The Accurate Reloading Forums THE ACCURATE RELOADING.COM FORUMS Guns, Politics, Gunsmithing & Reloading Reloading anyone using the Pressure Trace strain gauges?Go | New | Find | Notify | Tools | Reply | |
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These are a bunch less money than the Oehler setup, and I like the way the gauge can be left on the rifle. I'd like to hear from someone who's used them exactly how to set up the rifle, measure where the strain gauge is, how difficult to set up, anything pertinent about the use of their system. | ||
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btt (I am thinking of getting one!) jpb | |||
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Check posts by Denton, he has one. Check posts by Hot Core, he detests them. | |||
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In the June 2004 issue of Handloader, there is an article written by John Barsness that may be of interest to you. The article, titled "Reading Chamber Pressures," deals specifically with the Pressure Trace. It's a great tool if you are interested in knowing your rifle pressures during load development, but you must buy the correct brass for the tests. It's all explained in the article. | |||
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Quote: Now that is what I call a succint summary!!  Yes, I have one. I was using another product for a year or so before PressureTrace became available. It is a self-contained unit, which is nice. I mount my gages underneath the forestock, so they are out of sight when not in use. You can make a little pocket inside the forestock to store the wire and connector. If you have the curiosity gene, or if you are shooting some offbeat or wildcat calibers, they are marvelous. I have done the formal test, and they are at least as repeatable as published reloading data. If you are careful in your setup, you can get data with very respectable absolute accuracy as well. They do require a portable computer. The PressureTrace unit connects through the serial port. You can easily add gages to other barrels as well. I have two rifles instrumented, and am in the process of adding a third. The gage itself is about 1/2" x 1/4", and about as thick as paper. I use JB Weld, but RSI provides the correct type of "instant glue" with the kit. Either will work, but you do have to let it set for 24 hours. It's very cool to see the shape of the pressure waveform. You can see the faster risetime of quicker powders, you can measure the peak pressure, and the software marks the spot on the trace where the bullet exits. Hope this info is helpful to you. | |||
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Quote: Where and how do you runf the wire and connector? Any chance of a photo to illustrate? | |||
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I am planning on getting this system.. since the gauges are "only" held on with superglue, and I have acetone, I see ZERO reason to leave on, past load dev, and can be remounted afterwards jeffe | |||
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I suspect the reason Hot Core detests them is because they contradict the old wives' tales he has chosen to accept as "facts".  | ||
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I usually put the gage just a little forward of the center of the cartridge, but still behind the shoulder. That way, it is a little bit away from the shoulder of the barrel. It's a good idea to glue the wires to the barrel, near the gage, so you have a strain relief, and don't accidentally yank the wires off. The arrangement I am going to use on my next installation will be to put the gage on the shooter's right of the barrel, just down from the top edge of the stock, with the wire pointed up. Then, I'll cut a very small groove in the inside of the stock to bring the wire out the top of the stock, next to the barrel. When it's not in use, the wire and connector will fit in a little pocket inside the stock, out of sight. When it's not in use, you'd have to look pretty close to tell that anything has been modified. Make sure the stock does not touch the gage when making measurements. In one of my older installations, I mounted a very small male/female connector in the stock, just forward of the trigger guard. The strain gage plugs into the back of the m/f connector, from the inside. The meter plugs to the other side of the connector, from the outside. That works pretty well, and I didn't mind doing it, since it's a milsurp rifle. I think the other method, mentioned above, is better. If you have a junker stock around, you can just leave an hole in the bottom, and do your measurements from the junker stock. In that case, it is probably easier to dangle the wire out the bottom. | |||
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Quote: Actually, I think everyone should buy a bunch of them. Should help stimulate the economy even more. No doubt at all they are great devices for inside a Certified Ballistics Lab. ... Quote: There are only a few problems with them, Cost, either destroy a rifles accuracy potential or glue the Strain Gauge to the outside - cute (in either case you have to remove ALL grease or oil from that spot and get ready for the rust), no way to achieve an accurate Set-Up, can't be Calibrated and due to that, they will only provide "guessed at" mis-leading data. After a considerable amount of SERIOUS RESEARCH on my part, apparently the very best position for attaching the Strain Gauge is directly across the muzzle. At least that way you can get something "useful" from it as it will(hopefully) keep trash out of the Bore while Hunting. HSGS = Reloader's Pyrite(aka Fool's Gold) Keep the economy cranking, get a few!!! | |||
