Thermocouple probes, microwave temperature

A particularly difficult problem in microwave processing is the correct measurement of the reaction temperature during the irradiation phase. Classical temperature sensors (thermometers, thermocouples) will fail since they will couple with the electromagnetic field. Temperature measurement can be achieved either by means of an immersed temperature probe (fiber-optic or gas-balloon thermometer) or on the outer surface of the reaction vessels by means of a remote IR sensor. Due to the volumetric character of microwave heating, the surface temperature of the reaction vessel will not always reflect the actual temperature inside the vessel [7]. 

Fiber-optic thermometers can be applied up to 300°C, but are too fragile for real industrial applications. In turn, optical pyrometers and thermocouples can be used, but pyrometers measure only surface temperatures which in fact can be lower than the interior temperatures in reaction mixtures. Application of thermocouples which in case of microwaves are metallic probes, screened against microwaves, can result in arcing between the thermocouple shield and the cavity walls leading to failures in thermocouple performance. 

AU the modern scientific microwave units have the capabiUty to measure temperature during the course of a reaction. This can be done remotely using an infrared sensor located in the waU or the bottom of the microwave cavity. In many cases it is also possible to record the temperature inside a reaction vessel using a fiber-optic probe or thermocouple. Pressure measurement is also possible in many cases. The contents of a reaction mixture can be stirred by means of a magnetic stir plate located beneath the microwave cavity and a Teflon stir bar in the vessel. 

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