Abstract
In addition to bulk moisture, the measurement of moisture profiles is very important in pharmacy, the food, construction and car industries, the military, etc. In special cases, moisture in the upper layers, close to the surface and at the surface, affects various processes more intensively.Existing measuring principles, such as those based on nuclear magnetic resonance, are unsuitable for particular applications. For this reason, a novel, non-contact, light-reflection type of moisture sensor was designed and investigated (patent pending). It is based on a water molecule's light-absorbing characteristics in the near-infrared (NIR) spectrum. In order to be able to measure the surface and near-surface moisture on a complex geometry with thin walls, the light from a double laser diode is led through an optical fiber and reflected back 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's response was measured by a precision embedded data acquisition system NI CompactRIO and evaluated using a moisture analyzer, which measures the bulk moisture with a loss-on-drying technique.The results prove that it is possible to measure the moisture in polymer layers very close to the surface with an approximately 0.1% effective resolution of bulk moisture content below 2%.
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