Tuning PCP psi and cold weather

[Quad82]

I have posted about this in other threads. But this past weekend while hunting in 5* weather I noticed something about reg creep. I know if I fill my rifle in the 70* house and go outside in 32* weather I loose pressure. Usually about 25 bar on a 250 bar fill. So I leave my fill tank and rifle outside or in the truck overnight before a hunt to even out the temps. Fill the rifle to 250 bar and blast off a few shots to set the reg. By the way, it’s a WC MK3 with a reg set at 135/140 bar. While hunting I notice the reg is now at 160 bar. So after a couple shots, it is still at 160. It’s 5* out mind you, so I figure that the very low temps are frosting the washers in the reg with tiny ice crystals and letting air pass. Why not? The barrel, scope and other metal parts are coated with frost. So when I get home, I wipe the rifle down and set it by the heat register to thaw-out. Refill to 250 bar, shoot off a couple shots and see the reg is now back at 135/140 bar. Shoot a string, fill it back up and leave it over night. Still at 135/140 in the morning. I never really noticed this until the regulated WC. My other regulated guns don’t see much extreme cold weather. So I don’t think my regulator really has any creep so to say. I believe it’s just the expanding of the washers do to extreme cold buildup. 

 

[Nelson86m]

Not sure if it would help or not but try a hot hands packet or 2 wrapped around the area that the regulator is at. They don't put out a ton of heat but might be just enough to help the regulator.

 

[8thman]

All my pcp’s slows down in velocities when the temperatures drop. And yes my guns that have a read out gages also change a bit on the regulator.

 

[nervoustrig]

Hmm, interesting. Is this an OEM FX regulator with the plastic piston?

If so, perhaps it is contracting which requires the piston to travel further before reaching the valve seat, raising the setpoint. A quick search says the temperature coefficient of linear expansion is an order of magnitude higher for Delrin than it is for brass. Just a wild guess!

 

[8thman]

All metals plastics springs and lubricants are effected by temperatures.

 

[Khornet]

I've noticed some oddities when shooting in 3-5 degrees here in NH. Not to mention the oddity of some hammerhead out shooting in 3 degree weather. Once I bring my rifle in from the cold, the regulator will gain about 5 bar, then back to normal after one shot.

 

[Quad82]

Yes, it is the FX AMP reg. I just thought it strange that even though the temp and fill pressure dropped that the reg pressure rose. I can see a little creep if it was summer and hotter outside then it was inside when the rifle was filled. as the fill pressure would also rise. I am careful about leaving a full filled rifle in the truck when I drive north in the summer. I have a small 4500 tank I carry when hunting sometimes. I only fill it to about 3500 if it’s hot out. Left it on the picnic table onetime in the middle of the day. When I took it inside it felt hot. Looked at the gauge and it read almost 6000 psi. 

 

[elh0102]

Here is my thinking on it, drink a little good bourbon, room temperature. If the ambient temp affects your rifle to an unacceptable level, have a little more good bourbon, repeat until the temp variance is no longer an issue. Works for me every time! 

And yes, cold weather will have the observed effects. 

 

[AlaskaAirgunner]

I have noticed the same thing here in Alaska, but not quite to that level. My reg may creep from 140 to 150 and back, but thats about it. And the temperature ranges are similar to yours. I fill in my 55 degree garage, house is at 67 and outside temps are between -10 and 5 above typically when it happens.

 

[JohnnyPDX]

Here is my thinking on it, drink a little good bourbon, room temperature. If the ambient temp affects your rifle to an unacceptable level, have a little more good bourbon, repeat until the temp variance is no longer an issue. Works for me every time! 

And yes, cold weather will have the observed effects.

Excellent solution! That's the kind of sound thinking that will lead us into the future. I practice what you say.

 

[L.Leon]

I had a similar experience to @quad82, my Bantam HR shooting at the dairy for four hours. Weather was 19 degrees and snowing. My regulator is set at 150 bar. After a few hours back indoors that regulator had crept up to 170 bar. I let off a shot it reset to 150 bar and has been there the last few days. So I believe the reg washers were definitely affect by the cold.

 

[BigTinBoat]

I had a similar experience to @quad82, my Bantam HR shooting at the dairy for four hours. Weather was 19 degrees and snowing. My regulator is set at 150 bar. After a few hours back indoors that regulator had crept up to 170 bar. I let off a shot it reset to 150 bar and has been there the last few days. So I believe the reg washers were definitely affect by the cold.

Doubt it was the reg washers, more likely can be explained by the warming of the air within the reg.

Reg set to 150b and it allows 150b to pass thru when inside warmer air. You take it to colder air, the pressure inside the plenum decreased (due to temp drop) and the reg allows more air in to maintain that 150b. Once you take it back inside the pressure inside the plenum rises due to the temp increase. Since the reg is a "one way" valve, the air cannot go backwards so the gauge shows the higher pressure. The reg pressure remains at this higher pressure till you shoot it and it will then only recharge to the 150b.

 

[nervoustrig]

x2 on BigTinBoat’s explanation. A plenum rise corresponding with a temperature rise is the expected thing. The OP describes the reverse scenario...pressure rise when the temperature falls. 

 

[L.Leon]

Thank you BigTinBoat and Nervoustrig for the clarification. Very much appreciated.

 

[Guest]

Hmm, interesting. Is this an OEM FX regulator with the plastic piston?

If so, perhaps it is contracting which requires the piston to travel further before reaching the valve seat, raising the setpoint. A quick search says the temperature coefficient of linear expansion is an order of magnitude higher for Delrin than it is for brass. Just a wild guess!

I think you nailed it. This is a complex problem though. Cold reduces the pressure of the air in the cylinder but it also reduces the size of all the parts as well. Volume of air chamber is slightly less. Everything changes a little ... I'd be willing to bet this sort of change is gun and regulator specific ...

... and +3 on BTBs explanation of the effects of change on air pressures. These two factors together likely make up most of the difference.

 

[tor47]

Think when I shot my wildcat in very cold weather below freezing temp, the regpressure maybe was 5-10 bar higher (can not remember exactly) than on a warmer day. There may be many reasons for it. My theory is that the tip of the plastic piston in the reg, gets a litle harder in cold temp, leading to a litle higher pressure needed to get a proper sealing. The piston surface probably has microscopic unevenness which needs to be pressed down, to seal off the air. To illustrate what I mean, is that if you imagine a soft rubber surface, getting harder in cold temp, you have to apply more pressure to deform, or squeeze the harder rubber the the same amount as it would when it is softer on a warmer day. But it is my theory, it might not be the case

 

[Quad82]

Lots of good points of views. For my hunting purposes, It hasn’t made a hill of beans difference on shot placement. And I don’t shoot paper in the cold. I have a set up from the heated house for practice. But it just struck me strange having reg pressure rise in cold temps. Seems others have seen the same outcome. No creep in the warm shooting range. Thanks Darrell