This gas-sensor consists of a sensing element and a heater. The small sensing element is SnO2 sintered metal. When heated to about 400 C with no oxygen present, free electrons flow easily through the grain boundary of the tin dioxide SnO2 particles. Oxygen adsorbed on the particle surface forms a potential barrier in the grain boundaries that traps free electrons. This restricts their flow, causing the electrical resistance to increase. When the sensor element adsorbs reducing gases, their oxidation lowers the potential barrier, allowing electrons to flow more easily, thereby reducing the electric resistance.
The sensor mounted in the fermenter head space can be seen in the next figure. The
unit in a rubber stopper fits into the fermenter lid. Air from the
head space flows through the flame arrester to the sensing element. To get
renewal around the sensing element, gas is allowed to escape through the
back of the sensor. A stainless steel grid shaped as a cone prevents
splashing from reaching the sensor.
For aseptic operation, the electrode should be mounted in the exit gas line. The additional lag time would be small. Calibration of the whole system links the ethanol concentration in the broth with the sensor output voltage. Air stripping of ethanol from the culture fluid was measured and could be reconciled in mass balances.
An acetic acid bioprocess was the model for evaluating the ethanol auxostat. Ethanol was the growth-limiting substrate. Soil was the inoculum. The auxostat principle was demonstrated by runs of several hundred hours. All bioreactors were open, and only the feed was sterile. Use "BACK" to go with the flow.