Page 40
18. Appendices: Dissolved Oxygen Sensor Theory
The sensor used, is the amperometric type of Clark Sensor and is
suitable for the measurement of oxygen pressures in the range 0 to 100
cm of mercury. While the sensor actually reads partial pressure of
oxygen, the circuit is calibrated to be read in percentage saturation or
parts per million (Milligrams/litre). The operation of sensors of the Clark
type relies on the diffusion of oxygen through a suitable membrane into a
constant environment of 0.1 molar potassium chloride. Measurements
are best performed with a reasonable flow past the membrane. At
sufficiently high flow rates, the oxygen current is totally independent of
the flow (few cm / sec). The cell must not be shaken however or unstable
readings will result from electrolyte surge bringing new oxygen from the
reservoir to the working cathode surface.
18.1 Operating Principle
The Clark oxygen sensor consists of a gold cathode and a silver/silver
chloride anode, placed in an electrolyte solution. This solution is
contained behind a plastic membrane. In this case the plastic is 0.025mm
intermediate density polyethylene sheet. PTFE (Teflon) can be supplied
for special applications. It must be realised that using membranes of very
different thicknesses will result in an error in the temperature
compensation that is applied in the instrument for the membrane
permeability. This coefficient (here +4.2%/
o
C at 25
o
C) is for this thickness
polyethylene. A polarizing voltage of about 800 millivolts is applied
between the two sensors. The gold sensor is placed close to the
membrane and because of the polarizing voltage, oxygen diffusing
through the membrane will be reduced at the gold sensor.
Equation:
O
2
+ 2H
+
+ 2 electrons
H
2
O
2
This reduction process will produce a current through the oxygen sensor.
A load resistor (actually a thermistor in this case) situated in the sensor
itself, converts this current into a voltage proportional to the oxygen
partial pressure. The thermistor provided within the body of the sensor
can have a temperature coefficient of -4.2%/
o
C. This gives an accurate
temperature compensation for the temperature/permeability effect of the
membrane to oxygen, over a range of
20
o
C about a centre value of
25
o
C. Note this compensation is not for the solubility effects. A separate
sensor also included achieves this.