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Sidelock
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Sidelock
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Chuck,
I have no problem with either strain gauge or piezo transducer measurements to obtain pressure. I just know that if calibration to a repeatable standard isn't registered in the setup the data is no longer valid except to say this load is higher pressure than that load also measured on the same devices. The industry standard pressure guns are held to a specific set of internal dimensions and are regularly recalibrated to maintain accuracy of the output. I started dealing with both of these devices (before computers were available so the data was recorded by oscillascopes and Fastex cameras) in 1961 in connection with engineering military ordnance. When you ask an ammo company for data they are not able to relate it to your specific firearm. They can only tell you what pressure it should produce within certain statistically defined parameters in a gun of proper construction. The max avg of the lot tested in 12ga should be less than 11,500 psi, max probably lot mean 12,100psi and max probable sample mean 13,000psi. If the lot exceeds any of these values it is junk. Now we come to hobbists measuring a couple rounds from a lot and making pronouncements - doesn't cut it because the numbers tested are statistically insufficient on devices insufficiently demonstrated to be accurately calibrated. Fun hobby for sure and I have played with it myself on a friends equipment.
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Joined: Feb 2004
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Sidelock
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Sidelock
Joined: Feb 2004
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Jerry,
Your concerns are appropriate. That does not necessarily mean that the data collected by a "hobbyist" is invalid, less accurate or useless.
Having been around the metrology end of the biz a little as well as various types of test setups, I can see where the "repeatable standard" matra could be applied in more than one way. For example: Joe, the hobbyist, buys a strain gage calibrated by a manufacturer that produces them for many industries and produces high quality and reliable strain gages with calibration traceable to national standards. He then uses industry proven methods of measurement of his barrels using measuring devices traceable to national standards. Can he get acceptable data? Does he need to put a direct reading pressure gage on the barrel that has a traceable calibration to national standards? IMO, he's met requirements commonly used in many industries. Sure, some could be more stringent and require yet another level of the overall system to be calibrated, but I would say it's as likely as not.
My direct experiences suggest that having measurement methods/equipment traceable to national standards is good, but it doesn't guarantee a good measurement. I think you'll agree that there are other factors that can affect the outcome of measurment besides calibration of equipment in complex applications.
Bill used a strain gage and likely the micrometer or other measureing device that was traceable to known standards.
So, I think it's unfairly dismissive to suggest Bill's equipment and data is either inaccurate, unreliable, not applicable or invalid, as I don't believe there has been enough information shared here to make that determination by anyone. As to "statistically significant" quantities of tests, I dunno, I didn't note whether he tested one or some other quantity of a given load. But like many electronic devices of today, the device is probably more sophisticated than the elaborate setups to gather similar data many decades ago. It really does have tremendous capability. How the data is applied is no less important than the instrument used to collect it.
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Sidelock
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Sidelock
Joined: Aug 2004
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Bill used measuring tools for the barrel such that diameters are to 1/1000 in. The number of loads tested was about 100 per type.As to sampling statistics I did not (even though I taught a class on sampling and the math that is used).The glitch was that the original Pressure Trace was damaged,probably dropped.When I bought the current one and down-loaded the program,the original data was lost.The project a year ago was to take a box of AA skeet shells,number them,fire all and record pressures.Then a member of the club was to reload them and then repeat,for about 10 times.I wanted to see if the shells had a reloadable life.However, life changes and I gave-up my FFL which was for gunsmithing (mainly doubles).Now,I am into,sorta, 1000 yd.with the .308.Really, go to the Pressure Trace site,click on products and there is a good discussion on the product,including some stats.Remember,there are:Lies,Damned Lies,and Statistics (Queen Victoria)
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Joined: Feb 2003
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Sidelock
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Sidelock
Joined: Feb 2003
Posts: 4,109 Likes: 78 |
Chuck: It's all just a different kind of Tarot card, now isn't it?
To quote Ernie Gann explaining yet another aviation engineering failure, "... some totally unrecognizable genie has just unbuttoned his pants and urinated on the pillar of science".
"The price of good shotgunnery is constant practice" - Fred Kimble
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Joined: Feb 2004
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Sidelock
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Sidelock
Joined: Feb 2004
Posts: 13,883 Likes: 19 |
 Those genie's have cropped up from time to time. Or at least they seem to. More often than not, they're found to be "known genies" that we've corked up before. Rare to see a new one these days. That Air Frog Scarebus seems to have been a stall to splashdown. The chauffers tunnelled in on the wrong instrument and ignored the logical...power n attitude makes for flight, yankin around makes for fright. 'guess they chose wrong that day. I'da thought ISIS woulda saved the day.
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Joined: Feb 2003
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Sidelock
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Sidelock
Joined: Feb 2003
Posts: 4,109 Likes: 78 |
I thought you might enjoy that.
So, tell me again why we proof guns.
If I built a bridge for a one ton truck, and then drove a two ton truck over it just to 'prove' it would hold a one ton truck... and it sagged, but then came back to the original shape...
Haven't I just put more fatigue into the bridge structure than a million one ton trucks?
Seems like all I've 'proved' is that it held a two ton truck once.
As a result I now have a well used instead of new bridge, that could fail from fatigue any time.
On the other subject... three pilots sat there adding 'nose up' the whole time? If your attitude is +15, and your flight path is -15, take a wild guess as to the angle of attack...
