Brent
Well-Known Member
BB wrote:
And the only ez way to calibrate it is to fire over-pressured loads that show signs of overpressure - then you can use it to get a safe max.
Actually, neither PT or the M43 require calibration. Without a SAAMI spec pressure barrel chamber, even SAAMI pressure testing reference ammo is not going to calibrate one. The only way I know of that will truely calibrate your chamber/action/strain gage combination is to hydraulicly pressurize it to peak operating pressure. A specific offset pressure could "then" be determined. Firing over-pressured loads will only tell you that they are over-pressure, not by how much though. If you use over-pressured loads in order to calibrate a strain gage system, the SG system is only as accurate as the method used to determine the over-pressured load, and if you suggest using classic excessive pressure signs to determine this, you now do not even need the SG system. Truth is, the SG system is far more accurate than most can believe... with no calibration at all. The thick and thin wall pressure vessel formulas used in the M43 and PT software are public knowledge. The modulus of barrel steel and brass are well known and each strain gage is calibrated before you get it with a known gage factor. The rest is pretty simple. I would guess that PT is accurate to within 3 Kpsi, and more than likely within 1-2 Kpsi. Test box after box after box of factory ammo for multiple cartridges and you may decide it is just coincidence but, when ammo tested comes within this 0-3 Kpsi range of SAAMI rated pressure spec's for a given cartridge every time... you realy have to question how much ammo people have truely tested using SG systems when they talk about the inaccuracy or need for calibrating these SG systems mentioned. SAAMI reference ammo fired in yours or mine's chamber should be at the same, or lower pressure than what it produces in a SAAMI test barrel's tighter chamber, right? Yes, that's the purpose but, it does not mean that if your SG system registers 60 Kpsi and not the expected SAAMI 65 Kpsi this cartridge is rated for, and you expected to see, that your SG system is not calibrated correctly.
I have never seen factory ammo exceed SAAMI maximum average in any test that I've run, nor have I seen the average pressure between rifles chambered for the same cartridge vary in pressure more than about 3 Kpsi. Needless to say, this is pretty **** close to what I would expect, and I'm quite confident in developing loads from the ground up for a wildcat cartridge with the aid of the SG systems I use.
Getting MV by using an "accurate" chrono will probably get you closer to actual max load than CHE will in most cases, and it is a very good indicator of pressure contrary to the nay-sayers. However, it may allow you to go a bit beyond max load if using it as a sole indicator because it is not as conservative as CHE. Used in conjunction with other excessive presure indicators it can very often be the most helpful.
And the only ez way to calibrate it is to fire over-pressured loads that show signs of overpressure - then you can use it to get a safe max.
Actually, neither PT or the M43 require calibration. Without a SAAMI spec pressure barrel chamber, even SAAMI pressure testing reference ammo is not going to calibrate one. The only way I know of that will truely calibrate your chamber/action/strain gage combination is to hydraulicly pressurize it to peak operating pressure. A specific offset pressure could "then" be determined. Firing over-pressured loads will only tell you that they are over-pressure, not by how much though. If you use over-pressured loads in order to calibrate a strain gage system, the SG system is only as accurate as the method used to determine the over-pressured load, and if you suggest using classic excessive pressure signs to determine this, you now do not even need the SG system. Truth is, the SG system is far more accurate than most can believe... with no calibration at all. The thick and thin wall pressure vessel formulas used in the M43 and PT software are public knowledge. The modulus of barrel steel and brass are well known and each strain gage is calibrated before you get it with a known gage factor. The rest is pretty simple. I would guess that PT is accurate to within 3 Kpsi, and more than likely within 1-2 Kpsi. Test box after box after box of factory ammo for multiple cartridges and you may decide it is just coincidence but, when ammo tested comes within this 0-3 Kpsi range of SAAMI rated pressure spec's for a given cartridge every time... you realy have to question how much ammo people have truely tested using SG systems when they talk about the inaccuracy or need for calibrating these SG systems mentioned. SAAMI reference ammo fired in yours or mine's chamber should be at the same, or lower pressure than what it produces in a SAAMI test barrel's tighter chamber, right? Yes, that's the purpose but, it does not mean that if your SG system registers 60 Kpsi and not the expected SAAMI 65 Kpsi this cartridge is rated for, and you expected to see, that your SG system is not calibrated correctly.
I have never seen factory ammo exceed SAAMI maximum average in any test that I've run, nor have I seen the average pressure between rifles chambered for the same cartridge vary in pressure more than about 3 Kpsi. Needless to say, this is pretty **** close to what I would expect, and I'm quite confident in developing loads from the ground up for a wildcat cartridge with the aid of the SG systems I use.
Getting MV by using an "accurate" chrono will probably get you closer to actual max load than CHE will in most cases, and it is a very good indicator of pressure contrary to the nay-sayers. However, it may allow you to go a bit beyond max load if using it as a sole indicator because it is not as conservative as CHE. Used in conjunction with other excessive presure indicators it can very often be the most helpful.
