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What, are you going to make your box out of paper? You need a box, or the balloon would wobble and you'd go in the 2nd tier. And where did you plan on finding a 48 cm balloon, anyway?

An 18" balloon is 45.7 cm. Those are actually some of the more common sizes of helium balloons. Just google it, or search it on amazon.

Will an 18" latex lift 40 g? I know an 18" mylar lifts more like 5 g.

Flavorflav wrote:Will an 18" latex lift 40 g? I know an 18" mylar lifts more like 5 g.

I don't see any way to lift 40 g with a balloon that size. The best I've gotten with a 20'' latex was around 15 g.

So do you seem to be getting better results with a latex balloon?

Not really. I just haven't tried anything but latex balloons.

I have two tethers for my balloon. The permanent one required by the rules, and a temporary tether that I use to hold down the balloon, until a mechanism releases the temporary tether, allowing the balloon to rise. When the balloon rises, it lifts up the sign, a paper clip (used for attaching the permanent tether), and the temporary tether with it. So would the temporary tether be counted towards the sign mass? It is being pulled up with the balloon due to the balloon's buoyancy.

Primate wrote:There's actually a couple other factors, but they're hard to take into account. Use the ideal gas law, PV=nRT. (If you're not familiar, pressure x volume = number of moles x gas constant x temperature.) Balloons float for the same reason that people can swim; the weight of the balloon is less than the weight of the air it displaces. This buoyancy force is determined by ρVg, density of displaced fluid x volume x gravitational acceleration.

Manipulating the equations, you find that the weight you can lift is equal to ρgnRT/P - mg (of balloon). Disregard the gas constant, the gravitational force, and the weight of the latex for simplicity's sake, and you're left with ρnT/P. This is where you realize you're screwed. Air density itself varies based on pressure, humidity, and temperature, yet so does the volume of your balloon. If you could somehow regulate the number of moles so that nT/P was constant for your current location, you'd be all set. Show up with a calculator and lots of paper, bring your barometer, pressure gauge, and thermometer, and you'd know exactly how many moles of helium to add to provide the proper lifting force.

Or, on the day of competition, you could fill up a balloon all the way, and just add weight until it stops going up.

I don't know if you ever watched the old balloon race event, but in that event it often turned out that air movement in the room played a much bigger factor in the outcome then anything the competitors could calculate. The PV=nRT equation describes the physics very well but it tells you nothing about the aerodynamics of the balloon and what effect even a slight up or downdraft will have on the flight of the balloon. It is even entirely possible to mathematically model the aerodynamics of the balloon, but if you have no way to measure, or more importantly, control the wind then you have no way to calculate how much positive (or negative) buoyancy you will need to get the balloon task to work successfully and with the most possible points. In the old Balloon race many competitors were surprised to find that their balloon would actually start rising but then a wind would blow it back down again., or a team that didn't even know what they were doing would accidentally add too much mass yet the balloon would rise anyway due to an updraft from someone walking by at just the right time (or the A/C blowing just right).

I realize that in the end the event is what it is. I have been very successful in Science Olympiad competitions, receiving several national medals, but I wish that the events could be run in a way that did not favor random chance. The essence of a good scientific experiment lies in the control of variables and ones ability to measure outcomes with significant statistical certainty, but the balloon task in an open room with people moving about and doors opening and A/C systems cycling does not allow for such control. I don't mean to suggest that the winner of this event will not have a well designed and built device, but I do suggest that there will be significant random chance in the exact place that a team achieves. In regional competitions where the dividing line between the places is large this random chance in outcomes is less significant but at Nationals the difference between the top 10-15 teams is often quite small and environmental conditions will almost certainly play a major roll in the outcome.

Mission Possible is NOT Balloon Race, a point that has been made multiple times this season. If someone sets up their balloon task in a way that puts 250 points at risk, it's their fault-- certainly not the rules or rule-writers'.

Old wrote:at Nationals the difference between the top 10-15 teams is often quite small

Isn't that to be expected at a national level competition? Sporting competitions often have "photo" finishes where the top places separated by very small differences in time/score. It's not uncommon for random environmental differences to affect their outcome. This is accepted an part of the competition.

Throughout the history of science, random chance has often been the dividing factor between great scientists and unheard of nobodies. Generally, those who persevere to overcome random factors end up on top!