but the highest pressure measured, I can guarantee that you will seriously confuse the non-techie public. It will cause them to believe that pressure within a barrel works like this: ^^^^^, going up and then back down numerous times, rather than like this: \.

Don,

It has been a long time since I used a 'scope. I had an old antique that I tied to the "if" stage of a transceiver. I used it to tune in rtty stations. When I read Cox, I knew right away how hard those guys had to work to get the data.

Pete

You have to generally know the answer or have a priori knowledge from empirical data in order to measure the max deflection of the strain gauge or expansion of the tube. Pressure is an indirect measurement. The deflection is only measured along the finite length of the strain gauge. Seeing the tube is 3D and the measurement is pretty much 2D, I've often wondered how the location for the strain gauge was chosen. Maybe they knew where the majority of tubes had blown. I wouldn't think tube to be concentric, and the added effect only small, but why not choose a section somewhere else than the side, which is probably the easiest location for attaching the device.

Kind Regards,

Raimey
rse

I wonder why there is no pressure-change-color stickers around still?
Just imagine you attach sticker to shell, make a shot, pull out empty shell and reseach your sticker. Depending on sticker color you will be able to determ chamber pressure of your loads.
Cheap and simple for individual use.

OK, Larry, point taken. In the future, I'll refer to chamber peak pressure (which will be the single peak) with the term "peak." Peak pressure at other locations will be location maximum pressure. Now, just to be clear about what happens, as the wad passes any given point of the barrel, the pressure rises rapidly to the "location maximum pressure," and then decays to atmospheric shortly after the wad exits the barrel. How does that set of words play for you, Larry? Will it convey the technology to non-techies?

This is one of the things I love about this site. We can discuss in depth to get both the technology and the words right. Thanks for your input, Larry.

So just how does the wall thickness vary for the 1st couple of inches of the tube outside the chamber? I'm curious if early pattern welded tubes were as regular as assumed. I'd like to see empirical data from multiple strain gauges around the hoop, but perpendicular to it, to support your assumed case.

Kind Regards,

Raimey
rse

Rocketman, you're still using an unnecessary adjective. You don't need "maximum" either, and it can cause confusion. Pressure at ANY location in the barrel, anywhere after the pressure has peaked, is simply the pressure at that location. There is only one pressure at 6", 12", etc. "Maximum", like "peak", means you're comparing it to something. Well, there's nothing to compare it to--AT THAT GIVEN LOCATION. And it's lower than the peak pressure, so it's neither peak nor maximum. And even though the pressure rises behind the wad compared to what it is in front of the wad, that comparison is really of no consequence to someone evaluating the impact of pressure on his (perhaps old and thin-walled) barrel. What he's interested in is what's happening to the pressure in relation to other pressures developed BEHIND the wad. Gradually decreasing or decaying, as I understand it--all the way down the barrel. So while you're technically accurate in saying that there's a peak pressure behind the wad at any given location along the barrel, compared to the pressure in front of the wad, you're dealing with information that is of interest only from a technical standpoint--of interest perhaps to engineers, and maybe those dealing with how many angels can dance on the head of a pin--but having no practical interest to the shooter. The KISS principle works best when attempting to communicate with non-technical folks.

I think of it as I do my own results when I run a 5K road race. Let's say I ran one last weekend. Someone asks me how I did. "Well, I ran the fastest time," I reply. What I don't add is that I ran the fastest time in my age group. And since that age group is over 60, it's often the case that I ran not only the fastest time, but also the slowest time--because it was the ONLY over 60 time. How I did in comparison to the fastest runner in the entire race, or what specific time I ran, is likely what the questioner wants to know--rather than whether I picked up my standard "appearance award" for being the fastest (and "onliest") over 60 runner in the race.

Raimey,
While it's certainly possible to use a straingage on a commonly tapered double shotgun barrel, the makers of Pressure-Trace recommend using a singleshot H&R type of gun. This is because the chamber outside has a cylindrical shape which minimizes the possibility of measurement error for wallthickness measurments by providing a nearly parallel wallthickness for the chamber area, albeit the chamber has some small taper. I had similar questions to the makers when I bought mine.

Soooo, are you saying once you locate the general locale of the peak pressure hoop to attach the device parallel to the hoop?

Kind Regards,

Raimey
rse