If you are going to have any reasonable chance at controlling your solar kiln, you will need some basic instrumentation. The primary output variable that we're trying to control is the rate of moisture removal from the lumber. Monitoring that variable involves weighing kiln samples and possibly the use of a moisture meter. Those activities are covered in the discussion on operating your kiln. The rate of moisture removal is affected by several other related variables such as air flow rate, temperature in the kiln, and relative humidity in the kiln. With these simplified solar kilns, the air flow rate is mostly fixed based on the fans that are purchased and installed. Temperature can be somewhat controlled by adjusting the vents, but the primary control is whether the sun is shining or not. Relative humidity can be affected by the remaining moisture in the lumber, the outside relative humidity, the temperature in the kiln, etc. but our primary means of controlling humidity is also through venting.

The only way to know how the kiln is performing and how you are affecting its operation is to monitor the temperature, the humidity, and ultimately the moisture content of the lumber. The easiest way to monitor temperature and humidity is to grab a low cost thermometer/hygrometer and place it in the kiln. I bought one from RadioShack for around $20. It measures relative humidity from 25% to 95% and temperatures (indoor/outdoor) up to 122°F, although the outdoor sensor is rated to 158°F.

I placed the thermometer/hygrometer on the floor at the inlet to the lumber stack and near one of the end walls. The second temperature sensor (outdoor) is on a cable so it is stretched out to measure near the center of the lumber stack on the inlet side as well. With the thermometer/hygrometer on the floor and near the end wall, one of the vent doors can be opened and reading taken without having to open the main kiln door.

The RadioShack thermometer/hygrometer The view of the thermometer/hygrometer from the door of the kiln

For the sake of this website (and my twisted desire to track the kiln's operation) I also purchased two dataloggers from Lascar Electronics (see Resources). One is temperature only (costs around $60) and is used to get reference data on the external temperatures. The second is temperature and humidity (costs around $85) and monitors the internal conditions of the solar kiln. These dataloggers take multiple data points (16,000) at predetermined intervals (mine are set for 5 minutes). When it is time to gather the data for review, the dataloggers are simply plugged into the usb port of a laptop and the data downloaded to the computer.

Each sensor came with its own mounting bracket and protective usb cap. I modified the protective cap on the temp/humidity logger by cutting the end off. This allowed me to attach a usb extension cable, wrap the connection in plastic and seal with electrical tape.

Lascar dataloggers Protective caps removed showing the usb connections Modified cap with the end cut off
Modified cap on and the usb extension cable plugged in A little protective plastic and electrical tape to seal things up
The modified cap and extension cable on the temp/humidity logger allowed me to mount it at the inlet side of the lumber stack and then route the extension cable to the exterior of the kiln, near the temperature-only logger. This makes it easy to download data from both dataloggers without having to crawl around inside the kiln while it is drying.
The supplied mounting bracket installed inside the kiln The temp/humidity datalogger in the mounting bracket with the extension cable routed to the exterior of the kiln Temperature only datalogger installed on the exterior of the kiln under one of the pv panels for protection
A shot of the temperature installed datalogger and the temp/humidity extension cable routed under the polycarbonate A view of the extension cable under the pv panel before it was coiled up and tucked into the pv panel frame
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