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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Caution in the cold
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<blockquote data-quote="astronut502" data-source="post: 1564474" data-attributes="member: 108941"><p>"Cold Trigger Finger, post: 1563900, member: 56732"]At the risk of starting something.</p><p> But because I've experienced the results I will share this............................... "</p><p></p><p>Cold Trigger Finger, this is an interesting premise. I am a chemist, so I ran the numbers to see just what change in volume and thus pressure should be expected by a significant drop in temperature, and thus volume, in the chamber. Lets assume a few things:</p><p></p><p>1: The chamber is 1/2" x 3" in size at +40F. (a bit larger than a .338 Lapua Magnum cartridge]</p><p>2: The temperature drop is from +40F to -20F. [Burrr]</p><p>3: The chamber is made from carbon steel.</p><p></p><p>The thermal coefficient of expansion for carbon steel is 11.0x10e-6 to 12.5x10e-6 u/u degree C. [This value is available at many places on the internet] Lets use 12.0x10e-6. </p><p></p><p>The volume for the chamber calculates to 0.589 cubic inches [cuin] at +40F.</p><p></p><p>If the change in temperature is a differential of 60F (+40 to -20) [33.3C], then change in the chamber volume is a decrease of 0.000000000012 cuin (1.2x10e-11 cuin). So, the 'cold' chamber volume at -20F is 0.588999999988 cuin. </p><p></p><p>Stay with me.</p><p></p><p>The Ideal Gas Law is PV=nRT. It governs how a gas at varying volumes will change in presssure. Without getting too messy here, n is the number of molecules of gas in the container, R is the gas constant, T is the temperature in degerees Kelvin, V is the volume of the container and P is the pressure. So, we can say that PV/nRT = P'V'/nRT', where the prime(') values represent the chamber at the lower ambient temperature. Since the number of molecules of gas (exploding powder) are the same in each instance, n cancels out of the equation. The R value is a constant, so it cancels out, also. T should be the same in each equation since the amount of heat being generated by the chemical reaction of the exploding gas should be the same, so it cancels out, too. <span style="color: #0000ff"><span style="color: #660033">(One could argue that, at the lower temp, the reaction would proceed more slowly, thus the heat has more chance to transfer off into the chamber wall, but that would also lower the pressure. This is one of the reasons we see lower muzzle velocities at lower powder temperatures) </span> </span></p><p></p><p>That leaves us with PV=P'V' [Boyle's Gas Law]</p><p></p><p>[I'm going to use non-standard units for this calculation for simplicity, but the answer still works out becaue the units are consistent]</p><p></p><p>Lets assume that +40F chamber pressure of our round is 60,000 psi. We already know that the volume is 0.589 cuin.</p><p></p><p>So: PV=P'V' </p><p> 60,000 psi * 0.598 cuin = P' * 0.588999999988 cuin</p><p>This rearanges to:</p><p> P' = (60,000 psi * 0.598 cuin)/0.588999999988 cuin </p><p>Solve for p:</p><p> P' = 60008 psi when the temerature of the chamber is -20F. </p><p></p><p>If there are holes in this calculation, please let me know. ;-) </p><p></p><p>I wonder if there is more danger of the barrel/receiver steel becoming brittle at lower temperatures. Some of the mild carbon steels drop off rapidly in impact resistance at temperatures below zero centigrade. </p><p></p><p>For what it's worth..........</p></blockquote><p></p>
[QUOTE="astronut502, post: 1564474, member: 108941"] "Cold Trigger Finger, post: 1563900, member: 56732"]At the risk of starting something. But because I've experienced the results I will share this............................... " Cold Trigger Finger, this is an interesting premise. I am a chemist, so I ran the numbers to see just what change in volume and thus pressure should be expected by a significant drop in temperature, and thus volume, in the chamber. Lets assume a few things: 1: The chamber is 1/2" x 3" in size at +40F. (a bit larger than a .338 Lapua Magnum cartridge] 2: The temperature drop is from +40F to -20F. [Burrr] 3: The chamber is made from carbon steel. The thermal coefficient of expansion for carbon steel is 11.0x10e-6 to 12.5x10e-6 u/u degree C. [This value is available at many places on the internet] Lets use 12.0x10e-6. The volume for the chamber calculates to 0.589 cubic inches [cuin] at +40F. If the change in temperature is a differential of 60F (+40 to -20) [33.3C], then change in the chamber volume is a decrease of 0.000000000012 cuin (1.2x10e-11 cuin). So, the 'cold' chamber volume at -20F is 0.588999999988 cuin. Stay with me. The Ideal Gas Law is PV=nRT. It governs how a gas at varying volumes will change in presssure. Without getting too messy here, n is the number of molecules of gas in the container, R is the gas constant, T is the temperature in degerees Kelvin, V is the volume of the container and P is the pressure. So, we can say that PV/nRT = P'V'/nRT', where the prime(') values represent the chamber at the lower ambient temperature. Since the number of molecules of gas (exploding powder) are the same in each instance, n cancels out of the equation. The R value is a constant, so it cancels out, also. T should be the same in each equation since the amount of heat being generated by the chemical reaction of the exploding gas should be the same, so it cancels out, too.[COLOR=#660033] [/COLOR][COLOR=#0000ff][COLOR=#660033](One could argue that, at the lower temp, the reaction would proceed more slowly, thus the heat has more chance to transfer off into the chamber wall, but that would also lower the pressure. This is one of the reasons we see lower muzzle velocities at lower powder temperatures) [/COLOR] [/COLOR] That leaves us with PV=P'V' [Boyle's Gas Law] [I'm going to use non-standard units for this calculation for simplicity, but the answer still works out becaue the units are consistent] Lets assume that +40F chamber pressure of our round is 60,000 psi. We already know that the volume is 0.589 cuin. So: PV=P'V' 60,000 psi * 0.598 cuin = P' * 0.588999999988 cuin This rearanges to: P' = (60,000 psi * 0.598 cuin)/0.588999999988 cuin Solve for p: P' = 60008 psi when the temerature of the chamber is -20F. If there are holes in this calculation, please let me know. ;-) I wonder if there is more danger of the barrel/receiver steel becoming brittle at lower temperatures. Some of the mild carbon steels drop off rapidly in impact resistance at temperatures below zero centigrade. For what it's worth.......... [/QUOTE]
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