Hydraulic Retention Time in Seconds is referred as the ratio between the reactor volume and the feed flow rate, it represents the average time the cells and substrates stay inside the reactor. And is denoted by θs. Hydraulic Retention Time in Seconds is usually measured using the Second for Time. Note that the value of Hydraulic Retention Time in Seconds is always negative. Typically, the value of Hydraulic Retention Time in Seconds lies in the range from 5 to 20.