Water purity is often very important in biochemical processes. One of the best methods to detect the presence of salts or other electrolytic materials is to measure its resistivity. Conductivity or resistivity probes are capable of measuring conductivities as high as 20,000 microsiemens per centimeter and resistivities as high as 20 megohms per centimeter.
686 Fermentation and Biochemical Engineering Handbook 10.0 TEMPERATURE
Precise temperature control and profiling are key factors in promoting biomass growth and controlling yield. Temperature is one of the more traditional measurements in bioreactors so there is quite a variety of techniques.
Filled thermal systems, Fig. 6, are among the more traditional temperature measuring devices. Their operating principle is to take advantage of the coefficient of thermal expansion of a sealed fluid to transduce temperature into pressure or movement. This has the advantage of requiring essentially no power and therefore is very popular in mechanical or pneumatic control loops. Although the trend in control is toward digital electronic, pneumatic and mechanical systems are still very popular in areas where solvent or other combustible gases may be present and therefore represent a potential safety hazard. The primary constraint in these types of systems is that the receiver (indicator, recorder, controller) must be in close proximity to the sensor.
Figure 6. Filled thermal system assembly for temperature measurement. (Courtesy of the Foxboro Co., Foxboro, Mass.)
Thermocouple assemblies, Fig. 7, are a popular measurement choice in electronic systems or in pneumatics where the sensor must be remote. The thermoelectric principle, referred to as the Seebeck Effect, is that two dissimilar metals, when formed into a closed circuit, generate an electromotive force when the junction points of the metals are at different temperatures. This conversion of thermal energy to electric energy generates an electric current. Therefore, if the temperature of one juncture point (the cold junction) is known, the temperature of the hot juncture point is determined by the current flow through the circuit. Depending upon the alloys chosen, thermocouples can measure a wide temperature range (-200 to +350°C for copper, constantan) and are quite fast acting assuming the assembly doesn't contribute too much lag in its absorbance and dissipation of heat. Its primary disadvantages are its lack of sensitivity (copper, constantan generates only 40.5 microvolts per °C) and requirement for a precise cold junction temperature reading.
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