Firstly, I made this at my local hackerspace/makerspace Bloominglabs (where this unit is also installed). This is a follow-on to this instructable covering the electronics used in this weather station:
The first step in obtaining accurate ambient air temperatures is to build a structure which allows air to pass through it but blocks direct sunlight and moisture. With modern weather stations using electronics, keeping the moisture out is even more important. An old design that has stood the test of time is a the Stevenson screen.
Without the “screen”, the radient sunlight would drive up the temerature readings and direct contact with moisture could drive down readings. This is most important for getting consistent and correct readings on ambient air temperature.
There are two main parts. First, the “screen” needs two ceilings just like your house has. The top roof reflects direct sunlight and rain. The second roof is protected from most of the direct contact from both of those. The space between these works like the attic of your house.
Then there are the sides which are more complicated. There are many short-cuts people use to build these, usually involving window louvres or cut up plastic plant pots. However, if you look closely at a “real” Stevenson screen, you will notice that the side panels have louvres sloping downwards on both the outside AND the inside. It’s not a huge problem to skimp and only have one set of louvres per side, except the original design is to keep mechanical equipment mostly dry, not electronics which need to be kept completely dry. With a single set of louvres (per side), it will block direct sunlight which is half-good. However, blowing rain can still come directly inside if it is blown at an upwards angle which is very likely to happen in any storm. The double set of louvres in a full Stevenson screen should guarantee that any rain being blown at the sides from any angle hits the top or the bottom of either set of louvres and kills the kinetic energy of the droplets. Even if rain drops get to the inside louvres, they will hit the wood and then just drop harmlessly down at the inside edge without making any contact with the equipment inside.
So, in my opinion, taking the shortcut is more likely to cause you problems with a microcontroller driving the weather station. Also the amount of work required to build any of the “shortcut” screens is not that much less than building a full and proper screen (if you can laser cut the louvres that is).
Step 1: Tools and Materials for the Louvre Panels
You will need a laser cutter of course. 40W or higher power.
Material: 1/8” baltic birch plywood, at least 12”x20” in size. I belive 8 sheets of this will be enough to build four louvre side panel assemblies.
Water reistant wood glue (Titebond III or similar)
At least 12 small spring hobby clamps, 16 or more is better though
Papertowels because glue is messy.
Step 2: Laser Cut Some Louvres
Download the SVG files from here:
louvre_sides.svg - There are 4 parts cut from this SVG. Obviously the louvres fit into the slots to make one screen side. The blank pieces are to be glues with water-resistant wood glue to the outsides once the screen side is fully assembled. You will need to laser cut 4 complete copies of this for a complete screen.
louvres_x18.svg - This cuts 18 louvres. It is sized to fit a Full Spectrum 5th Gen laser which has a bed size of 12”x20”. You need 27 louvres for a single side, and 108 for a complete 4-sided Stevenson screen. So you need to cut 6 copies of this.
louvres_x9.svg - This cuts 9 louvres. Only use this if for some reason you plan to build just a single louvre panel. In this case, cut 3 copies of this to get the 27 louvres you need. Again, don't use this if you are making all 4 louvre assemblies, use the x18 svg file above instead.
Step 3: Assemble the Louvre Assemblies
Louvre panel assembly is a bit like basket weaving. Give yourself plenty of time and maby have an adult beverage to calm the nerves. First lay down the louvre side on a table. Then populate all the slots with louvres. Once you have all the slots populated, lay the other louvre side on top. Starting at either end (does not matter which), slowly start pushing the louvres through the slots of the top side part. You will need to use one hand to hold down the louvre side as you continue working more louvres into their slots. The first one is the hardest, once you get the hang of it, it becomes easier. Once you have all the louvres installed into all the louvre side slots, be vary careful. It will become a single rigit part at that point, but louvres can still pop out.
Glueing: lay out a couple papertowels on your worksurface, and extras for your hands. Take the blank louvre side and cover it well with a liberal coating of water resistant wood glue. I used a bit of folded papertowel to smear it around and completely cover the side. Then lay it on top of the side as shown. You MUST use clamps or else it will not bond properly and the sides will have an irregular widths which will doom your project in the next phase. Once you get one side clamped, turn the assembly over and sit it on the clamps. Repeat this step on the other side of the louvre assembly. Clamp this side too. I prefer doing both sides at once as I can make corrections to the assembly while the glue is still wet. Let the assembly dry for 24 hours or whatever your glue directs you to do.
Repeat this step for 3 more louvre assemblies.
Step 4: Tools and Materials for the Bottom, Roof, and Frame
For the rest of this build you will need a saw, drill, and a jobber set (drill bit set). A table saw and band saw will speed things up, but are certainly not required.
6' length of 2"x2" white pine board
Plywood, 12"x40", any thickness from 1/4" on up
2" and 3" drywall screws
1 can white primer spray paint
1 can white gloss spray paint
Ideally all the wood will be pressure treated, however we will be painting it so that is not required. I used scrap plywood and picked up a 2x2 from the hardware store.
All screw must have holes pre-drilled or else the wood will split. To size a drill bit for a screw, hold the bit in front of the screw so the bit covers the screw. It is sized correctly when it covers nearly all of the center of the screw but the threads extend wider than the drill bit. Never use a drill bit that is wider than the threads on a screw, then there will not be any material for the threads to dig into. Too narrow and you risk splitting the wood.
