SpudFiles

jeepkahn wrote:Choking... Key word here, I'm not sure ,you'd have to ask D-hall about his usage of the word in ggdt, but choke normally means in gas flows that the combination of pressure and limited orifice size cause mach, or faster than mach, speeds to be achieved, so technically, if it ain't choking it ain't going faster than SOS....

http://en.wikipedia.org/wiki/Choked_flow

I agree.. What I was seeing when I read the numbers is the COF of the valve is eliminated as it is huge in comparison to the entry to the barrel so full chamber pressure into the barrel is how I was interpeting it. I like the prediction of 1511 FPS, well into supersonic. I can't wait to finish it.

A note on the chronograph measurements, maybe a wire break is in order. If interested I can show how to build a simple one with inexpensive parts that uses a computer sound card and free software.

When you're dealing with valves think of it like this, if the air has to go THROUGH the valve, then it's COF has to be considered, if it's a toolie or coax or other "valve" that is actually more of a "stopper" like in a bathtub, when the stopper is removed the VALVE is taken out of the equation and you have to figure the COF of the ORIFICE, and then by calculating orifice/chamber/pressure/etc, you'll find that the COF doesn't add up to a true valve.... If you can measure time from valve opening until chamber is at atmospheric pressure and figure that with the volume you'll get a good idea of what I'm talking about....

BTB Came over to my place, and we tested some more.
We changed the pilot area of the gun (again) and removed the 25 bar gauge.
This allowed us to use 40 bar from his fridge compressor stack.


So shaking up the neighborhood again we got this:

Wow.... 295 m/s!
Thats 1063 km/h!

Sound barrier, we're on our way!

2 meter barrel will surely shatter it...
If not...lathe yourself some lightweight ammo...

Speedy just told me he went mach 1 with an airsoft bb...

So I'm guessing it's more of a mind-barrier than anything else

( nice design BTW... )

jeepkahn wrote:When you're dealing with valves think of it like this, if the air has to go THROUGH the valve, then it's COF has to be considered, if it's a toolie or coax or other "valve" that is actually more of a "stopper" like in a bathtub, when the stopper is removed the VALVE is taken out of the equation and you have to figure the COF of the ORIFICE, and then by calculating orifice/chamber/pressure/etc, you'll find that the COF doesn't add up to a true valve.... If you can measure time from valve opening until chamber is at atmospheric pressure and figure that with the volume you'll get a good idea of what I'm talking about....

Anybody have any idea on the COF of an ORIFICE?

I can't agree about muzzle blast having no effect on chrono-readings. I've seen tests in magazines that show airguns read unnaturally (even impossibly) high when readings are taken too near the muzzle - sometimes more than 30% over the real figure.

The muzzle blast may not cause an error on its own, but it can clearly interfere. I don't know how many of you have built variations on jimmy101's chrony design, but I've looked at the output curves from the light sensors on my version, and there's clearly generous distortion on them with dry fires.

Either way, bear in mind that aerodynamic drag in the muzzle blast is minimal (sometimes even negative) because the relative air speed to the projectile is very low - remember, the air in the muzzle blast is moving pretty much as fast as the projectile.
Given that there should be little to no velocity difference at the muzzle and at the end of the muzzle blast, I don't get why people feel they need to have the chrony as close as possible. I'd much rather know the right number than a large one.

Ragnarok wrote:I can't agree about muzzle blast having no effect on chrono-readings. I've seen tests in magazines that show airguns read unnaturally (even impossibly) high when readings are taken too near the muzzle - sometimes more than 30% over the real figure.

That is a concern which is why a wire break is on the table for when I build the 3 inch 200 PSI cannon. The wire break will be hard to fool.

I too have seen slightly high chrono readings. The apple speed posted earlier is faster than the shot group. It most likely is in error. The real speeds of the rest of the shots are about 100 FPS slower.

Magnetic coil pickups are also unaffected by optical debris.

An update to the controversy....

last nite I made a coupla shots with the coppercane, 4shots from 5ft, 4 shots from 2ft, and 4 shots from pointblank... the readings were all within a 15fps range with no particular order regarding speed and distance... So I'll have to say that muzzle blast definitely may or may not have an effect... I think humidity, condensation in the gun, how bright ambient light is, can decide whether muzzle blast effects it or not... Sometimes it will, sometimes it won't, so for the sake of cutting controversy we'll just say that shots should be from the recomended 5ft range... Happy Now???

p.s. all shots were between 1087fps and 1103fps, 500psi at the reg prolly 470ish in the chamber, and I didn't have vacuum hooked up...

Technician1002 wrote:

jeepkahn wrote:When you're dealing with valves think of it like this, if the air has to go THROUGH the valve, then it's COF has to be considered, if it's a toolie or coax or other "valve" that is actually more of a "stopper" like in a bathtub, when the stopper is removed the VALVE is taken out of the equation and you have to figure the COF of the ORIFICE, and then by calculating orifice/chamber/pressure/etc, you'll find that the COF doesn't add up to a true valve.... If you can measure time from valve opening until chamber is at atmospheric pressure and figure that with the volume you'll get a good idea of what I'm talking about....

Anybody have any idea on the COF of an ORIFICE?