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Hot Core, you have once again shown up as predictably as ants at a picnic. You have no facts, no figures, and no physics. You have an opinion, based on opinion you read somewhere. You claim that your PRE system is calibrated. You claim that the strain gage system is not. Please post some of your PSI measurements, and your method for getting them from PRE measurements. If PRE is really calibrated, that should present no problem whatever. You have said that factory ammunition is satisfactory as a limit sample, and you have said that it is not. Which is it? You can't have it both ways. You're not John Kerry, you know. You have congratulated Deke on getting a 23% increase in MV in his new rifle, defying all laws of physics. What are you going to tell Deke if he follows your advice, and destroys his gun? On what data do you base your claim that the strain gage destroys accuracy? Do you really mean that my 1/2" groups would tighten up if I removed my gage? Preposterous! Barnes Bullets, Dr. Brownell, and Harold Vaughn will all be hearbroken to learn that their strain gage measurements are uncalibrated, and just guesses. Have you written to them. and to NIST yet to get them up to speed with your new concept? I'm sure they would be fascinated to learn that they are doing it wrong. And what have you to say about your idol, Ken Waters' statement that strain gages produce real readings, and PRE does not? You are dragging a large pile of discrepancies and contradictions along with you. When are you going to start taking responsibility for those? From Mr. Waters himself: Quote: Since modern design has solved the cost problem associated with strain gages, and since your idol clearly says that they are superior to PRE, what objection do you have to moving to the better system? | |||
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Quote: Hey jstevens, I've never liked that aspect at all. This is not an attempt to argue with you about it, but I am curious as to what you "like" about it. Perhaps just that you don't have to put a new one on each time you want to test a new Load? Reality check: You can "Pre-Test" how much you like it by degreasing a spot on your barrel over the chamber, or gouge out the bedding on the bottom side of the chamber and gluing a couple of wires to that spot. (No, not being funny at all.) ... Quote: And you can also "Pre-Test" a couple of the serious flaws in the HSGS by going ahead and measuring the "thickness" of the barrel where you plan to hook up. Of course you won't be able to use a 0.0001 capable Micrometer because denton has pointed out numerous times in the past that "no one outside a trained expert" has the ability to use one. They really don't work well on the "Tapered Convex" Chamber shape. And of course they won't reach the inside chamber wall, nor could they be used on that concave surface if they could reach it. So, that moves you to a good old 0.001" capable Caliper. Nope, same problems as the Micrometer. And that leaves you with a 12" Ruler or a Yard Stick. Both of with will allow you to "guess" at the thickness. Reality check: Unless you have access to a CMM with a l-o-n-g arm Ball tip sensor, any measurement you take will be a "guess". Expecting to get valid usable data from a "guessed at" Set-Up isn't rational or logical. ... Next up is Calibration. Go attempt to purchase "SAAMI Certified Reference Ammo" and let us know how well it goes. Your next choice is to shoot regular factory ammo and "guess" at what the Pressure is. You won't know and they won't tell you what it actually is. Or as denton is fond of saying, the HSGS can Self Calibrate. I remember people thinking clinton would be a fine President too! Reality check: Unless you work in a Recognized and Certified Ballistics Lab, you won't be able to get Refrence Ammo. Therefore, you won't be able to Calibrate the HSGS to a known Pressure Standard. Reference: Speer Reloading Manual #13 page 53 under, "Calibration of Pressure Guns". ... So, you can "Pre-Test" the Set-Up and Calibration without having to buy a thing. See how well it goes and let me know if you have "any" problems. HSGS = Reloader's Pyrite(aka Fool's Gold) Best of luck to you! | |||
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Denton From the "30/06 and double pressure peaks" thread of 7/10/04 you said "I think there are two secondary bumps. One starts out as flat spot in the pressure decay, before the bullet exits. As you increase the load, it gradually grows to a damped, single cycle sine wave. The other happens after the bullet exits, and has a very fast risetime. When powder gasses exit the barrel, they have no oxygen, but are hot enough to glow. They are also rich in hydrogen. One fairly popular theory is that when the hot gasses hit the air, they re-ignite, and cause the secondary spike. " I would like to know if you really believe there is a spike in CHAMBER pressure AFTER the bullet leaves the barrel ? I tend to agree with Hot Core that a strain gage system cannot be calibrated (by the hobbiest reloader). That does not mean it can't provide you with some interesting comparative data or that it would not be fun to play with....... | |||
