Wireless
Humidity Detection Sensor

We have developed a Wireless Humidity Detection System (WHDS), which uses inexpensive sensors for wireless detection of humidity. The sensor is passive, so there is no need for batteries or other power sources. The WHDS can be applied to measure humidity in places that are hard to reach, for instance behind walls, in closed packages and for monitoring moisture in food, pulp and sand.

The key component in the sensor is a magnetoelastic metallic thin film. The magnetoelastic film can be made sensitive to many physical properties by coating it with various materials, for instance a humidity sensor can be constructed by coating the film with a humidity absorbing material. When the humidity sensor is placed in a humid climate, the coating will absorb water and become heavier. Similarly, when the sensor placed in a dry climate, the coating will release water and become lighter. A heavy sensor will vibrate with a lower frequency than a light sensor.

We have developed a hand held control unit that can read out a sensor at a distance of up to 40 cm. In a typical application, the humidity sensor is placed where one wants to measure moisture. The status of the sensor is monitored with a control unit that can be either stationary or hand held. The oscillation frequency is detected and hence the humidity is measured.
 

Remote measurement of humidity through a concrete plate using our hand held WHDS unit.

Wireless sensor networks will have a major impact on how and where we use sensors in the future. The relatively new ZigBee-standard based on IEEE 802.15.4 is a network standard developed with the properties and demands of sensor networks in mind. Battery powered sensor units can be embedded in the measurement objects and deliver data for years without any wires going to or from the units. A sensor unit becomes a part of the sensor network and the units can easily move within, leave or re-enter the network.
 
At Imego we are developing wireless sensor network systems customized for the customer’s application. We integrate ZigBee-ready modules with the sensors (often including a microcontroller and additional electronics), which together with a network management and data collection software makes a complete ready-to-use system.
 

We have developed and tested a ZigBee based wireless sensor network system aimed at measuring moisture and humidity on many locations in a building. The system monitors the moisture levels in wood and concrete and the humidity in the air during the construction stage. The sensor nodes contain humidity and temperature sensors, sensor electronics, a microcontroller and ZigBee modules. The sensor data is collected in a coordinating node, forwarded to a database server and presented through a homepage. The work was done together with the Division of Building Materials at Lund Institute of Technology and was partly funded by the Development Fund of the Swedish Construction Industry (Svenska Byggbranschens Utvecklingsfond, SBUF) and Boverket. The results have been summarized in a written report (in Swedish) and it can be accessed in the download area.

Imego has previously conducted a survey together with the Division of Building Materials at Lund Institute of Technology on the construction sector’s need for moisture measurement in buildings. That work also included a survey on moisture measurement techniques and sensors available on the market today and of network solutions which can be used in a system for wireless moisture measurement. The results have been summarized in a written report (in Swedish) and it can be accessed in the download area. The work was partly funded by the Development Fund of the Swedish Construction Industry (Svenska Byggbranschens Utvecklingsfond, SBUF).