reg pressure and temperature

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tor47

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May 18, 2018
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Other, Norway
 As there often is talk about reg creep, I wanted to see how much pressure can change with different temp, regardless of creep or not. As if you shoot the gun outside in a cold day, then store it inside, and then take it out the next day and shoot it. Since I have a wildcat, with an edgun digital gauge measuring the tank pressure, I did a simple test: Took gun and laid it on a table outside in 8 degrees C (its winter where I live), and left it for a couple of hours, and then read the pressure drop. The temp inside my house where the gun was stored was about 22 degrees C, and the bottle pressure was 159 bar. After it had been lying outside a couple of hours in 8 degrees, the pressure had dropped from 159 to 151 bar. 8 bar drop. I did not shoot the gun. But if I had a shooting session and the regpressure was set to about 150 bar (just for comparison), the next day i shot the gun from just bringing it outside in same temp (where it had litle time to cool down), the first shot would probably be 8 bar higher, even with a perfect reg with no creep:) Wonder what happens if you have a black gun, and bring it outside in a warm day for a hunting session? How much would the sun heat up the gun, after bringing it out from your air conditioned car? Just something to think about. So if you are totally hooked up in "perfect" stable pressure,there might be reasons to dryfire even with a perfect working reg:)
 
For the purposes of thinking about temperature effects on a regulated rifle, you can think of the regulator as a one-way valve. In other words, whatever happens to the regulated air, the regulator can only respond in one direction...to bring the pressure up to the setpoint. It can't bring it down.



It turns out this relationship is fairly serendipitous to the typical hunter who stores his gun indoors and then heads out into the cold. As the gun's temperature decreases, the regulated pressure will begin to fall and the seat will open up to allow the pressure to top off. I presume on a fine enough scale, there is some amount of hysteresis to it but from a practical perspective, it's not something to be concerned about (although the trajectory may still be affected by temperature since cold air is more dense).



However if the gun's temperature rises, so does the regulated pressure. That's because the air is trapped, so to speak...the increased pressure simply causes the regulator to close harder. Depending on how the gun is tuned and how much of a temperature/pressure rise we're dealing with, there may be a change in POI. But if it is tuned carefully, this effect can be minimized. That is achieved by tuning your regulated rifle more like an unregulated one, operating somewhere near the peak of the bell curve. In this way, pressure changes do not affect the extreme spread to any meaningful extent.



To give some perspective, a 30F increase in temperature will cause a 2000psi setpoint to climb to 2113psi. I would say that's a pretty extreme example for most of us, and it's not uncommon for conventional (unregulated) PCP to hold a 1-2% ES over a range of 500psi.
 
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