Industrial Implementation of the New Nir Laser-Based Sensor for Measuring Surface Moisture in Polymers

In special applications, such as the transient adhesion process over small polymer components used in electric motors, electronic circuit coating or moisture transport in wood; moisture in the upper layers, close to the surface and at the surface, affects the processes more rapidly. Existing measuring principles, such as those based on nuclear magnetic resonance, are cumbersome to use and they are also less suitable for some parts of the moisture range. For this reason, a novel, non-contact, light-reflection type of moisture sensor was investigated, designed and developed (patent pending). It is based on a water molecule’s light-absorbing characteristics in the near-infrared (NIR) spectrum. The light from a double laser diode is led through an optical fiber and reflected back from a sample to an InGaAs photodiode detector. The high stability of the relatively low-cost laser source was achieved through a constant-power laser-control circuit. The sensor was designed for measuring the surface and near-surface moisture on a polymer samples with complex geometry and thin walls, which proved to achieve an approximately 0.1% of effective resolution of bulk moisture content below 2 %. The tests showed that the mechanical set-up of the samples plays a significant role. For this reason the special measuring cell was designed in order to be able to use the sensor in an industrial process line, where every year several hundred thousands of polymer samples are manufactured. The advancements from first introduction of the sensor will be presented, together with the experience obtained with its industrial implementation. The later work is in close relation with the activities of the European metrology research programme (EMRP) joint research project Metefnet.