Tuning Pneuma (AT44) in anticipation of regulation

[wasatch]

The picture shows results of tuning my AT44 following posts by others, primarily Rsterne (thanks!).  I was surprised how powerful the AT44 is!  To get the green string I had polished the hammer and tube, removed the spring spacer in the hammer, set the spring preload to minimum and glued shut 3 of the 6 ports in the valve.

Later today I hope to shoot it to find an accurate velocity for the JSB heavies.  Lets say it turns out to be 880fps, would I then set my regulator to approx 115bar (the red circled shot in the string?  If I did that and filled to 220bar i think i could get over 70 shots 

The efficiency isn't as good as Rsterne got (http://airgunhome.com/agforum/viewtopic.php?t=8657&postdays=0&postorder=asc&start=0).  If I increase spring preload won't that shift the peak to higher pressure?  Should I try a smaller transfer port?

[Motorhead]

Odd your string profiles are belled
Under regulation ( If I read that correctly ? ) should be nearly flat and level.

what am I missing ?

[wasatch]

Not regulated yet.

I'm tuning now with plans to regulate.  Likely getting Altaros.

[Motorhead]

Ah, that thread header is deceiving to your posts inquire.
Go REGULATED ... you will be much happier !!

[wasatch]

That better ?

[Motorhead]

That better ?

That works too ...
Will say, you had asked if a smaller transfer passage would flatten the bell some .... YES it would 

[wasatch]

The Altaros is 84 euros shipped.  It installs inside air tube, butted up against valve.  It seals against the valve with an o ring.  The oring on the valve is removed and instructions say to not thread valve all the way into air tube to leave a 0.5mm gap to allow reg to breathe.  This will mean the 3 ports i didn't glue closed may not be pointed up.  The Altaros will also displace some air tube volume.  I'm not really keen on having that unclosed gap that could be accidentally closed (not to mention the port indexing issue).  Maybe I cut a couple small notches in the end of the air tube instead of leaving a gap?
http://www.altaros.cz/regulators/hatsan-at44/

The pekator and guntuning regs are well north of 100 euros + shipping and they install the reg in the end of the air tube with another section of tube as the plenum between the reg and the valve.  no 0.5mm gap but alot more cost, longer air tube but no loss of air tube volume.
http://www.guntuning.com/content.php?cms_id=84&lang=eng

[Voltar1]

The Altaros is 84 euros shipped.  It installs inside air tube, butted up against valve.  It seals against the valve with an o ring.  The oring on the valve is removed and instructions say to not thread valve all the way into air tube to leave a 0.5mm gap to allow reg to breathe.  This will mean the 3 ports i didn't glue closed may not be pointed up.  The Altaros will also displace some air tube volume.  I'm not really keen on having that unclosed gap that could be accidentally closed (not to mention the port indexing issue).  Maybe I cut a couple small notches in the end of the air tube instead of leaving a gap?
http://www.altaros.cz/regulators/hatsan-at44/

The pekator and guntuning regs are well north of 100 euros + shipping and they install the reg in the end of the air tube with another section of tube as the plenum between the reg and the valve.  no 0.5mm gap but alot more cost, longer air tube but no loss of air tube volume.
http://www.guntuning.com/content.php?cms_id=84&lang=eng

Your ports will align exactly as they do now.
Just the tube is backed off to let the reg breath.

[wasatch]

you are so right!    wrong frame of reference

[rsterne]

First of all, when you install the regulator, you will not get the same power at 115 bar as you do now, because of the limited volume between the regulator and the valve.... This is because the AVERAGE pressure during the shot will be less because the valve won't have the whole 180cc reservoir to draw air from, only the small volume between the regulator and valve....

Secondly, how did you decide that while Hatsan tell you that the maximum fill pressure is 200 bar, you can fill to 220?....

You are "hoping" to get 70 shots at 18 FPE (a total of 1260 FPE) from 180cc (11CI) of air with a drop of 105 bar (a total of 1155 CI).... That requires an efficiency of 1.09 FPE/CI, which is likely possible to acheive, even in .177 cal.... However, to do that, instead of settling for about 55 shots and filling to 200 bar, you are willing to take a chance on overfilling the gun for those extra 15 shots.... Depending on the volume of the plenum between the regulator and the valve, you could have to increase the setpoint by as much as 10 bar, or more, to achieve your goal of 880 fps, further reducing the amount of air available and the shot count....

I recommend that you install the regulator, shoot the gun with various preload settings, plot the results, and tune the gun so that it is operating on the "knee" of the curve and see what you get for velocity and shot count.... Once you have that concrete data, then we may be able to make further recommendations....

Bob

[wasatch]

Thanks for the helpful information.  Altaros reg is on order set to 120 bar.  I'll limit charging to 200bar.

By 'knee' do you mean the inflection point or peak of the unregulated bell?

Here is a revision of my original data adding a string (purple) that was shot after filling the volume over the 3 plugged holes in the valve.  Note it was about 20degF cooler than when I shot the green string.

[rsterne]

No, the "knee" I am refering to is where the plateau stops and the declining velocity takes over in a plot of velocity vs. hammer preload.... like this....



On that graph, from 4 to 5 turns out from coil bind is the "knee".... At more preload than 4T out (the plateau on the left) , the velocity doesn't increase, or barely so, the gun just uses more air.... At less preload than 5 turns out (the downslope on the right) the velocity drops quickly as you decrease preload on the spring.... At the "knee" (ie the transition between the two) you lose very little velocity and yet the air usage drops considerably.... That is where I tune all my regulated guns.... that QB is tuned at 4.5 turns out....

Bob

[ascarissuis]

hi
I own hatsan at 44 long .22 cal.
With altaros reg set to about 140bar I have 28 shots , 885 fps average using jsb 18 grain.
I fill the tube to 230 bar approximately.
I have  run thru my gun about 5000 pellets, without any problems with the reg.

[rsterne]

I repeat, where do you get the idea that it's OK to randomly fill a gun 500 psi over what the manufacturer states?....

Bob