Saturday, July 7, 2012

Outdoor Temperature

I've had an outdoor temperature sensor for quite a while now.  It's a simple temperature sensor hooked to an Xbee that gets power from a wall wart I picked up on ebay.  The description of this little device is here.  Well, I decided to make it work like it should and actually start measuring the outdoor temperature.  This means I have to enclose it and find a place to mount it that will allow proper operation.

So, I had to build a Stevenson Screen.  At first I had no idea how to do this, then after a few hours of searching and reading I figured out I simply needed a louvered box, painted white, setting somewhere on a pole.  So, I picked a fence post out in the yard and ran a conduit to it.  I mounted a short piece of pipe and installed an outlet.
Above you can see the wall wart a couple of wires and the XBee with its temperature sensor soldered on the side (click on the picture for full size).  I chose to use a wall outlet and regular wall power because I don't want to have to worry about changing batteries every few weeks.  Yes, I could use rechargeables and a solar cell, but those things wear out here in the desert.  Believe it or not, the coating over the top of the inexpensive solar cells used in yard lights turns yellow and starts to flake off in about a year of exposure.  The batteries cook and die after about 8 months; basically solar lights are annoying.  Yes, I could use a good solar cell (with glass) and a lithium ion battery, but pricing those devices and the charge controller that would be required brings the power supply cost to roughly $75.  Conduit and wire ran me around $8.  Additionally, there's not much information on the lifetime of a lithium ion battery in 100+ heat day after day.  Understand my thinking now?

I found several Stevenson Screen projects on the web; one even used flower pot bases which was an intriguing idea, but the cost of those made the enclosure run around $35.  So, I had some scrap wood left over from anther project and decided the good old wooden box painted with glossy white paint would be the best route.  However,  I didn't have enough scrap wood to build a regular Stevenson Screen so I decided to build it in a triangular shape to save wood.  Don't do this.  The angles and complexity of a triangle over a simple 90 degree box drove me nuts.  It did come out reasonably nice though:
There's enough room inside to eventually mount a sensor for humidity and the top is flat so I could mount a wind sensor as well.  The humidity sensor is a good idea, but I would rather sense wind speed and direction from the roof; that will be a complex project that will have to wait for another day.  Since I don't want birds building nests inside this new installation, I lined the inside with aluminum screen wire.  There should be plenty of air flow and the rain can't get through the louvers.  It has several coats of white enamel paint to reflect the sun and limit heating to the minimum so I should have accurate measurements of the air temperature.
This is a chart of my temperature recordings over the last 24 hours.  On day one (today), the readings reflect badly because the sensor was in the house, then in the sun, then in the enclosure, but it will be working properly after about noon on July 7th, 2012.

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