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Control Characteristics of pH Controller

Objective: To determine the control characteristics of pH controller (M7822N) of Bio-Engineering AG fermenter.


Theory: The adequate pH control action in the bioreactor is highly dependent on the several factors e.g., density, viscosity & volume of the fermentation broth, strength of acid and bases, on/off time of the pumps, pH zone (band) of control action etc. It is possible to design the controller settings of above critical parameters so that the control action is quickly established. Faulty settings of the above parameters will lead to oscillations of the measured pH value around the set point, or offset between controlled value and set point or even delayed control response. The Bio-Engineering AG, Switzerland fermenter has  M7822N pH controller which allows to manipulate above parameters to arrive at optimal set point. This needs to be established before the start of any cultivation experiment so as to feature quick & stable control action with-out over any addition of acid/alkali.


Description of the pH Controller: The different components of the controller are described below:

1. pH/mv potentiometer – This potentiometer enables adjustment of the slope of pH electrode by means of a calibration buffer.

2. Asymmetry potentiometer It is used to adjust the zero mV point of the pH signal (pH 7.0).

3. Temperature potentiometer A manual control to correct for temperature coefficient of pH electrode.

4. Set point potentiometer It is used to set the mV or pH set point.

5. P-band potentiometer It serves to adjust the width of the P-band amplification. A low P-band translates to strong amplification.

6. TON/TOFF Potentiometer These two controls are primarily used to adjust the valve opening or closing time in relation to the mid-point of the P-band. Keeping both the potentiometer at the same value will ensure that opening and closing times of the corresponding values are equal in the centre of the P-band. Under this situation the transfer function of the system would be linear. These controls of the system adjust the controller to the shape of a titration curve so that even with a titration curve having steep end point the transfer behavior remains linear and control system does not begin to oscillate. The dead band of this P-controller will be characterized by a situation in which none of the two values is actuated and it represents 10 % of P-band.


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