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Here's the real reality check: According to Hot Core, PRE is calibrated. He seems to be the only one who thinks so. I keep asking him for his PSI numbers, but he never provides them. Calibration problem, there, Hot Core? Hot Core says that we can't use factory ammunition as a reference for setting maximum pressure, and explains why it is a bad idea. A few posts later, he does a complete 180, and says that it works just fine. Which is it, Hot Core? You can or cannot? We're all waiting for an answer. Hot Core claims to have 20 years of experience with strain gages. When challenged with a few simple questions about them, he couldn't answer. Hot Core says that a strain gage will spoil the accuracy of a rifles. Do you really think my 1/2" groups would tighten up if I took my gage off, Hot Core? That's what you have said. Why do you believe that to be true? Hot Core says that strain gages only work inside "Certified Laboratories". So, Hot Core, are you trying to tell us that if you take down the sign on the front of the lab, the equipment will quit working? How does the strain gage know when to quit working? Who tells it that it is outside a laboratory? Then comes the ridiculous fabrication that you need a CMM machine to measure the OD and ID. Yet he claims to get 4 digit accuracy from his $20 micrometer. So which is it, Hot Core? The micrometer works, or it doesn't? Or are you special, so that it only works for you? I've never said that it requires a "trained expert" can use one. Another of Hot Core's fables. Several months ago, when this first came up, I said that I did not know whether PRE was a repeatable system or not. I suggested a test. We invited Hot Core to participate. He ran from the opportunity. The test results are in, and they show PRE and CHE to be very non-repeatable. I have posted an experiment that anyone, anywhere, can do to verify the results. How about some test results from you, Hot Core? Show us your numbers. Hot Core says that if you clean your rifle with solvent, to attach a strain gage, it will rust. Really? Right through the epoxy? Why have mine not rusted? Still another Hot Core fable. I have never said that the strain gage is "self calibrated." That is yet another Hot Core fabrication. What I have said is that it can be calibrated from dimensions and the properties of steel that can all be easily had to three signficant digits. Now Hot Core, if you know that you have traveled at exactly 60.0 mph, for exactly 1.5 hours, can you tell how far you have gone? In what sense is that distance not calibrated? If I can do that, why can I not use the same method to calibrate my strain gage system? NIST does it all the time. If it is good enough for NIST, why is it not good enough for you? Hot Core claims that in order to calibrate a strain gage system, you need SAAMI standard ammunition. Why would you need that, Hot Core? What would it tell you that you cannot get from the dimensions of the barrel and the basic properties of steel? And why is SAAMI ammunition NOT needed to calibrate PRE, but IS required to calibrate a strain gage? Hot Core's own idol, Ken Waters clearly said that strain gages and even the old CUP system are superior to PRE. If you're going to believe your idol's opinion, Hot Core, why do you not believe it all? By carefully applying your advice, Deke loaded rounds that, by all evidence, were quite capable of destroying his rifle. Nice job, Hot Core. Your advice sure helped him a lot. You can no longer say that the system has never failed you, because it just did, in a very public way. You had him running loads designed for a 60,000 PSI bolt rifle in a 38,000 CUP lever rifle. Does that bother you at all? As always, you have no facts, no figures, and no physics. You just have an opinion, based on an opinion that you read somewhere. You have an enormouse pile of fabrications that you have never supported with facts, and a number of self-contradictory statements. How about spending some time tending to the pile you've created, before you create some more? Most people have enough sense to quit digging when they find themselves in a hole as deep as yours. | |||
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sdgunslinger... Quote: We're still all scratching our heads over the "secondary spike" thing. I've got an experiment lined up to test the most popular theory, that it is caused by re-igniting hot gasses. Do I think there is a real chamber pressure spike after the bullet exits? Don't know yet. It's possible, since the hydrogen explodes so much faster than rifle powder burns. You get into "inertial confinement" at those speeds... explosion happens so fast that the chamber doesn't "know" that the muzzle is open. The strain gage system is easily calibratable. See the distance and time example given in the previous post. If you know speed to three signficant digits, and you know time to three signficant digits, you know distance to three signficant digits, too, because you know the formula the connects them. NIST does this all the time. The NIST standard for voltage is no longer a standard voltage cell. It is a setup that generates a frequency, from which voltage is calculated, by formula. We do it that way because it is more repeatable than using standard cells. Similarly, we know the hoop strain equation that relates chamber growth to PSI and some basic constants. We know all the input variables to three signficant digits, so we know PSI to about that same level of precision. It's calibrated. I mentally throw out the third significant digit, just to be conservative, but that still gives you a system as good as the ones commercial labs use. The crusher method, for example, can't reliably repeat to within 1,000 PSI. PRE, on the other hand, can't reliably distinguish two pressures 27,000 PSI apart, which is why Hot Core had Deke loading 60,000 PSI loads in a 38,000 CUP rifle, and failed to detect the problem. | |||