Step 5: Cut the Bottom and the Lower (inner) Roof
First cut your plywood. You need two 12"x12" pieces for the base and the inner ceiling. The top roof will be longer, we will wait to cut that last.
Step 6: Cut the Corner Posts, Attach to Base
I cut two posts at 10.5" and two at 11.5". I put a 5 degree angle on the tops to give the top roof a slight slope. I believe I measured the length at the bottom of the angle, so the total length will be greater than 10.5" and 11.5" at the high end of the angle. I don't think the angle is super critical, if you want to keep it flat to save the trouble that should work fine. However most photos I've seen of a Stevenson screen shows an angled roof so I went with that.
The bottom of the enclosure we will keep square (not cut out the corners). Take your finished louver assemblies and sit them on the base, then place the corner posts in position. We want to center everything, then mark the base with a pencil where the corner posts need to mount. I prefer fitting all the parts and then measuring so any small variations from what you have on paper won't throw off your final assembly.
I drilled two screw holes per corner post, drilling through the base into the corner post. Either 1.5" or 2" drywall screws should work fine here for attaching the posts.
Step 7: Cut Some Corners
The lower roof will need corners cut so the corner posts can extend past it to support the upper roof. You will want to use the louver assemblies to position the height correctly, just rest the roof on them to make drilling and screwing easier. I only used one screw per post to attach this roofa
Step 8: Install the Louver Assemblies
Lay the enclosure on its side with the louvre assemblies still in position from the previous step. I drilled two holes per side and used 3" drywall screws to mount the louvres in place. The screws will pass through the louver sides and be exposed. If you are careful the exposed part of the screws will all be hidden inside the enclosure.
Once you have three louvre assemblies installed, remember that your last one will be your access door. In the 2nd photo you can see that I sanded the top down so one side has rounded corners. This lets you tilt this assembly out if it does not want to slide. I ended up only drilling a single screw hole to hold the assembly in place. This is becuase any maintenance I perform after this enclosure is installed will be while I'm standing on a ladder 30' in the air.
Step 9: Cut and Mount the Top Roof, and Your Mounting System
Since the top roof in this build is at an angle, it will be slightly longer in one diemension. I just laid my roof section out on the corner posts and marked it so it would hang over the base slightly on two sides. I drilled a single screw hole for each post. Since we still need to seal the lower roof I did not screw down the top roof.
Also, about now you will need to think about how you want to suspend your enclosure in the air. For this one, it will be mounted on an antenna mast. I cut some extra strips of plywood to attach to the top and bottom. I then cut some extra pieces of 2x2 and attached to my plywood strips. This enclosure will be attached to the side of an antenna mast using hose clamps. Since it will be a single person operation up on the ladder, it's easier to have the hose clamps already connected around the pole. I can slide them over the cut pieces of 2x2 and tighten then down with one hand holding the enclosure and the other turning the screw driver.
Since you may be mounting your enclosure in a different way, you may not have a need for some of this detail.
Step 10: Seal It
I squeezed water resistant glue into the cracks around the louvre assemblies and let it set up. You could also use silicone glue, but the glue seemed to fill in fine gaps easier and you don't have to mess with sticky silicone. If the gaps are too wide then silicone sealer will be required.
Obviously you won't be able to seal around the removable louvre assembly. That's fine, just make sure there are no huge gaps.
Also look inside and re-seal any gaps there too.
Lastly, you will want to use silicone sealer on the lower roof around the corner 2x2's (on the outside of the enclosure). You won't want water running down inside along the gaps where you cut the corners out. Seal this up good and you should be well protected from water getting inside.
Step 11: Paint!
I didn't mention it earlier, but you will want a drain hole and a cable access hole into the bottom depending on your weather station setup. The drain hole I drilled at a 45 degree angle so rain can't blow up through the hole easily. Make sure your cable hole is big enough for the connectors on your cable, RJ45 in my case. The cable hole should be sealed up with silicone once you are done so it will not be an egress point for water.
Generally, you will want to use white pant so as much radiant heat from the sun will be reflected away. I used white primer and then a 2nd coat of glossy white.
On the louvre assemblies make sure you paint up into the underside of the peaks, just turn it upside down for one of your coats. You also want to paint the inside too, small amounts of moisture may still get inside, we don't want any bare wood on the main structure to rot away over the years.
Also make sure you paint in between the lower and upper roof sections. Rain can still get blown in there and we don't want that to rot in the future.
Step 12: Add a Metal Roof Topper
I had some spare aluminum flashing left over from another project. It was just over 12" wide so it fit perfectly. I trimmed it so the edges would fold over the sides. I drilled some horizontal holes through the flashing into the top roof to hold it in place. This will add additional protection from the elements and hopefully a longer life to the enclosure.
Step 13: Mount Your Electronic Components
I used one last section of 2x2 mounted in the center of the enclosure to mount the electronics. The black component is a RPi2. The white and green component is an Ardent weather station controller. This is all covered in this instructable here:
I drilled two holes through the bottom up into the 2x2 for mounting the post.
That's it! Retest your electronics or instrumentation, mount it in the air, and wire up your cables. You will now have protected sensors which will read true temp and humidity readings.