Are we allowed to use helium?

[Al Skierkiewicz]

Alan,
Whenever a team used a part that was not covered by the rules, the inspectors used the flow chart to determine it's legality. Remember that materials were far more restricted in the past than they are today. Teams actually used packing material in the KOP because anything in the kit was legal.

Tristan, how can you make a distinction between air and other gases when the rules do not. The rules speak to "air pressure" which is by definition a mixture of gases. Gas shocks are specifically excluded, other devices that contain pressurized gas are not excluded and therefore not legal for use on the robot. As to robot rules, I have electronic copies back to 2001 and in that year materials you could use on the robot were limited...

The materials that you are allowed to use to build the robot are limited. There are three sources of
materials:
Sources of Robot Materials Location
1. Parts found in the Kit of Parts Appendix B, Inventory list
2. Parts from SMALL PARTS, INC. SMALL PARTS, INC. Catalog
Appendix D, Ordering information
3. Parts from Additional Hardware List Appendix C

Now I don't have a Small Parts Catalog from that time but I don't remember them selling pneumatic tires. Wheels provided in the kit (and listed in Appendix C) were specifically "5"-8" Ø, Up to 6 non-pneumatic".
As to the 1 atm, will someone please explain how a deflated mylar balloon would magically fill up if the pressure inside and outside the balloon are the same? Assuming that gravity and temperature are ignored, convince me that helium gas would magically move to the interior of the balloon and fill it with no other force acting on it. And if that is the case, why would the balloon deflate when it is opened? (Ignoring of course the lighter than air helium gas)

[Siri]

Originally Posted by Al Skierkiewicz

As to the 1 atm, will someone please explain how a deflated mylar balloon would magically fill up if the pressure inside and outside the balloon are the same? Assuming that gravity and temperature are ignored, convince me that helium gas would magically move to the interior of the balloon and fill it with no other force acting on it. And if that is the case, why would the balloon deflate when it is opened? (Ignoring of course the lighter than air helium gas)

I'll try

Pressure is the sum of the forces from all gas molecules hitting the container walls, divided by the walls' area. Thus, it is proportional to the number of collisions per unit area, and the velocity of the particles. The latter is temperature, which we're ignoring. The former is based on the container surface and the number of gas molecules. In elastic balloons, these two do not rise proportionally, because the internal pressure is balancing not just the external pressure, but also the the elastic potential energy of the balloon. In a mylar balloon (not overly inflated), the internal pressure is not compensating for elasticity, so the two pressures are in fact equal--the increasing volume exactly compensates for the increasing number of gas molecules. So it's not that there's no force acting on it, it's that the force in P=F/A is increasing exactly proportionally to A.


...Maybe.
<Is about to be killed by Ether>


EDIT: Hey, hey! It's seems that someone actually did this experiment. See page 3, it is indeed constant at 1atm. Who'da thunk? I was not expecting to get that right.

[Al Skierkiewicz]

Siri,
Thanks for trying. What I am trying to understand is this. You have a volume of gas at 1 atm. You have a deflated balloon, flat, no internal volume. The inside and the outside of the balloon are also at 1 atm. The volume of gas is connected to the balloon with a hose or tube. Explain how the volume of gas will somehow expand the balloon and fill it if all pressures are equal. This is what was proposed earlier in the thread. I see no force that will impel the balloon to inflate and for the gas to pass from one volume to the other.

[Ether]

Al,

A 1 mil thick sheet of Mylar weighs about 5*10^-5 pounds per square inch. So the pressure inside a partially-filled Mylar balloon would be about 0.00005 psi greater than atmosphere. Is that what you are arguing about here?

[Alan Anderson]

Originally Posted by Al Skierkiewicz

Explain how the volume of gas will somehow expand the balloon and fill it if all pressures are equal.

Add molecules at constant pressure to a flexible but inelastic container. The volume increases. I don't see how this is an open question.

A typical mylar balloon has exactly ambient atmospheric pressure inside it. If you manage to defeat the fill port's built-in leakage and overfill it so that it's starting to swell, it actually becomes less buoyant. After a couple of hours, it will have leaked back to ambient pressure.

[Tristan Lall]

Originally Posted by Al Skierkiewicz

Tristan, how can you make a distinction between air and other gases when the rules do not. The rules speak to "air pressure" which is by definition a mixture of gases. Gas shocks are specifically excluded, other devices that contain pressurized gas are not excluded and therefore not legal for use on the robot.

I think the only mixture of gases the rules refer to is air, which isn't an arbitrary mixture, but one with a reasonably-well-defined range of compositions. I'm contending that there is an implicit distinction between air and all gases not mentioned.

Originally Posted by Al Skierkiewicz

The materials that you are allowed to use to build the robot are limited. There are three sources of
materials:
Sources of Robot Materials Location
1. Parts found in the Kit of Parts Appendix B, Inventory list
2. Parts from SMALL PARTS, INC. SMALL PARTS, INC. Catalog
Appendix D, Ordering information
3. Parts from Additional Hardware List Appendix C

Now I don't have a Small Parts Catalog from that time but I don't remember them selling pneumatic tires. Wheels provided in the kit (and listed in Appendix C) were specifically "5"-8" Ø, Up to 6 non-pneumatic".

I actually just looked back at the 1999 rules, and they suggest that the wheels came from Small Parts—which is not how I remember it either (and a slightly-newer Small Parts catalogue doesn't list them). Skyway provided wheels in some of those years via something like a PDV.

Also, in 1999 there was no language about "non-pneumatic" wheels—so I guess it did arise as an issue at some point.

[DMetalKong]

Originally Posted by Al Skierkiewicz

Alan,
As to the 1 atm, will someone please explain how a deflated mylar balloon would magically fill up if the pressure inside and outside the balloon are the same? Assuming that gravity and temperature are ignored, convince me that helium gas would magically move to the interior of the balloon and fill it with no other force acting on it. And if that is the case, why would the balloon deflate when it is opened? (Ignoring of course the lighter than air helium gas)

By changing temperature. Helium is pretty close to an ideal gas as far as approximations go, so it will follow the law PV/T = constant. Since the P is being held constant, when the volume increases, the temperature will decrease, assuming the process is conducted instantaneously with no heat transfer. Since the balloons conducts heat, and because the process takes time, you won't see as significant a temperature drop as the naive law predicts because heat will start transferring into the gas as soon as there is a temperature differential.

The force is generated from the difference in concentration of the gases. http://en.wikipedia.org/wiki/Diffusion.

[Ether]

Al is correct about the 1 atm thing, but only in a painfully pedantic way. It does take a tiny (and I do mean tiny, neglecting any fill valve characteristics) pressure differential to cause gas to flow into an uninflated flat Mylar balloon... because the pressure inside the balloon must lift the "top surface" of the mylar as the balloon changes shape from flat to an oblate spheroid. In an earlier post I gave an estimate of the theoretical pressure differential required to do this.