Hygienic Design of Temperature Measurement Devices

To measure temperatures in a hygienic application, bimetal, resistance, or thermocouple technology can be used. Bimetal thermometers can only supply local readings, while resistance thermometers and thermocouples are primarily used to obtain an electrical output for remote readings. However, devices exist that combine resistance and bimetal elements and provide both local and remote capabilities in one package. This allows the user to tap into the process only once, reducing the potential for contamination. Temperature measurement based on electronic detection of a change in resistance is the most common method. The actual temperature sensor elements used integrate either platinum thin-film resistors (Pt100, etc.), or employ other sensing elements with a varying electrical resistance against temperature (NTC or PTC resistors). Also, semiconductor devices are common (Cole-Parmer, 2009; Moerman and Kastelein, 2014)

The temperature-sensing element itself is inserted in a thermowell (Fig. 7.92), which is a closed-end reentrant tube provided with means for a pressure-tight attachment to a particular process equipment component. The dimensions of the tapered and straight thermowells must be chosen as a function of the vibration or stress caused by the process medium flowing through the pipeline. The protective sleeve (thermowell) is typically made of stainless steel and highly polished to a surface finish of Ra # 0.8 pm. However, special metallic overlays or polymer coatings can be applied to the surface of the thermowell for use in processes involving high-velocity particulates and acidic solutions that may cause, respectively, erosion and corrosion. A paste with high thermal conductivity is used inside the thermowell to bring the temperature sensor in close thermal and mechanical contact with the liquid (in a pipe, recipient, etc.) from which the temperature must be measured.

Design of an electrical thermometer with thermowell. Courtesy of WIKA Alexander Wiegand SE & Co. KG

FIGURE 7.92 Design of an electrical thermometer with thermowell. Courtesy of WIKA Alexander Wiegand SE & Co. KG.

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