Process Variables and Sensors in Bioprocess Operations

A process variable is a parameter of the current status of a process under operation, and, therefore, the measurement of process variables is a key to understanding what is happening in a bioreactor and to being able to control the process. Most of the sensors used for the measurements of process variables are inserted into the culture broth of a bioreactor. These must have qualities that will allow them to be repeatedly sterihzed (autoclavable) in order to prevent microbial contamination in bioreactors. Naturally, all the sensors must be calibrated before sterihzation. 

Process variables measured in bioprocess instrumentation can be categorized into one ofthree groups physical, chemical, and biochemical variables. Table 13.1 summarizes these three types of variables. 

Furthermore, using the primary measurements to obtain the secondary process variables (so-called gateway sensor ) is a form of bioprocess control. For example, the measurement of optical density (primary measurement) can be used for the estimation of cell concentration, and, subsequently, the time course of cell concentration can be employed for the estimation of specific growth rate (secondary 

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