Page 296 - Robots Androids and Animatrons : 12 Incredible Projects You Can Build

Table 14.1
English,
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cubic meter (kg/m )
lb/ft 3 Metric, kilograms per
Weight of hydrogen 0.0058 0.09
Weight of helium 0.0110 0.178
Weight of air 0.0807 1.29
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So what’s the lift of a helium balloon with 5 ft of displacement?
Weight of displaced air 5 (0.0807) 0.4035 lb
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Weight of 5 ft of helium 5 (0.0110) 0.0550 lb
Lift 0.4035 lb 0.0550 lb 0.3485 lb

That’s quite a bit of lift! The reason is that we didn’t subtract any
weight for the balloon. If the balloon weighs 0.25 lb, the usable lift
(0.3485 lb 0.25 lb) is reduced to 0.0985 lb, or 1.57 oz.
How does this compare to the lift using hydrogen? Well, the weight
of the air displaced is the same.

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Weight of 5 ft of hydrogen 5 (0.0058) 0.029 lb
Lift 0.4035 lb 0.029 lb 0.3745 lb
275
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The difference in lift for a 5-ft balloon is
0.3745 lb 0.3485 lb 0.026 lb, about / 2 oz
1
Because the difference in lift is small, it is not worth the added risk
of using hydrogen gas! I recommend using helium gas only.

Size
The piece of Mylar used to make a balloon after it’s folded in half,
lying down flat, measures 34" 56". The weight of the material is
3 oz (0.1875 lb). It’s difficult to estimate how much helium the bal-
loon will hold. To make a rough estimate, I use the volume of a
cylinder. I know a pillow shape is not a cylinder, but, like I said, it’s
a rough estimate. First find the diameter. The material is 34" 2
equaling 68" for the circumference. The circumference of a circle
is 2 times pi (3.14) times the radius. If you do the math, the radius
works out to 11". The volume of a cylinder equals pi times the ra-
dius squared times the height. The height in this case is 56". If you
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do the math, the volume equals about 12 ft .
The balloon will not be filled to its maximum capacity. In this case
I’d estimate the balloon will hold about 70 percent of the calcu-
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lated volume or about 8.4 ft of helium gas.
Team LRN Aerobots