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A microwave digestion system is basically used to heat samples in special digestion vessels. That is, it is merely a type of heater whose heating power is controlled by the microwave power radiated into the furnace chamber. This power is controlled in normal operation according to the sample temperature measured in all vessels by means of DIRC temperature sensor as well as the predefined heating program.
For safety reasons, the vessel pressures and vessel external temperatures are also measured continuously. As long as these pressures remain below preset maximum values, everything is fine and the values are recorded for documentation. The microwave continues to be controlled solely on the basis of the DIRC temperature. However, if a vessel approaches the maximum allowable pressure or surface temperature, the system will downregulate the microwave power for safety reasons, effectively avoiding critical operating conditions.
Microwave control with exothermically reacting samples
Thermometers must function at very high speeds. This becomes very clear, especially when observing spontaneously reacting samples. Typically, fast exothermic reactions occur in the heating phase, which can be illustrated by the example of the digestion of polymer granules. The attack of the acid and the incipient melting of the particles lead to a rapid rise in temperature. By continuously determining the temperature data, the microwave power is regulated accordingly so that a safe reaction can be guaranteed.
In addition, the rise in temperature is also accompanied by a rise in pressure (blue curve). The total pressure in the vessel results from the sum of the vapor pressure of the acid mixture at the respective temperature and the partial pressure of gaseous substances formed. In the example mentioned, the pressure is formed by the decomposition of the polymer granulate and the formation of CO2 as a decomposition product. The control of the pressure is also an important parameter, which is necessary for the regulation of the microwave power and a safe reaction control during the digestion.