To determine the accuracy of a digital rain gauge, you need to know the surface area of the funnel.

If the rain gauge has a rectangular inlet:

Area = length x width

If the rain gauge is circular:

Area = ( π x d2 ) / 4, where d is the diameter of the rain gauge and π = 3.14159

The calculation is as follows:

Total Rain = Water Volume / Area

Example 1:

Rectangular Rain Gauge:

Area = 11 x 5 cm or 55 cm2

If you pour in 100 ml of water (or 10 cm3) SLOWLY (over a 10 minute period), one would expect the rainfall to equal:

Total Rain = 10 cm3/ 55cm2 = 0.18 cm or 18 mm of rain (+/- the accuracy specification of the rain gauge).

Since 1 mm = 0.03937 inches, the total rain (inches) = 18 x 0.03937 inches/mm = 0.71 inches.

Take into account the resolution of the rain gauge.

If the resolution is 0.04 inches for example, the measurement would be 0.04 inches/tip x 17 tips = 0.68 inches.

Example 2:

Circular Rain Gauge (Osprey):

Diameter = 11.28 cm

Area = ( π x 11.282 ) / 4 = 100 cm2

If you pour in 100 ml of water (or 10 cm3) SLOWLY (over a 10 minute period), one would expect the rainfall to equal:

Total Rain = 100 cm3/ 100cm2 = 1 cm or 10 mm of rain (+/- the accuracy specification of the rain gauge).

Since 10 mm = 0.3937 inches, the total rain (inches) = 0.3937 inches.

Take into account the resolution of the rain gauge.

If the resolution is 0.01 inches for example, the measurement would be 0.01 inches/tip x 39 tips = 0.39 inches.

Important Note: Make sure the weather station display is set to total rain and not rain rate.