Canadian Airgun Forum

It's been a while since I posted a 'pie in the sky' idea. Hopefully you'll find it amusing if not helpful.
I read with great interest the posts by rsterne in GTA on 'Sonic Choke or Sonic Joke'. That doesn't mean I understood the math but I got the gist of the message.
It appears the bottle neck preventing higher air gun velocities is the that the air density increases as the pressure does. In other articles he explained how helium with it's low density is one way around that problem.
My idea simply is to heat high pressure air before firing the gun. Not only would that increase the pressure but it would do it WITHOUT INCREASING DENSITY ! So for instance if you start with 3000psi of air in a fixed volume chamber and heat it so the pressure rises to say 4000psi the density would stay the same as the original 3000psi air since no more air was added. You'd have 4000psi air with a 25% lower density then ambient air mechanically compressed to that pressure.
I found a formula that could tell us how much heat it would take to significantly raise the air pressure but I can't do the math. I'll copy it here for the those who can.

Relations between density, pressure and temperature
These three quantities are related in this way:
p = R r T

p is pressure,
r (Greek rho) is density
T is temperature (in degrees Kelvin),
R is the specific gas constant, which varies from gas to gas.
For dry air, R is 287 J K-1 kg-1.

This very important relationship is known as the Equation of State.

I get the 'degrees Kelvin' and the pressure but those r's really are Greek to me.
Once again it's just an idea to play with.

--Ed.

I was frustrated years ago and made a couple attempts at heating a QB78 for improved consistency in the winter. Last attempt I used two DS18b20 digital temperature sensors and a ATTiny85 risc controller for the brains. It was powered by a 20v remote battery pack. Heat was generated by two transistors and four wire wound resistors (2 in series for each transistor).
After insulating the main tube it worked rather well...

_________________
Daisy Red Ryder

To simplify.... to raise the pressure from 3000 psi to 4000 psi you would have to increase the absolute temperature by 4/3.... Starting at 70*F (530*R) you would have to heat it to 530 x 4 / 3 = 707 *R, which is 247 *F.... about 35 degress above the boiling point of water.... plus or minus....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

Thanks Bob. Surprisingly I found only one answer on the internet regarding the temperature of a car cigarette lighter.
It claimed it could reach 240 *F.
What I was really getting at was that under shop conditions with more powerful heating elements or with gas torches it could raise the pressure so high, without increasing density, that new air gun velocity records could be set using air.
What do you think?

--Ed.

Yes, you could set a higher record using heated air.... just as you could by using Helium.... The record was set at room temperature, that was part of the challenge.... 4000 psi air at 247*F would have about 25% less density, and 15-17% higher speed of sound and molecular velocity than air at 70*F...

An all out record, using Helium, would be MUCH higher.... At 70*F, Helium has 86% less density, and the speed of sound and atomic velocity is nearly 300% higher.... NO COMPARISON....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

I just read that helium leaks out of seals quickly and is not practical in pcp?
Heated air doesn't sound like it's a much better either.
Sure there's nothing wrong with an air gun being an air gun but it's fun to explore possible improvements.

Why use an inert gas then? The real punch would be something volatile. Take big risks and get the big numbers...

_________________
Daisy Red Ryder

Actually, Helium isn't that bad to seal up, and you don't even need exotic O-rings.... BunaN are among the best for He....

The reason for Helium is safety, compared to Hydrogen, and really no other gasses make sense from a performance point of view, unless you are going to burn them.... that's a whole other can-o-worms....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

If any other gas was used to break the AirGuns speed record, it shouldn't count. If using helium, for example, then wouldn't that ba a helium gun ?

_________________
If I had only one wish, it would be to be able to "unsay" things.

I agree, Joe.... There should be a record for air, and another open to all gasses such as Helium.... Apparently the current record for air (unconfirmed) is 2043 fps for a .22 cal shooting a rolled up piece of tinfoil (weight unknown), using 250 bar (3625 psi).... Sounds like the gun was a straight-through valve design, somewhat like a Condor.... 623 m/s backed up by a shot at 614 fps.... It's amazing what you can do if the projectile is light enough....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

Ok Bob one last question on maximizing velocity. So at a high enough pressure the density of 70*F air becomes more of a liability then the pressure is worth.
I was once looking at an Alfa Romeo engine and noticed the intake manifold gradually became narrower until entering the head. The idea was that air will accelerate to a higher speed if it's done gradually. Also it was rectangular shaped and was twisted like a drill bit. I guess to spin the air to force more of it trough like water going down a drain. These designs were implemented to get more air into the chamber much like us air gunners try to do.
What do you think of designing high air pressure ports that way?
Thanks, Ed.

If you had access to CFD programs, you might be able to get some boost.... There is a huge difference between air under a slight vacuum (less than 15 pis in an intake system) and air at 4500 psi (over 300 times the density)....

Air at 4500 psi still has more power/energy than air at 4400 psi.... but the losses are starting to build after you go past about 3000 psi.... The pressure is just as effective in generating additional force, but the number of air molecules is not increasing as fast as Boyle's Law for Ideal Gasses would predict, so your shot count starts to suffer.... Still not a great loss at that pressure, about 9%, but by the time you get to 6000 psi the loss in shot count down to 3000 psi would be about 20% from what you might otherwise expect....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

So if people go thru the hassle and expense of helium to get max fps, and I imagine they'll do just about anything to get it, then why not use hydrogen? The fire risk seems awfully low to me, almost zero compared to a real threat like gasoline which people use and store willy-nilly w/o much concern. Or is that Hindenburg video really that intimidating?
I was thinking about the heating thing too, but instead of heating the whole tank I was thinking about just the valve chamber. Like a ceramic lined valve with heating wires, which yes would be too much work for joe blo but kinda cool for hard core guys? The main point being much higher temps but optionally you could also use it to adj power. I can only guess at the realistic temps since I don't know what valving material you can get away with, but I imagine there's something out there that can seal and handle a worthwhile temp. Just a thought...

Aged; a cig lighter temp would be more like 1240F, or was 240 just a typo? It will vary drastically from lighter to lighter anyway but I guess the airgun relevant point is you're real limitation is battery power, not the heating element, which the above mentioned chamber heater would use less of. I think the hot air idea is excellent, but I guess the hassle is the question. So it would need to be so hot that it's thinner than He or H2 to be worthwhile, but if you did then it would also be easier and better to heat He/H2 which would then be the winner again.
The car eng intake is a different deal which has to do with the piston moving and valve opening and closing very fast, along with a bunch of other misc other things. Your question is rather good, but not applicable to an airgun in any way I can imagine.

Well if it requires a temperature above boiling water to drop the density of air by 25%..... and you can drop it by 86% by changing to Helium at room temperature.... and considering the insulation you would need to not get burnt by touching your hot air gun.... I'll stick to Helium, which is Inert, for any high speed experiments....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

I agree with Joe that a helium gun is not an air gun. I also think that heating the air no sooner disqualifies it as an air gun then does compressing the air - it's still just air.
Chevota made many good points. For instance adjusting the power. Also you could compensate for the air tube pressure dropping after so many shots. In fact you could start out with 2000psi or even 1000psi and have the same 4000psi at firing with enough heat and you'd have 50% and 75% less dense air! The batteries stored in the stock could be rechargeable - after all the air tubes need regular recharging. My pcp only shoots 20 shots at full power and I imagine enough batteries could last that long. If Chevota's right about the CLC temp of 1240*F then pressures of 5000+ are possible and a lot easier than pumping.