Wireless in situ measurements of moisture content and temperature in timber constructions

The aim of this article is to report on experience from a continuous wireless in situ measuring system. Today more than 600 sensors have been installed in wooden structures (beams, posts) and buildings at several sites for various research projects in Sweden. In order to measure a large amount of data, a wireless monitoring system was chosen with gateway connected to a GSM modem that sends the information to an Internet-connected computer database from whence it is transferred to the users. A wireless system is a comparatively cheap system and is easy to install, since there is no need of wires. When the plans for continuous measurements started, this was a relatively untested system, and experience from large scale measurements was limited. The purpose of the measurements is to verify models for prediction of long-term durability of wooden structures and wooden buildings based on periods of surface wetting, on moisture conditions related to climatic loads, coatings, wood processing and design and from the position inside walls or floors. With wireless HygroTrac sensors, relative humidity can be measured in the range of 10% to 90% ±2.5%, temperature can be measured with an accuracy of ±0.5°C at 25°C and moisture content in the range of 8% to fiber saturation point (FSP) ±1%. After nearly two years of measuring with the wireless system, the following experience was gained: the sensors was easy to set up and build in, some problems with the operation of the devices and the need for restarts after power failures; the effective range from the gateway to the sensors needs to be depends on the individual building construction.