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Having thought about it for a minute, I ask this question of Hot Core: How do you calibrate a speedometer? What standard do you use? Or are all speedometers uncalibrated? | |||
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Denton, Many thanks! I think I understand that! As a check on my comprehension, please tell me whether the following is correct. I am making two OD measurements (barrel and cartridge case) and one thickness measurement, presumably with a decent dial indicator or mike. The factory has done all the rest of the work and sent an individual setting for my instrument. As a check against the machine's having been dropped, or my brain having been dropped, I then fire some full-power factory ammo. My measurement of this ammo had better show it to be at SAAMI limit. If not, I either adjust settings upwards to make my measurement SAAMI max, or I reject the process and find out where the postal service or I, or even the factory, erred. If I adjust, and I had used ammo which the factory loaded not quite to max, I now have an instrument which is not quite right, but is clearly very safe. Am I understanding this correctly? Or do the makers instruct one simply to reject the process if the factory ammo doesn't read right? Or am I completely off? BTW, what was your experience with the gauge? Did it simply work correctly from the beginning, with the factory ammo measurement indeed just a safety check? Again, many thanks. | |||
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You're right, I don't want to have to glue one on every time I want to change powders of try a new load. They won't help the looks any, but neither do fiberglass stocks or sling swivels. I've built a bunch of 10000 rpm drag race engines, so I'm intimately familiar with using a mike, a lot of people are not, i'll agree. If I didn't feel competent I'd have a friend who's a machinist measure it, it only has to be done once. By the way, I also haven't had a lot of problems using the convential methods either, in a lot of caliber and a lot of rounds. Like a chronograph, it's just another tool that technology has provided. | |||
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Denton, I'm a bit new to this disagreement between you and HC on pressure measurement, which we reloaders are interested in primarily to avoid exceeeding safe maximum pressures (specified as a max allowable instantaneous pressure, I presume, although I know almost nothing of metallurgy). I think that HC recommends case head expansion, or maybe something else you abbreviate as PRE (pressure ring expansion?), but I am not interested in discussing those. I do have a question or two about the strain gauge method, which I presume you are the one best equipped to answer. 1) In the end, would the home user like me be calibrating and measuring so as not to exceed the pressure of some available factory ammo used in the calibration process, or would I be calibrating to get actual absolute pressures at each point along some time-pressure graph? 2) If the latter, what is the actual calibration process (I don't need to know the formula, but I wouldn't mind knowing the input variables and how they are measured), and if any of the measured input variables are other than length, what does the home user use for a standard? Hope my questions read like I have them in my mind, and many thanks in advance for your explanation. If it is easier for you just to post the factory's instructions, that's fine, too. Again, thanks. R3 | |||
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Avanti... Good questions, simple answers, verbose poster. If you just want the sort answer, read only the next paragraph. The strain gage pressure system is "absolutely calibrated". That is, it will give you meaningful answers in real engineering numbers, that you can compare with the numbers in reloading books, and with other reloaders. As a safety check, to make sure you entered the numbers correctly, and got the gage properly stuck down, you do need to shoot some commercial ammo. In the '06, a good one is Federal High Energy, since they tend to be close to the pressure limit. Super X, for example, seems to run about 52 KPSI. If a gun or casing is going to "let go", it will happen at peak pressure, so peak pressure is of particular interest. There are some cases, such as the M1 Garand, where the shape of the pressure curve is of particular interest. The software automatically finds the peak of the pressure vs. time curve, and reports that number to you. Calibration is very easy. There are just seven basic quantities in physics. Everything else is derived from those. So, if you're calibrating mass, or distance, which are fundamentals, you just compare with a known value. Speed is a non-fundamental quantity, and you can't get a standard speed from NIST, or anywhere else that I know of. So the usual procedure is to calibrate indirectly, by formula. In the case of a speedometer, you might measure a mile, time how long it takes to travel it, and calibrate by the formula speed = distance/time. You do the