Here we show how accurate such measurements can now be
made in modern well controlled cultivation processes. We will
show that these measurements can be performed during normal
production runs without changing the trajectories of the
key variables biomass and product concentration. By means of
a temperature-controlled fermentation processes it is now possible to make quite accurate measurements
of the power truly transferred to the culture during a fermentation
process. In this way the kLa results can be compared with the
available correlations.
It should be mentioned that the currently used measurement
techniques assume that the simple basic transport model depicted
in equation (1) can be applied. This requires representative values
of O and O* which characterize the driving force of the oxygen
mass transfer and kL and a which are the transport coefficient
and the specific transport cross section. At least at larger scales,
all these variables may considerably vary with the local position. This
is sometimes compensated by simple assumptions, e.g. by taking
logarithmic means.
The errors resulting from these assumptions are considered
small but are finally appearing in the kLa values obtained. In other
words inaccuracies in the driving force appear as errors in the measured
kLa. Its values thus neither distinguishes between kL and a,
nor between the transport coefficient kLa and changes in the driving
force (O*–O).