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SCREEN FOR A HOME WEATHER STATION
sensors need to be shielded from direct radiation otherwise an
incorrect temperature could be indicated. If the sun is
on the sensor then it would likely read higher
if the sensor is exposed to the night sky then it would likely read lower
radiation problem "official" temperature sensors are shielded by a "Stevenson
. This is
usually a white-painted wooden box with slatted sides for
Unfortunately these boxes
are rather too large
to be included with the average home weather station.
a home weather station is supplied with a rather rudimentary screen
which would be OK if the station were placed in a shaded
such as under the eaves of the house.
such a position is impractical if the station is also meant to obtain
accurate rain and wind readings.
shown a Maxkon
weather station to which a home-made "Stevenson Screen" has been added.
screen is made from 4 melamine bowls held together with threaded rod
and tubular spacers. The bowls can be obtained for about $1.00 each if
you find the right kind of shop. (For best performance the bowls should
be white both inside and out).
1. Four white melamine bowls 200mm dia x 50mm deep.
2. M6 galvanized threaded rod, 450mm long.
3. M6 galv. nuts and washers, 7 of each.
4. Aluminium tube, 10 (or 12) mm dia, 400mm long.
(Instead you could use stainless
steel tube or even stiff plastic tube).
5. Galvanized RHS, 25mm square, 200mm long.
6. M6 galvanized bolt, 35mm long.
a hole saw make a 50mm hole in the centre of 3 of the bowls. (The 4th
bowl will be the lid and should not have a hole).
on a pitch-circle-diameter of about 90mm drill 3 equally spaced holes
at 6.5mm diameter into each of the 4 bowls. (You could possibly do this
with the bowls stacked together).
cut the tube into 9 pieces at 40mm long. (This length will ensure about
the right amount of vertical overlap between the bowls).
the threaded rod into 3 pieces at 150mm long.
the whole assembly together as shown in the pictures (left) and fasten
each rod with a nut and washer at the top and bottom ends. (Nylon nuts,
as shown in one of the pictures, could be used in lieu of galv. nuts).
mounting arm described below will suit a weather station with a 20mm
dia pole). 1.
the RHS to 200mm long.
end is to be made into a split clamp
as follows: Drill a dia 20mm hole 22mm from the end.
Drill a dia 6.5mm hole 6mm from the end and at right angles
the axis of the first hole.
4. With a hacksaw or an angle grinder, slit the RHS
as shown in the picture.
5. The outer end of the mounting arm will look much
neater if a piece is cut
out such as to allow the top flap to be bent down and make an enclosed
end. (See pictures at left).
6. The arm can be attached to the top melamine bowl
with 2 screws (say M5)
tapped into the bottom side of the RHS.
You will need to work out the positions of these screws such as to give
the correct radial position for the screen assembly. (For the Maxkon
station the radial distance from the centre of the mounting pole to the
centre of the screen assembly is 160mm).
Readings from this weather station have been compared with official
weather bureau temperatures and also with 2 other (well shaded) sensors
and good agreement was found, leading to the conclusion that "radiation
errors" are likely to be less than 2 degrees Celsius (mostly
than 1 degree). (Errors with the original "as supplied" screen
sometimes exceeded 5 degrees Celsius).
descibed above does not shield the radiation coming from the ground.
Thus it is best if the ground underneath is grassed or covered
with something that has low emissivity.
This page last updated 19