A frequent task in mixing processes is the measurement of the product temperature, for which complex sensor technology is required. The reasons for this can be manifold. For example, the product temperature must remain as constant as possible or it must not fall below or exceed a minimum or maximum value. Often, a certain temperature profile must be passed through in the course of the stirring process. In any case, it is crucial to measure the product temperature as accurately as possible in order to obtain a representative actual value for the temperature control.
The positioning of the temperature sensor plays an important role in measuring the product temperature in the stirred tank. As a rule, the temperature sensors are mounted in a stationary position; they are then brought into contact with the product to be stirred, e.g. through the vessel bottom, the vessel wall or through the vessel lid. This is not unproblematic, especially with medium to high viscosity products, as no representative measured values can be determined due to the different temperatures over the height of the stirred vessel.
The flow field of the stirrer also has a not inconsiderable influence on the local temperature. Likewise, a temperature difference between the vessel wall and the centre of the vessel must be registered. An improvement of the sensor system is achieved by using several temperature sensors.
Optimum temperature measurement is achieved when the measuring sensor moves through the material to be mixed during the mixing process. This measuring method is used, for example, in planetary agitators. The measuring sensor and a transmitter are integrated in the stirring tool or wall scraper. This ensures optimum heat exchange and the temperature sensor measures a representative product temperature.
The transmitter sits above the product surface and the measured values are transmitted contact-free via radio to a receiver. The transmitter operates with a battery that lasts for several years. In addition to the temperature measurement, the speed, the vacuum or the pressure as well as the fill level in the container often have to be recorded.
Weitere Sensoren für z. B. PH-Wert und Leitfähigkeit sind möglich. Als ein Maß für die Viskosität wird manchmal auch die Stromaufnahme bzw. das Drehmoment des Motors ermittelt.
Wägezellen können zur Kontrolle der Mischerbefüllung und zur Überprüfung der Behälterentleerung genutzt werden. Bei mechanischen Maschinenbewegungen können Sensoren und Abfragen u. a. auch das Anfahren von bestimmten Positionen ermöglichen.