One of the simplest methods for measuring gas flow to a fermenter is by means of a variable area meter. The most commonly used example is a rotameter, which consists of a vertically mounted glass tube with an increasing bore and enclosing a free-moving float which may be a ball or a hollow thimble. The position of the float in the graduated glass tube is indicative of flow rate. Different sizes can cater for a wide range of flow rates. The accuracy depends on having the gas at a constant pressure, but errors of up to ± 10% of full-scale deflection are quoted (Howe et al., 1969). The errors are greatest at low flow rates. Ideally, rotameters should not be sterilized and are therefore normally placed between a gas inlet and a sterile filter. There is no provision for on-line data logging with the simple rotameters. Metal tubes can be used in situations where glass is not satisfactory. In these cases the float position is determined by magnetic or electrical techniques, but this provision has not been normally utilized for fermentation work. Rotameters can also be used to measure liquid flow rates, provided that abrasive particles or fibrous matter are not present.
The use of oxygen and carbon dioxide gas analysers for effluent gas analysis requires the provision of very accurate gas-flow measurement if the analysers are to be used effectively. For this reason thermal mass flowmeters have been utilized for the range 0 to 500 dm3 min '. These instruments have a ±1% full-scale accuracy and work on the principle of measuring a temperature difference across a heating device placed in the path of the gas flow (Fig. 8.1). Temperature probes such as thermistors are placed upstream and downstream of the heat source, which may be inside or outside the piping.
The mass flow rate of the gas, Q, can be calculated from the specific heat equation:
where H = heat transferred,
Fio. 8.1. Thermal mass flowmeter.
Cp = specific heat of the gas, Tl = temperature of gas before heat is transferred to it, T2 = temperature of gas after heat is transferred to it. This equation can then be rearranged for Q:
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