Skip to content NREL Buildings Research National Renewable Energy Laboratory (NREL)
Field Test Best Practices: A Resource for Practical Residential Building Science

Main menu

Moisture Content Sensor

For long term monitoring of moisture content in wood frames to detect mold or decay, it is common to measure the resistance of wood and convert that to a moisture content reading [1][2]. Capacitance-type temperature and relative humidity (T&RH) sensors are commonly not used in these situations, because they often fail after extended exposure to high relative humidity environment.

Graph shows resistance as a function of moisture content of Conifer trees. The X axis is Resistance (in log scale) starting at 1, with grid markers at 10, 100, 1,000, 10,000, and 1,000,000. The Y axis is Moisture Content (as a percentage) starting at 7% with gridlines in one percent gain up to 25%. Tree types shown on the continuum are: Baldcypress; Douglas-Fir; Fir, California Red; Fir, white; Hemlock, eastern; Hemlock, western; Larch, western; Pine, jack; Pine, longleaf; Pine, red; Pine, white; Pine, ponderosa; Pine, shortleaf; Pine, sugar; Redwood; Spruce, black; Spruce, Sitka. On all shown, resistance increases as moisture content increases.

By reading the electrical resistance from a data acquisition (DAQ) system's moisture content can be derived from moisture content and resistance correlations for the specific wood species. Corrections for temperature and other species or species group can also be obtained. The detailed methodology is detailed by Straube et. al [1]. Wood normally has a very high resistance. A standard circuitry used for DAQ system reading or on a hand held multi-meter is normally as follows:

Diagram is a square shape with an open end on the left side and two arrows on the right-hand side. In the open portion on the left, the letters Vx appear. The line from Vx goes along the top of the shape and as it dips down to the right-hand border of the square, the letters Rw appear. Just below Rw, it says Rp, below which one of the two arrows shoots off to the right, showing a plus sign at the end. Below that, the letters Rs appear, and below those is another arrow with a – sign at the end. Between the arrows is a capital letter V.

During short term investigation a hand held multimeter that works on the same electrical resistance principle is often used. These multimeters have ±2% accuracy with moisture content in the range of 7-25%.

Photo shows a yellow hand held multimeter that currently reads at zero.

References:

  1. John Straube et.al. Methodology and Design of Field Experiments for Monitoring the Hygrothermal Performance of Wood Frame Enclosures, Journal of Thermal Env. & BLDG. SCI., Vol. 26, No. 2 – October 2002
  2. Garrahan, Peter, "Moisture meter correction factors." Proceedings of In-Grade Testing of Structural Lumber, USDA Forest Products Laboratory, Madison, WI.