Capacitance probes (Fig. 14) are one means to accomplish this. The basic principle is to measure the charge between two conductive surfaces maintained at different voltage potentials and separated by a dielectric material. The construction of the probe provides an electrode in the center surrounded by an insulator, air, and a conductive shell. The length of the probe is from the top of the reactor to the lowest level measuring point. As the level in the reactor rises the broth displaces the air between the capacitance plates and thereby changes the dielectric constant between the plates to the level of the broth. The result is a change in the charge on the plate. If the vessel wall can act as a plate (is sufficiently conductive), the preferred approach would be to use an unshielded probe (inner electrode with insulator) to prevent erroneous readings resulting from fouling of the probe. Because of the uncertain dielectric character of the broth, this measurement should only be used as a gross approximation of level for instituting antifoaming strategies.
Several other forms of level measurement technologies are available. One is the float and cable system, where the buoyancy of the float determines the air-broth interface boundary and the length of the cable determines the level. The density of the broth may render this measurement questionable.
A second is hydrostatic tank gauging, where level is inferred from pressure. Again, density, particularly if two phases exist (aqueous and foam), may render this approach questionable.
A third is sonic, which computes the distance from the device to the broth surface based on the time it takes for the sound wave initiating from the device to reflect off the surface of the air-liquid boundary and return.
Several other ingenious variations of these basic approaches are commercially available as well.
696 Fermentation and Biochemical Engineering Handbook 16.0 REGULATORY CONTROL
Automatic regulatory control systems (Fig. 15) have been in use in the process industries for over fifty years.
Utilizing simple feedback principles, measurements were driven toward their setpoints by manipulating a controlled variable such as flow rate through actuators like throttling control valves. Through successive refinements in first mechanical, then pneumatic, then electronic and finally digital electronic systems, control theory and practice has progressed to a highly sophisticated state.
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