FIG. 4.

System responses. (A) Response time of the modified PID controller. The system was equilibrated with gas 1 prior to initiation of computer control; regulation was started with gas 2 fully on. (B) Oscillations in oxygen tension when the set point is just above the actual oxygen concentration for gas 1. Initially, there is little oxygen consumption because the density of the cell culture is low. Later, as the culture begins to consume more oxygen, the size of the oscillation in oxygen concentration is reduced.