same thing with a strain gage, since PSI is not a fundamental quantity. If you can get a formula that expresses pressure in terms of fundamentals and dimensionless constants that you can get, you win. In this case, the formulas are the hoop strain equation, and Hooke's Law, which are built into the software. The input variables you need are the gage factor (supplied with each strain gage to 3 significant digits), the OD of your barrel at the point where the gage is applied, and the ID of your barrel at the same point. Those are easily gotten to three signficant digits with dial calipers. The short and dirty way to get ID is to just measure a casing fired in that rifle, and add .001". You might be off .001"-.002", but that is less than half a percent, and negligible. You also need the thickness of your brass at that point. You pretty well have to whack one open, and measure a chunk. This measurement actually has very little influence on the final number. You don't have to do any math. You just put the numbers into the program, and it does all the work. You know the output variable, PSI, as accurately as you know the input variables, so you're absolutely calibrated to fundamental quantities. For each barrel you set up, you enter the appropriate input variables, and the software keeps track of everything. You just call up "Old Blunderbuss", and it knows the dimensions you have entered. I'm kinda focused on peak pressure, but it gives you a lot more than that. It calculates the risetime of the pressure, so you can tell if you have fast or slow powder, and it does some other cool stuff. So, you get a wealth of useful information, including pressure vs. time curves. I think manufacturers must tweak each large batch of ammo to the particular properties of the powder lot. There is enough lot to lot variation in powder that handloads with published PSI numbers aren't quite as reliable as commercial stuff. | |||
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Quote: Hey jstevens, You are already well ahead of the game. I've never found it took longer than even 5 minutes to teach someone how to read a micrometer. How anyone can view them as an inadequate measuring tool just dosen't make good sense to me. Sounds like you have a better opportunity of getting the Chamber Wall Thickness to an "accuracy level" that might provide a good start. If your buddy or anyone you know has acess to a CMM with a long Ball-Tip Locator Arm, you can really be in business. If you do try to measure the Chamber Thickness, how about letting us know how well it goes and the method you use? Still don't know how you can get it Calibrated if you had it. Any thoughts on that? Best of luck to you. | |||
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Quote: Let's see... that would be Hot Core fabrication 1.2, you need a CMM to measure the ID and OD of a chamber, and Hot Core self-contradiction 2.4, Hot Core can measure brass to four significant digits with his $20 micrometer, but the rest of us can't measure a chamber to three significant digits. See how much more compact this is now? 1. Standard list of Hot Core fabrications, unsupported by any known facts or credible references--- 1.1. Mounting a strain gage on a rifle will spoil the accuracy. 1.2. You need a CMM to measure the ID and OD of a chamber. 1.3. Mounting a strain gage will rust your rifle. 1.4. Strain gage systems cannot be calibrated, and the results are just a guess. 1.5. PRE is completely repeatable and reliable. 1.6. PRE is calibrated. 1.7. Claims to have 20 years of experience with strain gages. 1.8. Claims that strain gages don�t work outside a laboratory. 2. Standard list of Hot Core self-contradictions� 2.1. Claims that you can, and that you cannot use factory ammunition as a maximum pressure reference. 2.2. Claims that you need SAAMI standard ammunition to calibrate a strain gage, but that you don�t need it to calibrate PRE. 2.3. Claims that you cannot use chamber dimensions to calibrate a strain gage, but that you can use brass dimensions to calibrate PRE. 2.4. Claims that he gets four significant digits measuring brass with a micrometer, but the rest of us can�t get three when measuring a chamber. 3. Most childish Hot Core behavior� 3.1. Resorts to name calling when things aren�t going his way. < !--color--> Quote: Now this may qualify as a new one for the catalog. Of course, a micrometer is a useful tool. That's not the issue. The issue is whether it gives you an answer in units that are meaningful to the problem at hand, or that can be converted to such units. So this qualifies as a half-truth. Egad! A whole new category!! Hot Core, you provide me with practically endless, free entertainment. | |||
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Hot Core, you provide me with practically endless, free entertainment. He's more generous than that. | |||
