Richard Hunt wrote:Hope this is not a stupid question.

I'm about to purchase a copper rain gauge with a funnel diameter of 123mm (Metcheck CM1037), which is just short of 5 inch.

The gauge I have seen comes with an internal plastic measuring cup, but I would prefer the glass measure (CM1013) with finer 1mm increments.

The glass measure is calibrated for a standard 5" gauge, the question is could I use the glass measure in a 123mm gauge and still be (fairly) accurate?.

The gauge and measure in question is from Metcheck.

Hi Richard,

With a slightly smaller diameter rain gauge you will be collecting slightly less rain in comparison with a standard 5 inch gauge.

The question is, if you use a measuring cylinder for a 5 inch gauge what will the error be?

I have done some arithmetic, square of gauge diameter over square of cylinder diameter, and

For a 5 inch (127 mm) gauge, the factor is 1 in 17.92 .

In your case with a gauge of 123 mm,

the factor is 1 in 16.81 .

So the error is small, less than 6% under measurement.

The trouble is, if you sum daily measurements over a longer period of time, a month or year, the error would build up.