According to this " http://www.ifps.org/Education/WhitePape ... rifice.htm "

a sharp edged orifice is rated at 65% efficiency, but if your orifice has a delaval<sic>, venturi, or some other rounded profile you can attain up to 100% efficiency.... this http://www.ifps.org/Education/WhitePape ... 20VENA.htm
explains why square edged orifices only flow 65% and curved or radiused orifices will flow more....

Sorry if this shows as a double post....

jeepkahn wrote:

Technician1002 wrote:

jeepkahn wrote:When you're dealing with valves think of it like this, if the air has to go THROUGH the valve, then it's COF has to be considered, if it's a toolie or coax or other "valve" that is actually more of a "stopper" like in a bathtub, when the stopper is removed the VALVE is taken out of the equation and you have to figure the COF of the ORIFICE, and then by calculating orifice/chamber/pressure/etc, you'll find that the COF doesn't add up to a true valve.... If you can measure time from valve opening until chamber is at atmospheric pressure and figure that with the volume you'll get a good idea of what I'm talking about....

Anybody have any idea on the COF of an ORIFICE?

According to this " http://www.ifps.org/Education/WhitePape ... rifice.htm "

a sharp edged orifice is rated at 65% efficiency, but if your orifice has a delaval<sic>, venturi, or some other rounded profile you can attain up to 100% efficiency.... this http://www.ifps.org/Education/WhitePape ... 20VENA.htm
explains why square edged orifices only flow 65% and curved or radiused orifices will flow more....

Sorry if this shows as a double post....

Thanks!.. I'm going to work more on a large valve and venturi into a smaller barrel. 3 inch valve into a Golf Ball barrel should be insane.

I rounded the edges of my ports just to keep the o rings from getting caught on them. I didn't think it would help flow. Wow.

Two more things to consider concerning possible chrony errors caused by the muzzle blast.

1. Vibration through the Chrony itself. The SOS in a metal is much faster than it is through air. The SOS in most metal ranges from about 1300m/s to 5000m/s, roughly 4 to 15 times faster than in air. If there is enough energy in the muzzle blast wave the Chrony will vibrate and may give the second signal from the internal vibration instead of from the projectile crossing the second gate. Any loose connections, less than perfect battery contacts, cold solder joints etc. will make it much more likely that you get a false signal. With a commercial Chrony you don't get to see the waveform so you can't judge if there are two or three (or more) peaks.

Many types of capacitors, particularly ceramic disks, will give an electrical signal when they are vibrated since they are basically piezo transducers. A loud enough muzzle blast may be enough for the cap(s) to act as voltage sources and trigger the Chrony's second gate circuitry.

2. The affect of condensation (as others have mentioned) and pressure gradients in the air can also confuse optical detectors. Pressure gradients give a Schelerien affect (just like in the high speed photo's of projectiles striking things).

I would think there are ways to minimize the condensation and Schlieren affects. The closer the detector is to the projectile's flight path then the larger the percentage of the detectors "view of the sky" is blocked by the projectile. That will give a much larger swing in the detector output and, I would think, reduce the detectors sensitivity to the muzzle blast. In a perfect setup 100% of the detectors "view of the sky" would be blocked by the projectile. Commercial Chronys typically have a fairly small percentage of the sky blocked by the projectile (perhaps 10%?), that makes it much easier to get the projectile to actually pass through the detection region. But it also means the detector sees the very large blast cloud. The blast cloud is much less dense and blocks less light than the projectile but is also much (maybe much much much) bigger than the projectile.

With a commercial Chrony I don't think there is much you can do to get the projectile's path closer to the detectors, it's just to risky to the detector. You would only have to miss once to destroy the Chrony.

With a homemade chrony it is pretty easy to get the detector very close to the projectile path and easy to limit the field of view of the detector. That makes it easy to get the projectile's shadow to be much bigger than the detectors view of the sky. In my (and others designs) the detector is within ~3/16" of an inch (basically the thickness of the barrel wall) of the projectiles flight path and the detectors field of view at that distance is much less than 1/2" across. That setup gives a very robust change in the light detectors signal and I would think it would be pretty insensitive to the muzzle blast. The peak shape may get distorted a bit but the peak maximum (or center of the peak plateau for a long projectile) should still represent the center of the projectile.

Tech's inductive coils would also be expected to be pretty insensitive to the muzzle blast. To get a true muzzle velocity you would need to move the coils off the barrel.

What was the weight of that fast dutch marble ?

AFAIK it's 16mm barrel.. so it should be 5 gram each

Right so glass marble density of about 2.19g/cm3

I'll have to try shooting some lighter projectiles when I get back to France, I've only been using lead in a while.

GGDT simulations using glass instead of lead gives scary results.

al-xg wrote:Right so glass marble density of about 2.19g/cm3

More commonly, I find the density of marble glass is around 2.5 g cm^-3. 16mm marbles tend to weigh between 5.2-5.3 grams.

For example, if I take a 16mm marble out of my collection now, it weighs 5.25g. It's actually a hair under 16mm, so we'll call it 15.9 on average.
That gives 2.49 g cm^-3 - so 2.5 is a very good estimate.