I
put the weather station’s new rain gauge into
service last year. Before I built it, I did
a lot of research on the ’Net. I learned that
even the equipment meteorologists use is based
on the tipping bucket method for measuring rainfall.
I felt I needed to try it, so I followed a rough
sketch I found on the Internet and built the
prototype shown in Photo 1. As I built and tested
the gauge, it became clear why this method is
so widely used: it’s simple, it has amazing
resolution, and it produces a digital signal
that’s easy to process.
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(Click
here to enlarge)
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Photo
1—I used a plastic parts drawer to make
the tipping bucket. The hypodermic needle
(without the plunger) provides a measured
amount of water to slowly enter the tipping
bucket. A small piece of aluminum momentarily
blocks the optoisolator on each tip. Adjusting
screws at the ends of the tipping bucket
enable me to adjust the amount of liquid
per tip. |
My
main concern was getting the geometry right
so that the bucket would tip reliably. The prototype
worked on the first attempt. The geometry was
really forgiving. It would have been hard to
make one that didn’t work as long as common
sense was still part of the equation. The prototype’s
resolution was 13.5 tips for 2 oz. of water,
which translates into a resolution of 0.009²
per tip if you use a collector that’s 6"
in diameter.
The
finished version of the tipping bucket rain
gauge is shown in Photo 2. There was no reason
to change the geometry of the tipping bucket
in the final version. After a season in operation,
there is only one problem. The hole through
which water flows from the collection pot into
the tipping bucket is small. Its inside diameter
is that of a 16-gauge hypodermic needle, which
just takes a number 56 drill. That’s a diameter
of 0.046". This hole clogs (mainly with
bird droppings) from time to time.
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a)
b)
(Click
here to enlarge)
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Photo
2a—The main parts of the finished tipping
bucket rain gauge are housed in a piece
of 6² aluminum
pipe. Water drains out of the bottom, which
is screened to keep out insects. b—The tipping
bucket’s design is the same in the finished
unit as it is in the prototype. The optointerrupter
is blocked most of the time. It unblocks
momentarily when the bucket tips. I didn’t
use a connector because I was concerned
about water damage. |
The
gauge will remain accurate as long as the blockage
is cleared before evaporation occurs. I can
use a thin wire to clear the blockage. I haven’t
experimented with a larger hole, but you definitely
don’t want any splashing out of the tipping
bucket. With the current design, the water stream
on the bucket is gentle and there is no splashing.
All
of the weather stations I saw on the Internet
used a reed switch/magnet combination to signal
bucket tipping. It seemed to me a superior approach
would be to use an optical interrupter module,
so that’s what I did. It has been working well.
So,
measuring rainfall becomes counting pulses.
If the 6" collector pot is used, I need
to count 11 pulses for every 0.1²
of rain. Not bad!
