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January 2020 / 67 sensing probe to project the infrared beam into the dryer without being exposed to high temperatures. Radio frequency. In a radio- frequency (RF) device, a probe projects an RF energy field into the material, and a sensor measures the loss or change in the materi- al's dielectric constant as affected by the material's moisture content. This method is most effective when the material has a constant density and is deep enough to absorb the RF energy field emitted from the probe. The probe can operate in environments up to 480°F. RF mois- ture sensing has been used on belt conveyor dryers, fluid-bed dryers, and conduction dryers. Microwave. The microwave (μW) method is used mainly to determine moisture content in a large mass of solids, such as a bale of cotton or paper. In a bulk mass dryer, such as a hopper (or column) dryer, lens and the material surface can impact the measurement, this effect is typically minimal because the energy field is concentrated at the beam's set focal distance. However, dust adhering to the sensor lens can affect the measurement. One way to protect the NIR sensor from sticking dust deposits in a fluid-bed dryer is to install the sensing probe outside the dryer and a Pyrex glass window in the dryer wall, so that the infra- red beam is projected into the dryer through the window. This window will need to be constantly swept clean by moving material. The exter- nal sensing probe and Pyrex glass window can also be used in another application. The NIR sensing probe can operate in environments only up to about 130°F, so for a dryer with a higher-temperature environment, installing the sensing probe outside the dryer with a Pyrex glass win- dow in the dryer wall allows the ects a stabilized infrared light beam at a specific focal distance onto the material and then uses a sensor to filter the light reflected from the material at two wavelengths — one specific to moisture and one as a reference. The instrument, which is calibrated for the material's solids and moisture components, electron- ically analyzes these two reflected signals to determine the material's moisture content. The NIR technique can effec- tively measure moisture in parti- cles up to 1 millimeter in diameter. The material must cover the entire projected infrared beam to yield accurate moisture measurements, so the technique works best when the material moves across the infrared beam while on a flat surface, such as on the belt in a belt conveyor dryer or as a dense mass in a fluid-bed dryer. Although heavy dusting in the air space between the sensor