Accuracy of Thermistor Method for Measuring Wood Temperature

Moisture-meter readings are affected not only by the moisture content (MC) of wood but also by its species and temperature. In checking the final MC of lumber after kiln-drying, it is, therefore, necessary to correct the indicated MC for both species and temperature. Correction tables are available from meter manufacturers, from the two Forest Products Laboratories of Canada, and from the U.S. Forest Products Laboratory (James 1961, 1963). Combined species-temperature correction tables for several western species were recently prepared by Bramhall and Salamon (1974). A major problem in making temperature corrections is to determine the temperature of the wood. The final dry-bulb temperature may be used, if MC is determined immediately after the lumber is removed from the kiln. The ambient temperature is suitable after the lumber is cooled for about 24 hours. However, it is usually most convenient to determine MC some time between these two extremes. The thermistor method of determining wood temperature, as used in the Delmhorst TM series electronic thermometer with a TS-2 probe, requires a hole predilled with a No. 29 drill (0.135 in.). With the probe inserted into lumber to a 3/4-inch depth, maximum readings were obtained in 40 to 45 seconds. However, due to heat capacity of the probe, these readings were about 9?F low where wood temperature was 90?F above ambient. The errors in temperature corrections to moisture meters due to this cause are considered negligible.

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