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

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. No, it is the pressure at that location at that time/displacement. The pressure at location X will be maximum for X immediately after wad passage and it will be lower for X a split second later or fraction of an inch of additional displacement. There is only one pressure at 6", 12", etc. Nope, there is some maximum and any pressure lower than that maxium and higher than atmospheric depending on time after ignition or displacement of the wad. "Maximum", like "peak", means you're comparing it to something. Yes, we are comparing "location X's maximum" to location X's pressure some fraction of a second later or some additional wad displacement later. Well, there's nothing to compare it to--AT THAT GIVEN LOCATION. The pressure at that location at a later time or additional wad displacement. And it's lower than the peak pressure, so it's neither peak nor maximum. It will definitely be lower than "chamber peak" or just "peak" as we agreed. However, the pressure immediately after wad passage will also be the maximum pressure the location experiences for that shot. At any later time or additional wad displacement the pressure at that given location will be lower. 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. Pressure differential across the wad (front to back) is not what is under discussion. What he's interested in is what's happening to the pressure in relation to other pressures developed BEHIND the wad. At any given instant or wad displacement, the pressure is relatively uniform within the volume of gas; chamber to wad base (back to front) and across the barrel (side to side). The pressure for the whole volume varies with wad displacement and can be displayed as varying with time. Gradually decreasing or decaying, as I understand it--all the way down the barrel. Do you understand that if the wad exited the muzzle at, say, 400psi, the chamber pressure at the instant of wad exit would also be 400 psi despite a peak chamber pressure of, say, 10,500 psi? 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. Not what we are talking about. The KISS principle works best when attempting to communicate with non-technical folks. If we can get you on the same page technically as me, I expect you can help with the KISS wording- - - I'm counting on it.
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. Try this. Lets say you run a "race" where you are required to run as fast/hard as possible for you until you reach some set distance. You would quickly reach some peak speed and then taper off. The correlation to pressure would be how fast are you running at any given point along the raceway? If we were to plot you speed at time or distance increments, the graph would look much like a pressure trace for a firearm - IMO.

Soooo, are you saying once you locate the general locale of the peak pressure hoop to attach the device parallel to the hoop? Yes. Hoop strain to hoop stress to pressure is the most straight forward approach to cylinderical pressure vessel, which is a fair to middlin' description of a firearm. Locating the gague around the barrel eliminates a lot of variables we really don't need. With a generally round barrel of modest taper, both internally and externally, we get an accurate enough system for our application.

Kind Regards,

Raimey
rse

Rocketman, you're still trying to make this whole thing unnecessarily complicated, except to engineers. No, not I!! I'm trying to make it real simple - as simple as possible. Nobody--except engineers--gives a tinker's damn that the pressure behind the wad happens to be higher than in front of the wad. I've absolutely no idea where the reference to pressure differential across the wad came from. Certainly, I did't mention it. Obviously it's higher behind the wad than in front of the wad, so obviously it changes at location X when the wad passes. But that's both irrelevant and potentially confusing to shooters. And not part of this discussion, at least from my view. What SHOOTERS want to know is: 1. Where does the peak pressure occur, and what is it? Yes.and 2. How quickly does that peak pressure decay? "Quickly" relative to what? Time? Displacement? The pressure at a given point with which shooters are concerned is the pressure behind the wad. Yes. That's the ONLY pressure they're interested in at that point, and referring to it as "peak" or "maximum at that location" will only create confusion. I'll accept that the use of the term "peak" for more than one pressure might be confusing. However, I surely don't see the term "maximum at that location" as anything but clear and concise.

My wife, with advanced degrees in both English and journalism, spent several years as a webmaster and editor working with engineers. Her job was to take what they wrote and make sure it could be understood by non-engineers. Rocketman, if you're presenting a paper on the technical aspects of pressure within a shotgun barrel, you're doing just fine. But trying to present essential, useful information to everyone else . . . you ain't there yet. OK, then I'll keep working on it.
All of this reminds me of the story about executions taking place during the French Revolution. Lawyer has his head under the guillotine, but the blade does not fall. The authorities determine it would be illegal to attempt to execute him again, so he goes free. Next is a priest. Blade does not fall. Authorities figure divine intervention, so he goes free. Next is an engineer, who looks up at the blade and says "Hey, I think I see your problem . . . " Moral of the story: Sometimes it's better not to present all of your technical knowledge.

The problem continues to be multiple uses of a word--"peak", "maximum", take your pick--that have the same meaning. And it's easy enough to convey precisely what one means, and precisely what the shooter wants to know, without using words that are synonymous--and therefore potentially confusing. Like this: "This load produces a peak pressure of 8,542 psi at 1.06 inches from the breech. At 6 inches, the pressure has decreased to 6,342 psi, and it has further decreased to 5,063 psi at 12 inches." Because all the shooter is interested in is the highest pressure at each of those points down the bore, it's UNDERSTOOD that those readings are "local peaks", if you will. Well, if he is interested in the "highest pressure" at a given location, I will accept use of "location peak", "location maximum," and/or "location highest pressure." We need to designate one as we need a term. "UNDERSTOOD?" I wasn't aware that all shooters had a common understand of this topic and a common need. But if you say that they're local peaks or maximums, then the reader may visualize that the pressure is rising again to reach those peaks, rather than steadily declining--which is what pressure does, in every graph I've ever seen. You go up to a peak and you come down from it. If you repeat peak or maximum, then the image you are projecting is like a range of mountains rather than the descending slope of a single mountain: coming down, then going back up, then coming down, then back up, etc.
A range of mountains, each peak slightly lower than the previous one, can look like that. Pressure within a gun barrel cannot. It's one continuous descent, from one peak. The rate of descent can certainly vary, but it's always headed in only one direction. If I place a strain gauge at any location ahead of the chamber, it will show a sudden pressure rise to barrel pressure as the wad passes and then a decline. The trace would show a clear peak and decline. So, one must make it very clear what part of the overall trace one is focusing on. What statement/term clarifys the difference between the overall trace and a location trace?