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Avanti... I think you've got it absolutely nailed. I have two instrumented rifles, and both worked perfectly, first time, so I'm two for two. I'm just finishing a 6.5x55, for which there are no published "adult" loads, so I'll slap a gage on that one, too. I bought two different boxes of commercial ammo for the '06, and the hotter one was the Federal High Energy. It was cruisin' at about 58.5 KPSI, but, then, it produces an honest 2900 fps with a 180 grain bullet. The Super X was 52 something. I checked my Finnish Mosin with some S&B ammo, that has a reputation for being on the "hot side". After just a few shots, I pulled the bullets on the rest, because they were at 58 KPSI, which is probably really OK, but more than the 52 KPSI that is safer in a gun with no safe gas escape route. Things to watch: Do your diameter measurements 4-5 times, and take the average. They will vary .001" or so, and the average will give you a better number... doesn't matter a lot, since it is a fraction of a percent, but it's just a little better. Make sure you don't get any air bubbles under the strain gage. I clean carefully with acetone, and use JB Weld, specifically because it takes a while to set. I roll my thumb over the gage to squeeze out any bubbles, and stretch some electrical tape over the gage, while the epoxy sets, to be sure I get a thin, uniform layer. You do have to make sure that the gage is straight... long way goes around the barrel. Barrel temperature is a major factor in chamber pressure. I generally do my testing at a constant 90-100 degree barrel temperature. If it's safe at that temp, it's safe for whatever I do. Just check barrel temp with your wrist, and take it slow, especially this time of year. You'll be fine. (Roughly 200 PSI per degree F on the barrel!) It's something that "can't not work" if you carefully follow the directions. | |||
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Quote: Just a thought for your experiment based on a basic understanding of the physics and chemistry involved, I could be WAY off (first time ever  )During the burning of the powder and bullet travel phase, the pressure causes the barrel to expand. In a nutshell, that is what the strain gauge is measuring. When the bullet leaves the barrel, could the springback and vibration (due to sudden release of many kpsi) be sufficient to account for the secondary spike(s)? Easy check to my mind would be to examine the raw pressure waveform and compare to bullet exiting time...dip then spike and rapidly damping sinusoid wave thereafter would indicate a simple elastic material vibrating. Just tossing an idea at ya! | |||
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The explanation of the how to take the measurements, etc. is appreciated. It'll be interesting to see how close some of my old reliable loads are to what I've estimated using the old CHE and PRE. My guess is they'll be pretty damn close. | |||
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CDH... I think that what you are suggesting is very possible, and that it needs to be checked out. Before the bullet exits, the rifle is a closed system, so the center of mass has to stay in one place unless you have outside forces. That says that the receiver lug pushes back on the stock, and that creates a force that tends to push the muzzle up. As the bullet exits, the system becomes open, and that upward force goes away. That could be a source of the spike. The other thing that would logically flow from your idea is that the rifle is putting a lot of spin on the bullet, so the bullet is exerting a torque on the barrel, which gets released as the bullet exits. Interesting ideas.... not sure how to test some of them. There is a technique used in large machines to measure torque and torsional vibration, but I'm just not curious enough to go to quite THAT much trouble. | |||
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Quote: Well, Denton, that raises an issue which I imagine Hot Core wold want to pick up, but I'll see if I can beat him to it. Is your chronograph electronic? How can you believe the figures produced by your chronograph, have you calibrated it and, if so, how? Or are you using pendulum chronograph, equivalent to HC's PRE technique (only actually reliable and repeatable)? <-- humour | |||
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LOL!! Perceptive question!! Good one! There are two issues worth checking. One is the repeatability of a measurement system--can it get the same answer over and over again, given the same input? That one is fairly easy. You put two identical chronographs in line, shoot over them, and whip a little math on the numbers to see how big the random error is. I've done that with a pair of Shooting Chronys, the cheapest alternative, and the Effective Resolution is just a trifle above 1 fps. That last digit is meaningful, and, under constant conditions, there is very little random error. The second question of interest is whether the numbers match reality. Basically, a chronograph has a crystal controlled oscillator inside. Mine happens to run at 12 MHz. When the first photocell "sees" the bullet, the chronograph starts counting pulses. When the second photocell sees the bullet, it stops. If the CPU knows the frequency of the clock, how many clock pulses happened, and how far apart the photocells are, it can calculate speed. The chronograph is calibrated just as a speedometer is... you know distance, you know time, so you know speed to the same accuracy as you know distance and time. For the "terminally curious" you can check this with a pair of LEDs, one over each photocell, and some common electronics bench equipment. If you press Shooting Chrony for an accuracy number, they will tell you .5%, but I'm sure they don't have a clue. They bought the design, and are producing it, but have no design staff, or, as far as I can tell, anyone who actually comprehends the question, let alone the answer. Unfortunately, in the less than ideal real world, lighting conditions do influence how "crisp" the bullet shadow is, and that will influence the precision of the start and stop signals. I do my chronographing only on sunny days, and not very early or late, so that I avoid that problem. There are now some models with LEDs above the photocells, and I think that might be a worthwhile improvement. Even the really cheap crystals they use in chronographs have something like a 10 ppm per degree C tempco, so they are basically set and forget. If you're as wacky as I am, you can even slip a little pickup loop down around the circuit board, and measure the clock frequency. | |||
