Temperature measurement

In recent years, the use of sensors in reference vessels has become more common. Here, thermosensors coated with fluoropolymers are immersed directly in the sample solution. The reaction control is limited to one single vessel, mostly for cost reasons. In the case of highly inhomogeneous samples or samples with different reactivities, sufficient temperature monitoring is not possible. Additionally, the time of the reaction, which often involves multiple protective sleeves around the sensors, is limited by the transition of heat through the coating. This results in delayed detection, making it difficult to monitor spontaneous exothermic reactions. For this reason, additional IR sensors are often used to measure the outside temperature of the digestion vessel. However, this does not provide any information about the actual sample temperature. With the modern in situ IR temperature method, the sample temperature is measured directly and is characterized by the following advantages:

• Fast response time ->Physically, the speed of the measurement is only limited by the speed at which the infrared radiation from the sample hits the detector. Since there is no material between the sample and the sensor, the temperature is measured without delay.
• No contamination ->The sensor is located outside the digestion vessel and the microwave field.
• Easy handling ->The sensor does not have to be mounted.

How the DIRC non-contact temperature measurement works

Physically, any body above absolute zero emits radiation dependent on its temperature and emission coefficient. This radiation has its maximum at temperatures <500°C, in the frequency range of the infrared. The exact relationship can be understood by the Stefan Boltzmann law. However, many bodies do not only emit the infrared radiation but also absorb it. To determine the temperature of a body, it is therefore usually necessary to have a clear view of the surface. There must not be any strongly infrared absorbing medium between sensor and object.

Measurement of the internal temperature of a vessel

In the past, the first attempts to measure the internal temperature failed due to the fact that conventional broadband IR measurement techniques were used. In this spectral range, the vessels themselves absorb the radiation and do not allow the measurement of the sample temperature inside the vessel. The measurement of IR radiation in a frequency range in which e IR radiation is not absorbed by the  vessel material has turned out to be a feasible solution. Thus, the actual temperature of the sample in the digestion vessel can be determined in real time without any detour. Both TFMTM-PTFE and fused silica are transparent to IR radiation in a certain spectral range and allow monitoring of the internal vessel temperature. Improved accuracy of the method is achieved by filtering out IR radiation emitted from the surface of the vessel. Thus, in a simple way, the temperature of all samples in the microwave oven can be determined in the measurement range of 50-300°C with high accuracy (+1°C at 200°C). Only by the temperature information of all containers an optimal monitoring and thus also regulation of the microwave power can be ensured