Table 2: Methods of measuring oxygen transfer rate.

Measurement methodBasis of the methodProsConsRef.

Sulfite oxidation methodMonitoring pH changes during the oxidation of sodium sulfite to sodium sulfate controlled by oxygen depletion rate(i) Simple and low cost
(ii) Can be used for the determination of the interfacial area between gas and liquid
(iii) Being accurate for minivolumes of less than 1 mL
(i) The kinetics of the homogeneous catalytic chemical reaction should be known
(ii) Limited accuracy by visually determination of color change
(iii) High salt concentration (usually 0.5 mol · L−1) reduces the maximum solubility of oxygen
(iv) Not appropriate in large scale bioreactors
(v) High surface tension causes the underestimation of potentially achievable OTR
[7275]

Dynamic methodMonitoring the dissolved O2 concentration during the aeration of the system(i) Consistent measurement
(ii) Does not depend on a zero or reference measurement
(i) Requiring a rapidly responsive, sterilizable, dissolved oxygen probe
(ii) Limited application for minititerplates (MTPs)
(iii) Not costly favorable
[76, 77]

Optical methodMonitoring the color changes during the sulfite oxidation reaction using a pH sensitive dye (e.g., bromothymol blue)No need for pH electrode which frequently disturbs the hydrodynamicsNot accurate due to being time dependent of the color shift which indicated the time of the oxidation reaction[7880]

Gassing-out methodMonitoring by direct measurement of the rate of increasing dissolved oxygen concentration, after neutralizing the system by flushing nitrogen through the vessel to achieve an oxygen-free solution(i) Can be applied to different media (for investigating the effect of media composition on oxygen mass transfer)
(ii) Does not involve chemical reactions that could impact the measurement precision and the liquid film resistance
A nonrespiring system which is not in exact correspondence to real culturing conditions[28, 81, 82]

RAMOS (intermittent online) methodMonitoring OTR by periodically repeating an automated measuring cycle composed of a measuring phase and a rinsing phase (i) Online monitoring system
(ii) Can be used simultaneously for measuring OTR in 6–12 parallel vessels
(i) Large amount of sample required for measuring
(ii) Not applicable for small volumes (microliters)
[8385]

Exhaust gas analyzer (continuous online) methodCalculating the OTR by specifying the oxygen concentration difference between the inlet gas stream (O2, in) and the outlet gas stream (O2, out) using magnetomechanical exhaust gas analyzer (EGA)(i) Continuous method
(ii) Can be used to measure OTR in one to five parallel culture vessels
Only applicable in high volume bioreactors[86, 87]

Respirometer (offline) methodMeasuring of decreasing dissolved oxygen concentration with time after aerating the culture vesselCan be used for bioreactors of any shapeDifficult manual handling[88, 89]