In conventional fermenter, measuring cell concentration at one time is sufficient, but modern bioreactors will require real-time measurement and control of cell concentration. This monitoring will be achieved by removing cells trough a cell harvest line (if concentration is too high) and by adding cells through a cell return device (if concentration is too low). Methods for monitoring cell concentration is directly measured or it can be indirect, where cell concentration is measured indirectly through the measurement of oxygen uptake rate or carbon dioxide evolution rate as an index of metabolic rate.