PET bottle rain gauge

creating a rain gauge from a bottle of sparkling water and a piece of transparent tubing Ø24, 

you can use any tube diameter, the graduation calculation is done as follows:

it suffices to measure the radius in mm of the opening of the funnel (Ra) and that of the bottom of the bottle (Rb). Then apply the formula Ra2 divide by Rb2 to find the distance in mm between each graduation on the rain gauge, your graduation = 1mm of fallen water.

enjoy!!