Page 1 of 1

GGDT isn't showing the same velocities I get with a chrono

Posted: Wed Feb 13, 2013 5:32 pm
by whoa044
Is there something I'm doing wrong here or did the chrono somehow misread the shot?

Everything I entered in the GGDT matches up fairly well with the conditions I had, and everything was measured with digital calipers and a digital scale. I couldn't figure out the exact chamber volume or pilot volume, but they didn't seem to make too much of a difference so I guessed what was closest.

I used a 54 caliber plastic sabot, and trimmed it as much as I could. It weighs 0.58 grams.


Chrono reading

The plastic slug stopped at 40 pages. It punched a circle through the next 60 pages, and tore another 80 pages after that.

The gun


An unrelated shot: 15.4 gram 4 inch nail at 220 psi with a 30 inch barrel. Book is 750 pages.

Both books are old and not being used anymore.

Posted: Wed Feb 13, 2013 5:38 pm
by Zeus
The muzzle blast would make that reading invalid, repeat it with the muzzle a couple of meters away. Don't shoot your chrony either.

Posted: Wed Feb 13, 2013 5:43 pm
by MRR
This is what I wanted to tell you all along after you have posted on my youtube video. The numbers don't really add up (did it in GGDT too).

You should follow Zeus advice and see if it reproducable at greater distance from the chrony.

Posted: Wed Feb 13, 2013 5:46 pm
by whoa044
Yeah I thought about it some more after I commented and figured the numbers were a little too crazy, so I posted here. I'll definitely redo the shot from a few feet

Posted: Wed Feb 13, 2013 6:46 pm
by MrCrowley
I'm not sure if you have entered some the GGDT parameters correctly either. Though, I don't think they will change the output readings by much.

'Outer diam' should be the inside diameter of your chamber and 'Inner diam' should be 0 unless the cannon is a coaxial in which case it is the outside diameter of the barrel. For this cannon, inner diam. should be 0.

Valve coef. might be more accurate if it were in the 35%-50% range. I'm not really sure about this as I've never bothered to calculate the flow of a valve in terms of litres per minute with a drop of 1 bar in pressure, but I usually go for a number in that above range and other members seem to as well.

I think you should get chrony readings in the 800-1100fps range. You might get close to the SOS but I doubt you will get much higher than that.

Posted: Wed Feb 13, 2013 6:56 pm
by whoa044
Thanks for the tip.

I'll be redoing this shot in a few days, and I'll also dry fire the gun through the chrono to see if muzzle blast alone can create a reading. Is water vapor really dense enough to trip the chrono's eyes?

Posted: Thu Feb 14, 2013 1:50 am
by CS
(^"air blast prior to projectile exit. " Picture by Technician1002. From unrelated thread. link)

I've had high readings shooting too closely to the chrono. As in the muzzle an inch before the first gate.

Posted: Thu Feb 14, 2013 1:55 am
by jackssmirkingrevenge
Holy enormous pictures batman!
Is water vapor really dense enough to trip the chrono's eyes?
Is steam dense enough to set off the optical sensor in a fire alarm?


The above is a very small chamber at just 100 psi, filmed at 1000 frames per second you can see that the vapour released is substantial.

Posted: Thu Feb 14, 2013 3:42 pm
by whoa044
^Nice video

I redid the shot with the muzzle 3 feet from the chrono's first sensor and hit 1102 fps.

Learned my lesson here.

Based on this shot and another with a lead ball, my flow coefficient is somewhere between 30-50%. What are other people using in GGDT for the flow coefficient of their barrel sealers?

Posted: Fri Feb 15, 2013 7:55 pm
by Technician1002
A high speed camera and the rule in the background did provide good data on the projectile speed as well as the shockwave expanding before the projectile emerged. Much of the "Steam" in that shot is applesauce from a broken projectile apple. In broke on the way in, so we shot it anyway. Most shots don't have that much applesauce in the photos from an air cannon. The leading edge of the sauce is near Mach 1. It surprised me. The average bulk is about 2/3 Mach 1