Author Topic: Tuning a Regulated PCP  (Read 8747 times)

mcoulter

  • Jr. Member
  • **
  • Posts: 69
Re: Tuning a Regulated PCP
« Reply #30 on: April 28, 2018, 07:44:08 AM »
Since in almost all cases when a regulator is installed in a PCP, you are reducing the pressure at the valve, there should never be a situation where you can't get to the plateau, assuming you had any reasonable bell-curve in the unregulated version.... More likely, you may not be able to tune the preload LOW enough to get below the plateau.... However, here is that information....

If you are stuck on the plateau and no matter what you do you can't reduce the velocity, you need to either shorten the hammer spring or replace it with a weaker one, and/or fit a lighter hammer.... Alternately, if you want more velocity, you can increase the setpoint pressure....

If you are on the downslope and cannot reach the plateau (and hence are not sure where the knee is), you need a stronger hammer spring, or a spacer to increase the preload (if the spring isn't at coil bind), or you can try a heavier hammer.... Alternately, if you have too much velocity, you can decrease the setpoint pressure....


Bob


Hi Bob,

I have done some testing on my FX Royale which came regulated from the factory.  I have replaced the regulator with a HUMA.  (FX replaced the original regulator once for me but I've not had very good luck with them.)  I shot four pellets over the chrony and took the average for each at six hammer spring settings (zero to six turns out) and got this data.  I never see the plateau.  Does this indicate that I should be thinking about doing what you describe above in the blue text?

Pellets = JSB 18
Temp = 55f
Regulator = 125 bar (I'm suspect of this and wonder if it's actually much higher)
Chrony distance = 14 feet

Google Sheet Link - http://bit.ly/2vWA7u8



Any advice would be appreciated!
« Last Edit: April 28, 2018, 07:45:48 AM by mcoulter »
  • Central New York

rsterne

  • Member 2000+fps Club
  • Global Moderator
  • Hero Member
  • *****
  • Posts: 1490
  • Mozey-On-Inn and see what Coalmont has to offer!
    • Mozey-On-Inn
Re: Tuning a Regulated PCP
« Reply #31 on: April 28, 2018, 08:27:16 AM »
You are getting close to the plateau at your maximum preload, you can tell by the flattening of the curve on the left end.... If you extrapolate the curve, you can imagine it going horizontal at about 960-970 fps.... One more increase in preload (if you weren't already at coil bind) would probably make little if any difference to the velocity.... which for practical purposes is the plateau....

OK, so let's assume the plateau is 970 fps (I really don't think it would be any higher) with your current setpoint.... You want to tune the gun to 3-5% below that, which would be 920-940 fps.... If the plateau is actually 960 fps, then you would tune at 910-930 fps.... Therefore there is no need to increase the setpoint, unless you want to achieve a higher velocity than about 940 fps.... If you want a bit less, you could tune down 7% or so below the plateau (~890-900 fps) and get more shots and higher efficiency, but you are likely to start seeing a slight increase in velocity below the setpoint.... If you tune 10% below the plateau (ie about 860-870 fps) you will get lots of shots, but the chances are your ES will be quite large (high shot-to-shot variation) and once the pressure drops below the setpoint you will likely see quite an increase in velocity.... This is because the gun would be acting like an unregulated gun, with a bell-curve, below the setpoint....

Bottom line is, if you are happy anywhere between 890-940 fps, you probably don't need to change the setpoint pressure.... Just shoot a string at each preload setting, keeping track of the number of shots and the pressure drop, so that you can calculate the efficiency (FPE/CI) at each setting.... Also, pay attention to what happens to the velocity at the end of the string, when the pressure drops below the setpoint, and how many usable shots you can get below the setpoint, before the velocity goes too high or too low.... That will give you all the information you need to make an informed decision to balance against your desired velocity.... I find the "best" tune is where you can shoot a few shots below the setpoint pressure, with the velocity staying constant or rising not more than 1% before falling.... and terminate the string when the velocity drops 1% below your average.... That is usually 100-300 psi below the setpoint.... This gives the best balance between power, efficiency and shot count at that pressure....

Bob
« Last Edit: April 28, 2018, 08:32:55 AM by rsterne »
  • Coalmont, BC

rsterne

  • Member 2000+fps Club
  • Global Moderator
  • Hero Member
  • *****
  • Posts: 1490
  • Mozey-On-Inn and see what Coalmont has to offer!
    • Mozey-On-Inn
Re: Tuning a Regulated PCP
« Reply #32 on: April 28, 2018, 09:46:41 AM »
With time comes increased clarity and understanding (ie I'm always learning).... so I thought I would revisit this subject to add a chart that might help you understand what happens as you tune a regulated PCP with progressively less velocity than the plateau.... Here is an updated version of the chart in the first post of this thread....



The 3%, 5% and 10% below Plateau velocities are approximate only, but will give you an idea of what happens as you reduce the hammer strike, and hence the velocity, on a regulated PCP.... It will show you exactly why I like to tune 3-5% below that plateau.... because (at about 5% down) you get a few more usable shots below the setpoint before the velocity drops off.... or at the very least (at about 3% down) you save a lot of air compared to tuning up on the plateau, with very little loss of power.... The chart also shows what happens if you tune about 10% (or more) below the plateau.... Note that below the setpoint the velocity increases significantly.... and while operating in that realm gives you extremely good efficiency, for some applications (eg. Field Target, or in some countries where there is an FPE or velocity limit) that bump could put you over the limit....

