On Mon, 15 May 2006 11:23:08 GMT, "Dave.C"
wrote:

As far as I can tell with mine as soon as the first bucket tips the second
bucket is immediately in place to catch the rain draining through. it just
rocks backwards and forwards on a pivot

This is certainly the case with recent Davis type tipping gauges - the
two buckets are essentially just one continuous bucket with a thin
vertical partition at the centre point. So if the rain is not dripping
into one bucket then it's dripping into the other. There are therefore
no drops lost while the bucket tips in the sense of drops falling
outside the buckets.

However, what does in theory cause small errors is that extra drops
may continue to fall into the bucket that is already tipping out of
the way and so do not contribute to the new tip.

Although this effect may seem unlikely, the buckets probably tip only
relatively slowly. Just before they tip the buckets will obviously be
close to being in balance and so one more droplet in the heavy bucket
will only just tip the balance. I've seen estimates of 0.5-1 second
for the tip to pass the midway point, though I've never seen any hard
evidence and it wouldn't surprise me if it were typically either
slower or quicker than this.

Anyway, just doing some back of the envelope sums:

Assume heavy rainfall of say 12mm/hr = 0.2mm/min or 1 tip/min for a
0.2mm increment gauge

One 0.2mm tip for a Davis gauge would require - if my maths is correct
- 4000ul (that's microlitres) of accumulated rainfall. So how big is a
single drop? A standard laboratory rule of thumb is 50ul but can vary
considerably depending on eg the size of the nozzle, especially for
small/tiny nozzles. But for the sake of convenience let's call it 40ul
so on this basis maybe 100 droplets could be needed for one bucket tip
- close to 2/second. So it's not inconceivable that somewhere up to
2-3% under-recording could result during heavy rainfall from
over-filling each bucket. And if the droplet size were bigger than my
estimate then it would still be possible for a droplet to land on the
partition wall mid-tip and split between the two buckets.

(Can you tell I had a few idle minutes at lunchtime, though I'm not
guaranteeing my maths?)

John Dann
www.weatherstations.co.uk