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Use the spreadsheet to calculate the temperature rise needed to make the four sheet tetroon from the book fly. The spreadsheet was written using Microsoft Excel, but loads perfectly into Gnumeric (the open source spreadsheet program). Enter the following numbers into the boxes at the top of the spreadsheet:

sheet short edge length
Measure along the short side of one of the polythene sheets made from a cut up trash bag. Measurement must be in metres.

sheet long edge length
Measure along the short side of one of the polythene sheets made from a cut up trash bag. Measurement must be in metres.

sheet turnover length
Enter how much you overlap the individual sheets when making joins and how much of the edge you turn over when making a join at the edge of the envelope. Enter a value of 0.02 (20mm) when making a taped tetroon, and 0.006 (6mm)when making a welded tetroon. The numbers must be in metres.

tape weight
Weigh a one metre length of your adhesive tape (scotch tape) and enter the number in the box. The weight should be in grams per metre.

return tag weight
Weigh the postcard carrying your email address along with the plastic bag cover. Enter the weight in grams into the box.

ambient temperature
Measure the air temperature in the shade near to where you want to launch the balloon. Enter the temperature in degrees Celsius into the box.

temperature rise in balloon
Measure the expected air temperature rise using the solar power meter described in Chapter 2. Enter the temperature rise in degrees celsius into the box. You should enter the temperature rise above ambient, not the expected air temperature inside the balloon. For example if the ambient temperature is 20 degrees Celsius and the solar power meter predicts an air temperature inside the balloon of 55 degrees Celsius then your should enter a temperatre rise of 55-20 = 35 degrees Celsius.

You can use the spreadsheet to explore the effect of different ambient temperatures, sheet sizes and material weights. You can see by leaving the temperature rise at a fixed figure and changing the ambient temperature that a solar balloon can actually work better on a cold day!