Every gun is a bit different, and yours might produce a curve like the "5%" one above only 3% below the plateau, or it might be 7% below.... but to date those would be about the outside limits I have found to produce a tune that has a usable shot string (ie within a ~1% ES) extending slightly below the setpoint, which is the tune I prefer.... I hope this chart allows you to visualize what I have tried to confer in this thread....

Bob
  • Coalmont, BC

kayakr

  • Newbie
  • *
  • Posts: 2
Re: Tuning a Regulated PCP
« Reply #33 on: April 28, 2018, 05:29:49 PM »
I picked up a used .177 mrod which I would prefer to run at 800FPS and get more shots versus 1000 and fewer. I would think about giving it a regulator and a light weight hammer, but I think i just discovered the hammer is an unknown, because the tension is already backed out all the way, shooting as if tuned for high pressure with the backed out hammer and I can't find the inner 1/8 in hex adjustment screw inside.  So maybe I should just be happy with the 50ish shots I get as is between 2750 (about as high as I can pump) and 1800. I'm not sure how much more efficiency I would get without being able to pump it higher. I might have to get the light weight hammer as well as a bunch of components because of the unknowns with this hammer.

Any opinions about the efficiency being seen here?  Pellet is Crossman premier heavy 10.3 grains, 15 ft/lbs

(shot string graph attached)
« Last Edit: April 28, 2018, 05:39:24 PM by kayakr »
  • boston, ma, us, earth, milky way

rsterne

  • Member 2000+fps Club
  • Global Moderator
  • Hero Member
  • *****
  • Posts: 1490
  • Mozey-On-Inn and see what Coalmont has to offer!
    • Mozey-On-Inn
Re: Tuning a Regulated PCP
« Reply #34 on: April 28, 2018, 06:13:43 PM »
Respectfully, this thread is about tuning regulated PCPs in general, not an unregulated MRod.... I am far from an expert on the latter, so I would suggest you ask your question in the PCP Section....

Bob
  • Coalmont, BC

mcoulter

  • Jr. Member
  • **
  • Posts: 69
Re: Tuning a Regulated PCP
« Reply #35 on: April 30, 2018, 04:13:56 AM »
Bob, thank you very much for your insight and comments on this process.  Your posts above brings considerable clarity to this process! 

The overall velocities that I am was seeing did not seem right to me.  (Too high.)  So I took the regulator out again.  I found this...  Somehow in handling / installing the regulator, I am also inadvertently actuating the pressure adjustment.  I had installed it at 130 bar...



I turned it back to 120 BAR and shot a full-string but this indicates that i'm at least a full turn (counter-clockwise) past the scale. 

I accidental overfilled by 10 bar so this string is from 230 bar to 148 bar.  My normal starting fill is 220 bar.  Velocities seemed to drop around 179 bar which was about 50 shots:

http://bit.ly/2vWA7u8




For reference here is the target that I was shooting during the above string.  53 yards, 12-shot groups.  The winds were gusty and variable so it's hard to judge which velocity worked best, but it *felt* like 910 fps were best (bottom left group).



My next step is to remove the regulator again and screw the adjuster in one-full turn and repeat the above tests  :-)


« Last Edit: April 30, 2018, 04:19:59 AM by mcoulter »
  • Central New York

rsterne

  • Member 2000+fps Club
  • Global Moderator
  • Hero Member
  • *****
  • Posts: 1490
  • Mozey-On-Inn and see what Coalmont has to offer!
    • Mozey-On-Inn
Re: Tuning a Regulated PCP
« Reply #36 on: April 30, 2018, 08:07:14 AM »
The shot string indicates that the regulator it not regulating, IMO.... It looks like the most stable operation (least shot to shot variation) is occurring at around 900 fps.... If you know what the pressure was at that velocity, that is where to set your regulator, or slightly above that pressure....

Bob
  • Coalmont, BC

mcoulter

  • Jr. Member
  • **
  • Posts: 69
Re: Tuning a Regulated PCP
« Reply #37 on: May 01, 2018, 05:04:27 PM »
Yes, fortunately I did keep track of the pressure as shown by the gauge on the rifle.  Here are some points:

Shot #56 = 171 BAR = 912 fps
Shot #67 = 161 BAR = 899 fps
Shot #78 = 151 BAR = 887 fps

The second to last string here is the best group and includes most 900-ish fps velocities.  This was in the neighborhood of 160 BAR.



Oh, I should also mention that I have reached out to HUMA (admitting my mistakes here...) and have asked if they can advise on what the typical number turn in/out should be on the regulator or if there is a typical length to get the reg back in the normal  90 to 160 bar range.  Again the marker was on 120 bar but was more likely delivering 160 bar due to my inadvertent "adjustment".

5/9/2018 - Update

Please see this post while I work through some adjustment "issues" with my HUMA regulator...  I'll likely post back here again when I feel like I've stopped chasing my tail  ;-)

http://airgunguild.com/pcp-c02-and-helium-powered-airguns/a-new-huma's-in-the-house/msg20588/
« Last Edit: May 09, 2018, 10:06:22 AM by mcoulter »
  • Central New York