The flow of non-sterile liquids can be monitored by a number of techniques (Howe et al., 1969), but measurement of flow rates of sterile liquids presents a number of problems which have to be overcome. On a laboratory scale flow rates may be measured manually using a sterile burette connected to the feed pipe and timing the exit of a measured volume. The possible use of rotameters has already been mentioned in the previous section. A more expensive method is to use an electrical flow transducer (Howe et al., 1969) which can cope with particulate matter in suspension and measure a range of flow rates from very low to high (50 cm3 min~l to 500,000 dm3 min1) with an accuracy of ±1%. In this flowmeter (Fig. 8.2) there are two windings outside the tube, supplied with an alternating current to create a magnetic field. The voltage induced in the field is proportional to the relative velocity of the fluid and the magnetic field. The potential difference in the fluid can be measured by a pair of electrodes, and is directly proportional to the velocity of the fluid.
In batch and fed-batch culture fermenters, a cheaper alternative is to measure flow rates indirectly by load cells (see Weight section). The fermenter and all ancillary reservoirs are attached to load cells which monitor
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