View from the Garage

March 16, 2016

Killing the Bugs

Filed under: Woodworking — thefusionguy @ 11:45 pm

Turns out, the lumber from the fallen Torrey pine has bugs. That’s not too unexpected. But when I moved the 4″ thick boards from the field to my garage, I found several kinds of new insect life around. This put me in a bit of panic, so I turned to Google. I can fumigate the boards; well, no. Or I can heat them to 140 F (60 C) for an hour or so. Well, that didn’t sound so hard, and might help with the drying as well. I heated a smaller cedar board a few years ago by enshrouding it in a plastic box and using the sun to heat it, and inside a day it was plenty hot. That did seem to kill the boring insects that I know it had. But the Torrey pine lumber is much wetter, and therefore has much greater heat capacity. So I decided to try using an electric heater.

First I made a bottomless box out of 1/2″ CDX plywood and lined it with R-13 fiberglass wall insulation.  This box barely fit the particular boards I had. For a heat source I started with a $10 hair dryer, which I put in the box with it aimed into an aluminum foil tube made for clothes dryer vents from Home Depot. I perforated the tube in 7-8 places. The idea of the tube was to distribute the heat along the board and to keep the hair dryer end from getting too hot. To measure the temperature, I used a TMP36 temperature sensor, a marvelous $1.50 device that looks like a transistor. You put 3 to 5.5 volts across two terminals and the voltage on the third is 0.5+T/100, where T is the local temperature in deg C. Works great! I bored a hole in the center of the end of the board, about 2″ deep, and put the temperature sensor there.


But hairdryers aren’t made to run continuously for 6 hours or so. And I had to enclose the hairdryer completely inside the box, because I didn’t want the wood to dry out, just get hot, so keeping minimal air transfer to the outside is beneficial. So the entire dryer also gets to the cooking temperature. First thing, there’s a thermal cutout that I had to bypass. Then with that bypassed, there’s a thermal fuse that I also had to short out. Also a ground fault interrupter that I deleted. So all the safety mechanisms were removed. And I knew the dryer might fail, I was just worried about what the failure mechanism might be, so I kept a fire extinguisher nearby. But surprisingly, it seems to work.

I estimated the time it should take to heat the wood. Guessing the plank as 120 lbs or 55 kg, and the temperature rise as 60-25=35 C, and assuming the rather wet wood has an average heat capacity half that of water, the heat needed is 2 J/gC * 55000 g * 35 C = 3.85 MJ. The dryer puts out 1.5 kW, or 3.6 MJ/hr, so the heating should take about an hour. But another thing that affects the time requirement is the time it takes for heat to diffuse into the core of the wood. Heat diffusivity in dry wood is about 0.16E-6 m^2/s, according to experts. Here the half-thickness is 5 cm, so the soak-in time is 4.3 hours. But since there’s 2 sides to the lumber, should be about half this time. So several hours can be expected to be needed for the heat to reach the core of the wood. In practice, it looks like this, which should be enough temperature to kill the bugs.



1 Comment »

  1. “kept a fire extinguisher nearby” :DDD

    Comment by xi — June 30, 2016 @ 5:40 am

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