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Denton, Thinking out loud here but if the pressure spike is due to barrel vibration as CDH speculates then I have a suggestion to test the theory. Place two strain gauges 90 degrees apart around the circumference of the barrel. If the barrel is deflected in a direction that produces a strain on one gauge, it should not affect the second strain gauge. If the barrel is deflected in a direction that causes the greatest strain between the two gauges the results would be similar on both readings with a slightly smaller peak. (I guess for this case 4 gauges would be a better test! ....... or another option would be to move the location of the second gauge and measure again)I know you can only read one gauge at a time but if a given load produced a sharp peak at a consistent time interval, it would be interesting to monitor the pressure produced by that load at 2 points (4 even better) around the perimeter of the barrel and compare the readings.. I would guess one could determine if the pressure spike was vibration induced using this method. Thoughts? HogWild | |||
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I think your idea, or something like it would probably work. To do it right, I'd have to lug a real oscilloscope to the range, so I could look at four traces simultaneously, but that is not impossible. We now know of an incident where someone was getting a big secondary spike, and kept pushing the load up anyway, until the spike was pretty fierce. He took it up too far, and blew the end of his barrel off. That really seems to support the gas re-ignition theory, and that theory is easier to check, so I'm going to try that first. If that doesn't pan out, then we'll be trying to figure out something like you have suggested. Whatever we find, I'll post the results. If the secondary spike is gas re-ignition, it is a danger that nobody has paid attention to before. | |||
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Any of you guys know who stocks the pressure trace stuff and possibly sell it at a discount? | |||
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I'm familiar with the story you told about blowing the end off the barrel. That barrel had a muzzle break (IIRC) on it that was blown off. (I wonder if the muzzle break vent ports figure into the equation?) I'm a little skeptical of the reignition after the bullet leaves the muzzle theory. Before the bullet leaves the muzzle, it's essentially a closed system where pressure can build. After the bullet exits, the pressure in the barrel should cause the gases to rush out the muzzle. It's difficult to see how oxygen would rush into the barrel (against the outbound flow)allowing the gas to ignite inside the barrel. The other possibility is that the gas ignites outside the barrel. In this case, I would guess the gas would expand in the low pressure atmosphere. Can't really see how it would "blow back" into the pressurized barrel while the remaining gasses are escaping. I guess there is a possibility that gasses rushing out the muzzle could siphon oxygen inside the barrel through the muzzle break ports and ignite causing the pressure spike (I would guess gases are rushing out the break ports and the muzzle.....not going in). If the same pressure spikes occur on barrels without muzzle breaks it kills that theory. Anyone know if the spikes are the same with or without a muzzle break? HogWild | |||
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I think you've pretty well pegged the arguments against the re-ignition theory. Wonder if that guy's muzzle brake weakened the steel around the muzzle, and acted as a venturi, sucking oxygen in?? What we know so far is that it is more prevalent in long barrels, and that it is temperature sensitive. The hotter your barrel gets, the more the spike grows. It seems to happen more with lighter bullets. The risetime seems very fast. I'll have to stick a square wave into my PressureTrace, to check, but it might be even faster than the risetime of the PressureTrace. Every theory seems to have some issue that makes it seem like it is not the correct answer. However, re-ignition is a real phenomonon, that has been documented for decades with artillery. The question is, is it the cause of the spike? Don't know yet. | |||
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Denton You need to get a good book explaining fluid dynamics and pressure waves. The secondary pressure spike you are seeing on your strain gauge is a reflected pressure wave traveling through the powder gases from the case to the bullet base and back to the case then out the end of the barrel as the bullet base has by now exited the barrel and so can't reflect it a second time. Get a real long (5-6 foot) barrel so that the bullet is still in the bore when the pressure wave hits it the second time and gets reflected again and you will see the pressure wave (spike) show up a second time at the chamber. If the powder gases re-ignite (and that really depends on the composition of the powder being used, how much nitroglycerine if any, how much flash suppressants etc) when they hit the atmosphere it won't show up again back at the chamber in the form of a pressure spike, but as extra flash and noise in the muzzle blast. The re-ignition is happening in an unconfined space (the open air at the end of the barrel) and so can't build any significant pressure. | |||
