residence time (hydraulic retention time), t r in biotechnology

https://dx.doi.org/10.1351/goldbook.R05310
The average time a particle or volume element of the culture resides in a bioreactor (or other device) through which a liquid medium continously flows. Residence time of the liquid is the reciprocal of dilution rate.
Source:
PAC, 1992, 64, 143 (Glossary for chemists of terms used in biotechnology (IUPAC Recommendations 1992)) on page 164