Relative humidity along with time and temperature affect wood drying. Normally these parameters are based on previous drying experience, with the goal being to optimize drying time while minimizing wood cracking. The process parameters are unique to the system used, with different woods and seasonal variations affecting the result. This makes wood drying a demanding process. To achieve the best results, relative humidity needs to be continuously monitored.
Wood is a porous material, i.e. it is hygroscopic, absorbing moisture to reach an equilibrium with the ambient environment. The mechanical properties of wood vary with the temperature, pressure and moisture content. In most applications the moisture in the wood cells must be removed to allow further processing and to protect wood from blue stain, fungi and mold, as well as from cracks and checks. The initial moisture content of green wood, i.e. freshly sawn timber, varies from 50 to 100%, depending on the species. When used as a construction material in, for instance, a normal temperate zone climate, the timber is dried to a moisture content of 18%, while for joinery and carpentry a moisture content of 10% to 15% is ideal.
The accuracy of relative humidity measurements really matters. It directly affects the quality of the drying process and indirectly affects facility capacity. Inaccurate measurements can cause the following problems:
Three main factors help to keep measurements accurate: use of the appropriate sensor, ensuring that the measurement device is installed correctly and regular calibration of the measurement device.
Vaisala HUMICAP® Humidity and Temperature Probe HMP7 is designed for applications which involve constant high humidity or rapid changes in humidity
Vaisala HUMICAP® HM70 Handheld is designed for demanding humidity measurement in spot-checking applications. It is also ideal for field checking and calibration of Vaisala's fixed humidity instruments.
Vaisala Indigo200 series transmitters are host devices for displaying measurement values from Vaisala's Smart Humidity, Temperature, Dew Point, Moisture in Oil, CO2 and H2O2 Probes.
The Vaisala Indigo500 series transmitters are host devices for Vaisala Indigo-compatible, stand-alone smart probes. The Indigo500 series include multi-functional Indigo520 transmitter and Indigo510 transmitter with basic features.
The Vaisala DRYCAP® Dewpoint Transmitters DMT345 and DMT346 are designed for humidity measurement in industrial drying applications with particularly high temperatures.