Tuning Dwell time follow up (where’s the pellet)

[AndyH]

I found an article by Bob Sternes in Hard Air Magazine that answers my question about pellet acceleration and some rationale about dwell time settings vs pellet travel down the barrel. Not sure how this effects accuracy but seems like closing the valve when the pellet is halfway to the muzzle is worth playing around with. 

“This chart is essentially three of the above sets of graphs, where the only thing changed is how long the valve is open.

The red lines are for the valve being open for 1 millisecond (0.001 sec.). The green lines are with the valve open for 2 mSec., and the purple lines with it open for 3 mSec. The solid lines are the air pressure (psi), the dashed lines the velocity (fps), and the dotted lines the energy (FPE).

First, notice how little difference there is between the velocity and energy achieved between a dwell of 2 mSec. and one of 3 mSec.

With a dwell of 2 mSec. the valve is closing when the pellet is about halfway to the muzzle. That extra 1 mSec. of dwell buys hardly any increase in velocity, and only about 1 FPE in energy. Now look at the pressure curves!”

 

[AndyH]

Graph didn’t paste. 

 

[JimNM]

Faster valve times also mean lower sound profiles. FWIW

 

[Motorhead]

Exactly why Travis W and Myself did so much R&D on the "Cobra Tech" air spring mods which led to development of what would be the Gen 2 JSAR balanced valves.

Pressure pulse AMPLITUDE launches the projectile and quicker pressure peaks behind a stationary projectile, higher the break away speed of projectile will generally be.

Once the projectile is moving the volume of space the pressure must keep up with to continue accelerating the projectile is equal to the distance it travels.

Putting MORE AIR behind it at a point gains you little in speed because you can maintain the pressure pushing on it, you only use more air getting a louder muzzle report. 



Case study of this coming together very well was the Gen 2 JSAR balanced valves. Easy to open and releasing massive amounts of air, yet correctly set up CLOSE super fast chopping the dwell cycle and saving air, low muzzle report etc ...

LAUNCH Projectile HARD !!! and shut the poppet ..... CRACK or SNAP muzzle report being very easy to muffle / moderate.



Love it !!!



Scott S

 

[Revoman]

So, in layman's terms, this is why the velocity is adjusted to the highest FPS, then backed off around 5%?

Thanks for the info!

mike

 

[AndyH]

Revoman,

It’s more about how you tune down the 3-5%. Use the hammer strike or dwell assuming you have that choice. For example I am getting tuning to 865fps from a plateau of ~890fps. On the Delta Wolf I can use a dwell of 2.4 ms and hammer at 84V or 3ms and 82V to tune to the same 865fps.

Clearly the shorter dwell is more efficient but I am more interested in accuracy effects. Initial results show an extreme spread of 19fps with the short dwell and 10fps with the long dwell. I am trying to understand why?

 

[jaird]

Andy, I agree about accuracy. As Townsend Whelen stated…”only accurate rifles are interesting “. In my limited experiments, I have not made much improvement in accuracy with DW 527. I certainly hope that you and others have. Blessings 

 

[AndyH]

Hey Jesse,

Still playing with the relationship between the voltage and dwell time. Biggest change is dropping the reg pressure down to 130bar following Centercuts feedback. The Daystate manual direction on setting the reg at 170 had me chasing my tail for a while; and consumed many pellets.

At 130 bar the WARP setting is producing 895 fps so I am targeting 865fps (3% down from the plateau seems to be the conventional wisdom). Centercuts tune is shooting faster in my DW (890fps) but still producing 10fps spreads over 30 shots. So I plan to tweak the voltage and dwell to get the speed down without increasing the spread. Seems like a bit of a random walk without any understanding of the algorithm but fun to explore.

 

[jaird]

Hello Andy,

last week I set the reg at 148 BAR, used JSB 33.95 gr. MK1 25 cal pellets at 860 fps and at 50 yds shot some decent “round” groups. All in the factory setting. The next day, I could not duplicate this and have not since. Wind was not a factor. I referred to “round” groups as I had been shooting “string” groups previously. As a result, I ordered PRS accessories for my DW in an attempt to reduce the effects of my poor technique. The length of pull and cheek weld have been a problem. Still trying to improve. Tried some advanced settings but no progress, decided to go back to basics.

 

[jaird]

BTW Andy, I also have a Daystate Safari in .25 caliber. While my DW was back to Daystate for service, I shot the Safari as follows: Rangemaster 33.95 grain pellets at 900 FPS and got good results at 100 yds. 

 

[jaird]

Hey Jesse,

Still playing with the relationship between the voltage and dwell time. Biggest change is dropping the reg pressure down to 130bar following Centercuts feedback. The Daystate manual direction on setting the reg at 170 had me chasing my tail for a while; and consumed many pellets.

At 130 bar the WARP setting is producing 895 fps so I am targeting 865fps (3% down from the plateau seems to be the conventional wisdom). Centercuts tune is shooting faster in my DW (890fps) but still producing 10fps spreads over 30 shots. So I plan to tweak the voltage and dwell to get the speed down without increasing the spread. Seems like a bit of a random walk without any understanding of the algorithm but fun to explore.

Andy, have you seen the “Heliboard Tutorial” by Micheal Mahoney on YouTube? If you have, is there any correlation with Advanced Settings on the Delta Wolf? Micheal was very easy to understand. Hope you are well.