This is my project to build a Stevenson screen. I use temperature readings a lot in my projects (please take a look at www.flowrc.co.uk) and I noticed that the readings were really influenced by direct sunlight etc. So, I needed to get a more standardised reading and the answer is a Stevenson screen. I looked at the prices of a ready-made one – and quickly decided to make my own. The hardest part seemed to be building the louvered sides. I found some gas vents in a DIY store that looked perfect. The size of the vents dicates the size of the screen and hence it is not strictly a standard Stevenson screen.
Wikipedia gives us the following definition (and I couldn't have put it better myself!)
A Stevenson screen or instrument shelter is an enclosure to shield meteorological instruments against precipitation and direct heat radiation from outside sources, while still allowing air to circulate freely around them. It forms part of a standard weather station. The Stevenson screen holds instruments that may include thermometers (ordinary, maximum/minimum), a hygrometer, a psychrometer, a dew-cell, a barometer and a thermograph. Stevenson screens may also be known as a cotton region shelter, an instrument shelter, a thermometer shelter, a thermoscreen or a thermometer screen. Its purpose is to provide a standardised environment...
For this project I have used,
1. 18x32mm baton
2. 5mm Plywood
3. 4 no. gas vents dimensions 245x175mm –open area 137cm2
4. 2 no. hinges
5. 6mm Wooden dowels for jointing the batons
6. Screws to fix the vents
7. Paint - Undercoat and white top-coat
In terms of tools I used a drill, hand saw and a paint brush. The cost will depends on whether you have any spare materials to hand. The gas vents cost around £3.5 or USD $6 each.
Step 1: Cut & Join the Batons
The screen is a box with the columns extended to allow a 35 mm air gap. I haven’t built any legs for the screen as I intend to suspend it from the roof of my greenhouse.
As mentioned previously the box has the width and height of the gas vents. The wood is cut to also allow for a hinged door on one side. I cut the following from 18x32mm pine baton,
4 no. 210mm – columns
4 no. 200mm – for the side abutting the 18mm edge of the column
4 no. 180mm – other side
The door is made from
2 no. 180mm
2 no. 175mm
The batons were joined with 6mm dowels. I drilled the holes with a drill press to make sure they were vertical and secured them with glue.
Step 2: Fitting It Together and Painting
Step 2a – Cut the plywood and add the door
The door was fixed to the fame using 2 no. small hinges.
I traced around the top and bottom of the cube to get the sections of plywood the right size. These were then pinned to the frame. It might be best not to secure them until after painting as painting inside the box after was a little tricky.
Step 2b – Painting
The screen needs to be painted white to reflect radiation from sunlight influencing the temperature readings. I used two coats of white gloss on top of an undercoat.
Step 2c – Fix the vents & Roof
The final job is to screw the gas vents on to the frame and nail on the roof.
What would I do differently?
I would improve the design of the door. It rubs slightly on the abutting vents.
I would not fix the door and roof on before painting!