Stevenson Screen - Weather Station

Stevenson screen

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!

Comments

[hmithushara1983]

I like to know why the gap between roof and body and how much it should be?

[DaleM85]

Having just completed the building of a Stevenson box, I've encountered a few problems which may be of some interest or help to others building their own box.
I used single plastic vents and I believe that there may be a heat radiation problem from the vents during low elevation direct sunlight, thus warming the air inside the box slightly. I've increased the roof ventilation to "hopefully" create a ventri stack effect by using 4" PVC with a T at the top leading to elbows angled downward at ~35 degrees (to help prevent rain from entering).
I've covered the top with white tin cladding to reflect the sun and help keep the box relatively maintenance free and roof water from entering. Under the tin covering, I have 1 inch rigid styrofoam insulation to help prevent heat being transferred from the metal roof into the box.
At this writing, I've not yet checked my modifications to see if they are effective in preventing the interior from heating beyond the outside temperature in direct low elevation sunlight.
If this fails, I may change my venting (at least on the sun facing sides) to wooden vents to see if that solves any heating problem which may occur.
For what it's worth...

[pcooper2]

If you're going to feed your temperature readings to a weather service, a
home-built weather station would need to be carefully vetted and tested
against one built in the traditional way. The original Stevenson screens were made of wood, painted white and were in use for more than a century. Some stations have recently been set up with vinyl Stevenson screens, and it is not clear if their performance is identical to the original wood screens. There is a suspicion that part of the "global warming" phenomenon may not be real and may be due to differences in materials used in thermometer housings. If you're building this screen for your own use, though, it will most likely to the trick for gathering consistent local relative readings.

[diy_bloke]

it might be easier to have the door hinge at the bottom. that way you dont have to keep it supported while working in the weather station

[Mindmapper1]

This is an intersting design and idea. As far as I know Stevensons screens usually have double louvers ie inside and outside. I think I will be making one of these soon.

[85rocco]

This is an official government issue Stevenson screen and you are correct, they do indeed have double louvers, that helps to prevent radiant heat from the ground influencing temperature reading. The author's design could easily be modified to achieve this by placing two gas vents back to back on each side

[mattinflow]

Mindmapper1 - thanks for leaving a note. My intention was to make something that was a simple as possible but also functional - hence the idea of using standard gas vents. If you come up with some design improvements that please let me know and post a picture!