Tuning Regulator Pressure Creep?

[TG59]

I recently installed a HUMA XL regulator in a 25 caliber Marauder. This is my first PCP and first regulated gun.

The regulator was set at 130 bar/1885 psi when installed.

After the first few charges to 3000 psi, the factory pressure gauge read a few needle widths below 2000 psi - which looked about right. I know that the gauge may not be highly precise. I took some phots of the pressure gauge for records.

During a few chronograph strings I recharged the pressure five times. After the last charge I decided to run a test to chronograph 5 shots a day over a few days to see how stable the velocity was over time with a single charge. On the third day (and shots 10-15) the velocities were notably (~ 20 fps) higher than where they had been running. A glance at the pressure gauge showed that it now read 2100 psi. 

Is this typical, or it it some issue with the regulator?

 

[corny]

Higher reg/plenum pressures on a properly tuned gun will typically give you a reduction in Fps. Because the hammer can’t open the valve.

I would suggest you tune ur hammer to 95-97% of max power at said reg pressure. Then redo your testing. 
You will find that the slight discrepancies the reg puts out don’t matter so much. 

 

[Jayson]

What was the temp difference? That is one of the biggest reasons IMO you hear about creep and poi change! That being said, they can settle a little after the initial set up, that is not uncommon. Good luck and don’t panic! Your just fine

 

[Ezana4CE]

What was the temp difference? That is one of the biggest reasons IMO you hear about creep and poi change! That being said, they can settle a little after the initial set up, that is not uncommon. Good luck and don’t panic! Your just fine

Good suggestion concerning POI when thinking about changes in fps. I need to check some of my notes now. How do changes in temperature relate to reg creep?

 

[TG59]

Temperature was fairly stable: I keep the gun and chronograph in "heated" shop at ~ 60 F and step outside to quickly fire 5 shots.

After installing the regulator I cranked the hammer spring in 3.5 turns from fully out to get "max" fps (which was ~ 907 fps with 25.4 gr pellet) and then backed off the spring incrementally to get 20 fps less (which is ~ 97% of the max recorded). A 25 shot string then averaged 888 fps with an 8 fps spread. 

What surprised me is that when the velocity (and regulator) pressure jumped up on day three of the last trial described above, it was back up to ~ 900 fps.

I did not run the spring in further than 3.5 turns, so may not have really hit the maximum. Based on feedback I'm concluding that the next step is to crank the hammer spring back in to find the new max and then back off again.

 

[nervoustrig]

Creep is a slow climb of the output pressure over a period of minutes or hours. Send a few shots downrange, note the pressure, set it aside and check it again in an hour or more.

However what you described does not sound like creep. Like Jayson said, I think what you’re seeing is the regulator breaking in…the plastic seat will take a bit of a compression set, requiring the piston to move a bit further before it shuts off the transfer of air from the high pressure side (reservoir) to the low pressure side (plenum). The result is the setpoint moves up a bit.

At this point it has probably taken about all the compression set it is going to take. So as Corny suggested, take a few minutes to set the hammer spring tension for 95 – 97% of peak velocity. When adjusted in this manner, slight variations in pressure will have no meaningful effect on the velocity (the variations I am referring to here are not a permanent setpoint shift like the seat breaking in, but rather things like temperature induced changes or cycle-to-cycle regulator repeatability).

 

[nervoustrig]

How do changes in temperature relate to reg creep?

In most cases, temperature has little to nothing to do with pressure creep. At least not to the true form of creep which is a slow weeping of air from the reservoir past microscopic imperfections in the regulator’s seat.

However there are a few related scenarios worth considering:

1. Moving the gun from a colder temperature to a warmer temperature will cause a 1-time increase of the plenum pressure.
2. Item 1 leads to a related topic. Exposing a gun to a massive temperature rise puts a harder squeeze on the valve seat and may contribute to greater dimpling of the valve seat. The result being anything from the setpoint shifting up permanently, to actual seat damage that causes creeping to develop. The effect is very similar to what can happen if you have an externally-adjustable regulator and you attempt to reduce the setpoint while it is pressurized. BTW when I say massive temperature rise, I’m thinking of something along the lines of taking it from the cool basement and placing it in the direct sun on a hot day…going from 65F to maybe 135F.
3. A small number of regulators are built with plastic parts having a high coefficient of thermal expansion. The Delrin piston of FX regulators is probably the most familiar example. This material expands at 5 – 6x the rate of brass, aluminum, or stainless steel. So here we are talking about a potential shifting of the setpoint with temperature, not creep per se.

 

[TG59]

Thanks all for the helpful replies!

Now I understand that the topic should have been "Regulator Pressure Shift". I would edit if possible.

 

[TG59]

Update from the OP:

Cranked hammer spring in to find the "new" max after the regulator shift. Hit 932 fps with 25.4 gr pellet.

Then backed off spring and ran a few chronograph strings over a few days.

The regulator pressure appears to have settled in at 2000 psi, down a bit (splitting hairs).

At current setting velocity average is ~ 94.5% of max. FWIW I can get 30-32 shots before velocity begins to fall off.

Last string was 32 shots: 881 fps average with extreme spread of 17 fps ( <2%). SD was 4 fps.

The large majority of the velocities in this string were between 881 and 888 fps, with 5 shots ranging between 871 and 877.

I am not sure what good performance is for this setup. Pellets were not weight sorted if that matters. 

I am tempted to crank the hammer spring in a small amount to see if that reduces extreme spread.