I have to believe the fly-by-magic systems were preventing the crew from unstalling it. Like the Paris airshow... the airplane thinks it's smarter than the crew. Stalled for three minutes and no clue? I wouldn't accuse even Air France of that.
"The price of good shotgunnery is constant practice" - Fred Kimble
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Joined: Feb 2004
Posts: 13,883 Likes: 19
Sidelock
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Sidelock
Joined: Feb 2004
Posts: 13,883 Likes: 19 |
We proof new guns more out of tradition than most any other reason. Material control, dimensional control, etc. is more than enough to assure safety. I sure as heck hope nobody overstresses an airplane to "proof" it. But a very low number of high stress cycles on a low strength steel like barrels and frames have is not going to cause fatigue. The frog fright was more like +35* alpha. The article suggested it was the crew choice to go nose up and power off. http://www.flightglobal.com/blogs/learmount/2011/05/air_france_447_the_facts_and_w.html
Last edited by Chuck H; 06/04/11 12:17 PM.
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Joined: Jan 2002
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Sidelock
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Sidelock
Joined: Jan 2002
Posts: 5,954 Likes: 12 |
So, tell me again why we proof guns.
If I built a bridge for a one ton truck, and then drove a two ton truck over it just to 'prove' it would hold a one ton truck... and it sagged, but then came back to the original shape...
Haven't I just put more fatigue into the bridge structure than a million one ton trucks? Not necessarily!! Test loads (think "proof" pressure for guns) must be designed at appropriate levels for the material and the safety factor designed in. An airplane structure (application is very weight sensitive) might have a 1.4 safety factor where a bridge might have a safety factor of 3 or 4 (weight is of no consequence in application). Test loading above rated load would be quite different unless you were funded to run to fatigue limits. In the case of the bridge, that might be somewhere near infinity, OK, at least many, many millions of cycles. Since stress/strain characteristics are pretty well understood for metals, as is cyclic loading (fatigue), we can test a component to failure and walk away with a pretty good understanding of failure load, maximum working load, normal operating load, and the fatigue associated with each.Seems like all I've 'proved' is that it held a two ton truck once.
As a result I now have a well used instead of new bridge, that could fail from fatigue any time. No, not unless the load of double rated represented a fatigue life of one cycle. If the bridge is steel and no yielding of components occured (plastic deformation), then you have a many millions of cycles of fatigue life left.
On the other subject... three pilots sat there adding 'nose up' the whole time? If your attitude is +15, and your flight path is -15, take a wild guess as to the angle of attack...
I have to believe the fly-by-magic systems were preventing the crew from unstalling it. Like the Paris airshow... the airplane thinks it's smarter than the crew. Stalled for three minutes and no clue? I wouldn't accuse even Air France of that. Paniced people do unexplainable things.
A properly installed strain gauge on a decent test bed gun will give a reasonably accurate reading for pressure. Within the scope of quality control, other guns will experience a very similar pressure for this load. However, as Chuck has noted (several times), knowing the pressure of a load doesn't tell you about its suitability for use in any specific gun. You can use generically/anicadotially derived pressure limits for the gun. Or, you can calculate hoop stresses and set pressure limits that don't exceed the tensile strength of the barrel material.
Last edited by Rocketman; 06/04/11 10:58 PM.
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Sidelock
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Sidelock
Joined: Jan 2002
Posts: 5,954 Likes: 12 |
Simple question from a non-technical type: If the reading provided by the gauge depends on the dimensions of the metal, then how could a single strain gauge provide an accurate reading anywhere other than where it's placed? The strain gauge provides a trace of pressure vs time. The pressure inside the barrel "combustion chamber" will be relatively the same throughout; the pressure immediately behind the wad is about the same as in the chamber. This being true, the strain gauge pressure reading at any point of the trace will be useable as the pressure for any point behind the wad. Assuming it's reading pressure in the chamber, there would not be any way to tell the pressure farther down the barrel, where the dimensions of the metal change significantly . . . would there? Yes, but we need to convert the time axis to displacement. This requires a fairly complex set of calculations; just the sort of thing computers do well. I'm not sure if any of the pressure insttruments provide an option for pressure vs displacement --- yet. As I understand, that's why Bell used several strain gauges at various positions on the barrel. This is a good approach, sure fire, if you will, but not the only approach wsith strain gauges and piezo gauges, both of which give pressure vs time traces. With crusher technology it is the only way due to the crusher giving point peak pressure only and time or displacement base data; hence the barrel bristling with crusher ports. His tests were made in an effort to determine whether the peak pressure really does vary significantly between black and smokeless, and between fast-burning and slow-burning smokeless powders. From his results--and I think also from the earlier Dupont tests--it does not appear that there is much variation at all in the location of the peak pressure (always in the chamber), although there is some in how quickly the pressure drops. But, not generally enough to reliably suggest switching from one powder to another to protect thin profile barrels.
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Joined: Aug 2004
Posts: 411
Sidelock
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Sidelock
Joined: Aug 2004
Posts: 411 |
The big tell me is:who has the absolute perfect instrument that will measure psi in a gun while firing?I reported on a method that I bought and used.The comments were not perfect.The one that concerned me the most was that I could be liable (at the time and now I covered that by being an LLC and buying liability coverage).I find that I am now very unlikly to ever post on this board.
Last edited by William E Apperson; 06/06/11 10:09 PM.
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