Temp drop = Regulator drop?

Jump to latest Follow Reply
GA_LeadHead

GA_LeadHead

Member
Jan 10, 2022
564
678
Perry, Georgia, United States
Just making sure that all I'm seeing is thermal dynamics at work here. I have my impact m3 set at 160 for my heavy slugger. Went outside to punch a few holes and touched my digital gauge and noticed as I was sitting the reg pressure was little by little dropping. The outside temp is in the mid 40's F. Went back inside and put my hand on the cold to touch plenum. As I was holding the plenum in my hand, I watched the reg, easing back up. Again, little by little.

I'm assuming this is normal?
 
Yes. The same thing happens when you fill your tank or the guns bottle. Filling to 250 BAR at say 72 F ambient temperature, heats everything up. Let it sit for a while and the pressure will stabilize at a lower point in the bottle, than when you just filled it.

If you fire the gun when the pressure drops or rises due to quick temperature changes, the pressure on the output side of the regulator should be at the set pressure, but it could still change fairly quickly, depending on the ambient temperature and the temperature of the air in the bottle.
 
Upvote 0
Geezerhood said:
Yes. The same thing happens when you fill your tank or the guns bottle. Filling to 250 BAR at say 72 F ambient temperature, heats everything up. Let it sit for a while and the pressure will stabilize at a lower point in the bottle, than when you just filled it.

If you fire the gun when the pressure drops or rises due to quick temperature changes, the pressure on the output side of the regulator should be at the set pressure, but it could still change fairly quickly, depending on the ambient temperature and the temperature of the air in the bottle.
Click to expand...
Sorry if I'm a bit dense.
If I understand this correctly, what you are saying is that the small volume of air in the plenum can/will change pressure with changes in temperature.
This is normal and does not indicate a problem with the regulator.
Correct?
Thanks
Ed
 
Upvote 0
Yes, temp will effect the pressure in the plenum. I have my rear reg set at 120 at my house temp of 68 right now. Digital gauge to fairly accurate and consistent. When I went out to shoot that last couple of days, it's in the low 30's, it had refilled outside on the last shot. A few hours inside and it's up to 123. If I let the gun sit for a few hours and dry fire it resets at 120.
I fill it in my basement that is cooler than the upstairs. If I fill it and fire it in the basement it sets at 120, but when I bring it upstairs the pressure rises to 121.

Cool air contracts, less pressure, warm air expands, more pressure.
 
  • Like
Reactions: SilentSquirrel
Upvote 0
When the plenum cools and the pressure falls, the regulator’s seat should very slightly separate and allow the pressure to top off. Depending on the condition of the seat, it might take some time for it to equalize.

Another thing is the materials the regulator is made of can have differential thermal expansion rates, leading to a change of the setpoint. For example, FX used a Delrin piston up until recently. Delrin has a thermal expansion rate over 5x that of aluminum and brass. So in that case you could expect the pressure to increase as the temperature falls.
 
  • Like
Reactions: Seeker
Upvote 0
Very true. Reg will equalize the pressure if the pressure drops in the plenum. We know that the pressure drops when the temperature is lower. So if you have 160bars in your plenum at 20C and you go outside where there is 0C - you will notice the pressure drops but after a while the pressure should equalize. It may not be exactly 160bars due to the thermal shrinkage of the regulator components as well as a slight change of its physical characteristics but pretty close. If you want to minimize it then you need to use a brass piston instead of a delrin one:

Tuning - FX Impact/Maverick/Crown brass regulator piston done right

Hi. We know that a standard FX Impact regulator piston is made of delrin. It has a pretty poor coefficient of thermal expansion comparing to brass. Brass is a much better choice. It is more rigid and tough which is pretty important when we think about high reg pressures. FX offers a brass...
www.airgunnation.com www.airgunnation.com

There is also more to it. 160bars at 20C in your reg will give you more v0 of your projectile than 160bars at 0C. The air particles at 20C are more excited than the ones at 0C. Temperature is the amount of energy accumulated.

Lastly, when you go from 0C to 20C then your reg pressure will go over 160bars. Reg allows the air to pass in one direction only. It will not remove the excess of the pressure hence you need to dry fire once or twice.

HTH
 
Upvote 0
tcarlson777 said:
So, if we let our rifles sit outside for say 30-45 minutes (to equalize), at 35F temps, then a PCP should be just as accurate as when shooting at 70F?
Click to expand...

I don’t think this question can be answered for the general case. Temperature changes affect many things all at once, any of which could have a positive or negative influence on accuracy. The only way to know is to test under actual conditions.

At the very least there will likely be a change in POI as a result of the difference in air density. Meaning even if there is no noticeable difference in precision (group size), there will be a change in the location of the group…which means a change in accuracy until some sort of compensation is made (e.g. turrets or holdover/holdunder).
 
  • Like
Reactions: tcarlson777, Seeker and Centercut
Upvote 0
Depending on tune and what you are trying to accomplish, pressure change due to temp change can be very problematic. It is extreme the way I have my Texan 357 tuned. I shoot 158 gr semi wadcutters in it at around 905 FPS starting at 3100 PSI. I get two shots within 3-4 FPS at that fill. I start getting a little valve lock around 3150 psi fill. If I creep up on it just right, I get 3 shots that pretty much all look like this, 900:907:903. If I shoot a 4th shot it drops by at least 70 fps. But creeping up on that 3150, if i go over by less than 25 psi, first shot will be more like 880. So I just fill to 3100 and have 2 great shots off the bench, a 3rd drops radically. I discovered before hunting with it the first time that filling to 3100 in the climate controlled house is a big problem if it is going to be a warm day, never mind sun hitting the black bottle. While still working on setting up the texan, I had filled it in the house, went out on the deck and set everything up when a friend stopped by. He left half an hour later, I went and turned on my labradar, sat at the bench and fired. bloooop. It was less than 400 fps. That half hour or so delay caused the pressure to rise enough to be in serious valve lock territory with the outside temp in upper 80's, low 90's range. I made a spreadsheet for temp/pressure calculation in a closed system and use it when I go hunting. I know absolute max psi I can deal with is 3150 and lowest for one good shot is roughly 2700 bear min, preferably higher than that. Luckily it rarely gets cold enough that low temp is a problem, but rising temp is huge since I'm close to valve lock. Depending on outside temp and how it is going to change while I'm out hunting, I fill my Texan accordingly to always have one good shot.

Same thing happens in one direction with regulated pcp's. Getting colder shouldn't be a big problem, eventually the regulator will open a little and top off the plenum. What pressure differential that is I don't know, but it shouldn't be that much. But getting hotter can be a big problem with first shot, the extra pressure in the plenum has nowhere to go. Hello valve lock. Happened several times to me with my Uragan compact shooting squirrels in my pecan trees. I'd leave the Uragan on the front deck, and a few times I missed a squirrel horribly due to a very slow pellet. Don't know why it took several times to make the connection after dealing with my Texan and same thing. Now I don't leave the Uragan outside in the early morning ready to shoot a squirrel in the pecans.

Everything you need to calculate pressure/temp change in a closed system.
pressure change.jpg
 
Upvote 0
You must log in or register to reply here.
Top Bottom
Back
Home
Forums
Resources
Menu