Electrical resistance moisture meters

Electrical Resistance Moisture Meters. The electrical resistance of wood is extremely sensitive to its moisture content, approximately doubling for each 1% decrease in moisture content over the hygroscopic range of moisture contents. The development of a successful resistance moisture meter may be attributed primarily to the pioneering work of Stamm (9) who first measured this relationship quantitatively. Because of the nature of electrical conduction in wood there is also a strong increase in resistivity with a decrease in wood temperature. Figure 1 illustrates how the electrical resistivity of wood varies with both moisture content and temperature. 

Other Physical Properties. In addition to its important effect on the strength of wood, moisture also affects wood s other physical properties. Moisture s effect on electrical properties was described in the section on Electrical Resistance Moisture Meters (p. 130). Other properties such as specific gravity and thermal properties are discussed here. 

Resistance moisture meter scales manufactured for use in North America read directly in moisture content, based on calibration data for Douglas-fir at 27 C. Figure 2 shows the range in electrical resis-... 

Electrical moisture meters provide a quick and reasonably accurate nondestructive alternative. The direct-current resistance of the timber is measured or either the alternating-current capacitance or power loss can be measured. Direct-current resistance moisture meters are more common a pair of needles, a fixed distance apart, is driven into the wood across or along the grain (depending on the manufacturer s instructions) and the electrical resistance measured. The procedure is reasonably accurate between the fibre saturation point (defined later) at 30% and about 6% moisture content (at which point the resistance becomes too great to measure with reasonable accuracy). In this moisture content range, the relationship between electrical resistance and moisture content is represented by a log-log plot. 

There are also recording instruments based upon the relationship between the moisture content and the electrical properties of the material. The Shirley moisture meter (Fig. 2.15) depends upon the effect of moisture on the resistance of a sample of material compressed between two electrodes. 

Electrical resistance measuring devices together with dielectric and infrared instruments are available as non-destructive commercial products for field use, notably by small boat surveyors [38]. Clegg [39] has reported comparative tests on three of the most popular UK/Ireland moisture meters which use capacitance measurements. 

Several tin-based alloys, usually containing lead, antimony and copper, with a melting point of about 230 °C, are also die-cast where the very highest accuracy is required and great strength is not of importance. Their excellent corrosion-resistance to moisture makes them suitable for such components as number wheels in gas and water meters, and they have also found use in small complex components of electrical instruments. 

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