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Quote: Thinking out loud again, and it is late so this might not be the most coherent, but your barrel end blowoff, if repeatable by a significant number of other incidents (or repeatable, any takers to try...Clark???) could also be checked. To some degree, if taking a barrel with a significant spike and progressively cutting it down, inch by inch, the secondary spike should remain to a point where is disappears fairly rapidly, and that would be when the bullet has left the barrel. That would support Mc's reflection theory...a slow change/dampening out would support my harminic theory. The problem I see immediately with the reflection theory is that to get a wave required a pulse, and I would expect most loads to be a steady climb in pressure, then a steady dropoff...nothing to reflect, no sudden local high pressure points to cause a pressure wave. Its late, and I have babbled enough, but we engineers get a probelm in our head and have to throw out solutions, right!? | |||
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Very interesting comments.... When the fellow blew the end off his barrel, he cut it off and recrowned it, and the secondary spike completely vanished. I have noticed that the spike is a strong function of barrel temperature. Maybe the reason it went away was actually a cooler barrel??? Or shorter??? Or shorter and cooler??? I've wondered about pressure waves. But what would the source be? The pressure curve is "nicely rounded", with no sudden spikes. So where in that does the intial pressure wave start? I can't get a "source" that makes sense to me. I've even wondered about the effect of "uncorking" the barrel as the bullet exits. That's a very abrupt transition, that might propagate through the system. But why only with certain combinations of bullets and barrel lengths? I think the "unconfined space" observation is a very good one. My assumption has been that the gas explosion was so fast that inertial confinement could let you get real pressure... but would it? Don't know. I had a very brief flirtation with trying to model shock wave propagation in the barrel, but you've got a very dynamic situation, with a steadily lengthening cavity, and hot, highly compressed gas that is cooling and expanding.... didn't flirt with that for too long. Just to give you an idea of some of the bizarre thoughts I've had in connection with this... There is a region just downstream from the exhaust of a jet engine, where there is enough CO2, and it has the right energy state, that you can use the gas as the medium for a very powerful laser. It's called a Gas Dynamic Laser. So does the same region exist for the exhaust gas from a rifle? Oh dear..... Anyway, there is an old artillery trick for suppressing muzzle flash. I'm going to try it on a smaller arm. If it makes the secondary spike go away, then I think the re-ignition theory wins. Meanwhile, I keep looking for theories that can explain this, because every theory I can come up with has one or more problems! | |||
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Can one determine from the pressure curve graph and timeline if the pressure spikes are occuring after bullet exit or prior? Was just thinking one might compare a milder load with no spike and determine when the bullet exits. If one studied several loads, you may be able to make a reasonable prediction of bullet exit point on the graph (assuming it's fairly consistent or some pattern to it). If one knew the exact point the bullet exits, it seems half of the pre/post argument could be eliminated. HogWild | |||
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Where in the secondary peak does the bullet leave the barrel? That little fact alone would go a long way to defining the possibilities... | |||
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To the best of our ability to estimate, the spike happens after the bullet exits. Sometimes, at exit, you'll get this beautiful little rampdown to zero, as the barrel "drains" gas, and then a sudden spike, quite possibly faster than the risetime of the electronics, that can exceed the peak pressure while the bullet was in the barrel. On "well behaved" loads, you'll often see ~10 KPSI pulses lasting 100 microseconds or so after the bullet exits. I'm thinking that one way to investigate is to drag my 'scope down to the range, and set up some photocells near the muzzle on a couple of channels, and a strain gage on another channel. Big project, but Harold Vaughn dragged a tube type Tek 555, with a gas generator, around in the back of his pickup. (Note: The PressureTrace automatically marks an estimated bullet exit point on the trace, but there are a couple of reasons to believe the exit might actually be a bit earlier.) | |||
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The Accurate Reloading Forums THE ACCURATE RELOADING.COM FORUMS Guns, Politics, Gunsmithing & Reloading Reloading anyone using the Pressure Trace strain gauges?
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