[Weather] conversion help

[Mark J]

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In message <00d701c70c95$b35ebfa0$6700000a at main>
          "Matt" <mjoyce at iinet.net.au> wrote:

> Mine's a Lacross too.
> 
> I thought perhaps of getting a large funnel, or just modify a bucket, and
> have a tube out the bottom which pours in to the existing aperture.
> 
> I'm not sure how much flow it could take before becoming inundated and
> inaccurate.

Discrete drops from the funnel might be considered to have similar
size and kinetic energy. The more water leaving the funnel, the more
the kinetic energy. That kinetic energy adds to the downward force due
to the mass of water already in the bucket, tipping it increasingly
prematurely. Too great a stream and water will splash out of the
bucket, and the poor thing will be overwhelmed. There is also the
water lost at tip - at mid tip water will pour into either, neither,
or both buckets. Single bucket designs could well stay tipped, and
stop counting...

IMHO the greatest precision (repeatability) will occur when only one
drip enters a bucket as it is tipping - perhaps no more that 4 or 5
drops per second. The standard 7048 bucket tips at 0.25mm rain/3.7cm3
or thereabouts, and that contains about 80 typical drips, which at 4-5
drips per second is about 18 seconds per tip, which is about 0.25mm
rain in 18 seconds or 300mm/hr (12"/hr). 

Doubling the funnel area will halve the rain rate which the above
suggests is acceptable. Steady rain versus downpours? I personally
haven't logged any downpour greater than 6mm/hr within a 10 minute
period, but that disguises very short bursts that could be very much
higher...

At the other end of the scale you could have <0.25mm rain with no tip,
and that could dry up in the bucket and never be counted. Doubling the
funnel area there will halve that amount.

It's one great big compromise :-)



> 
[snip]
-- 
E.Mark Jolliffe
www.poppy-land.co.uk