A stirred fermenter is used to culture microbial cells isolated from umbilical cord of blood. The liquid

volume is $15L.$ The simple dynamic method is used to determine $k_{La}.$ The air flow is shut off for a few

minutes and the dissolved oxygen level drops; the air supply is then reconnected at a flow rate of $0\mathrm{.}25$

L$/$s$.$ The following results are obtained at a stirrer speed of $50rpm.$

When steady state is established, the dissolved oxygen tension is $78\%$ air saturation. In separate test

experiments, the electrode response to a step change in oxygen tension did not vary with stirrer

speed at $40rpm.$ The probe response time under these conditions was $2\mathrm{.}8s.$ When $k_{f,a}$ measurement

was repeated using nitrogen sparging to deoxygenate the culture, the results for oxygen tension as a

function of time were similar to those listed. Estimate $k_{fa}.$