Flow-through sensors measurement modes

Electronic ratio controller. In this type of controller, a proportion of both gas and air is diverted through a bypass in which a thermistor sensor measures the flow. The air and gas flows can be compared and the ratio calculated and displayed. A ratio control valve in the air or gas supply, depending on whether the mode of operation is gas- or air-led, will automatically restore a deviation from the pre-set ratio. The electronic controller maintains ratio over a 19 1 turndown. The principle of operation is based on mass flow, so that it can be used with preheated air in recuperative systems. 

Microadsorption detectors (Table 3.5) have not been used to any large extent for column monitoring. These detectors operate by measuring the heat of adsorption of a solute as it passes over a temperature sensor. The thermistor may be located in the analytical column or in a smaller chamber containing an adsorbent and through which the effluent flows. These detectors are operated in a differential mode and changes as small as 0.001°C can be easily detected. The main reasons for the lack of use of such detectors... 

A turbidostat mode of operation relies on the use of a sensor to measure the biomass concentration in the photobioreactor. Such a sensor can be based on the online measurement of culture turbidity, for example, but any other biomass sensor can be used as well. Based on such a continuous biomass measurement the flow rate through the system then is automatically adjusted such that the biomass concentration is maintained at a preselected value (Fig. 29). 

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