Monitoring Progress


When it comes to operating a solar kiln, there is not much that can to affect how the kiln runs. Adjusting the intake and exhaust vents is the extent of control you can have. As with all dry kilns (and most everything else in life), you can only adequately control what you are adequately monitoring. Temperature and humidity are the two primary variables that are monitored. Aside from the dataloggers (which are really meant for long-term research as opposed to daily operation), a simple RadioShack thermometer and hygrometer give me a reasonable idea of the kiln conditions.

I placed the thermometer/hygrometer on the floor near the front wall where the warm, dry air enters the lumber stack. This particular thermometer/hygrometer has a second temperature sensor at the end of a cable. The cable is stretched out with the sensor near the center of the front wall to give me a backup reading. The thermometer/hygrometer was located near one of the end walls because it makes it easy to view it through one of the vent openings without opening the kiln doors. I'm not sure this will work with 10 foot boards, but it works great with 8 foot boards.

$19 RadioShack thermometer-hygrometer The view from outside peeking through the vent The thermometer-hygrometer can be seen through the vent without opening the doors

However, what I realized after a couple weeks of drying is that the temperature and humidity are just a small part of the picture. The more important variable to keep an eye on is the moisture content of the lumber. I believe a solar kiln could be operated entirely by moisture content, with no temp/humidity monitoring if it were necessary.

It is a bit like losing weight. Ultimately, the most important number to be aware of is your weight. That's the variable you are trying to reduce. Sure, the number of calories you take in each day is important and the amount of calories you burn each day with exercise is important, but if you only monitor calories in and calories out, you can only guess what your weight is doing. You could certainly watch only your weight, but it doesn't hurt to track the input variables of calories in and out. Same with drying lumber. Moisture content and daily moisture loss is most important - the final goal. The biggest concern is that you do not exceed the safe limit of moisture loss per day. If you do, the wood will dry faster, but you run a high risk of damaging the boards.

The only real way to monitor moisture content when it is above 30% is to use kiln samples. Cutting and using kiln samples is covered in detail at many other sites (like this one) so I won't repeat it all here. You will need scales that can weigh to an accuracy of 1 gram for the sample sections and 0.1 lb for the sample board. I found a digital kitchen scale that could weigh to a gram and also up to 11 pounds. I also bought a cheap $18 toaster oven to dry the sample sections.

Weighing sample sections immediately after cutting them Weighing the sample board immediately after cutting it and end-coating Drying the sample sections to get to an oven-dry weight

Once the sample sections are oven-dry and all calculations are made, it's just a matter of weighing the sample board as often as possible to determine moisture content and moisture loss rate.

Sample boards in the lumber stack Digital scales ready to weigh the sample board Weighing the sample board 

After the moisture content gets below 30% it becomes possible to use a moisture meter. I have a wagner pinless meter. Even with the moisture meter, I still weigh the kiln sample. The moisture content as measured by the meter has correlated quite well with weighing the sample. It is important to realize the moisture content determined by weighing is an average across the entire board while the meter gives readings at a specific location. Don't get caught up in the fact that the meter reading can change from spot to spot.

Moisture meters can also be used but are usually limited in range Close up of my Wagner moisture meter being used 
 
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