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19. Appendices
19.1 Dissolved Oxygen
19.1.1 Dissolved Oxygen Sensor Fundamentals
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 the Clark type sensor relies on the diffusion of oxygen through
a suitable membrane into a constant environment of potassium chloride. Solution 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 (a few cm/sec is sufficient). 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.
19.1.2 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
0.001 inch PTFE (Teflon) sheet. 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 is +4.2%/
o
C at 25
o
C for this thickness membrane.
A polarising voltage of about 800 millivolts is applied between the two electrodes. The gold
cathode is placed close to the membrane and because of the polarising voltage, oxygen diffusing
through the membrane will be reduced at the gold cathode.
Equation :
O
2
+ 2H
+
+ 2 electrons
→
H
2
O
2
This reduction process will produce a current through the oxygen electrode. 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
has a temperature coefficient of -4.2%/
O
C. This gives an accurate temperature compensation for the
temperature/permeability effect of the membrane, over a range of about 5 to 45
O
C about a centre
value of 25
O
C. Note this compensation is not for the solubility effects. A separate sensor also built
into the tip of the sensor achieves this.
19.1.3 Sensor Storage
The Oxygen sensor should be kept moist when not in use to prevent the thin film of electrolyte
behind the membrane from drying out. To achieve this, the sensor can be stored with the tip in
water or in a humid environment.
For long term storage of several weeks or more, remove the membrane and empty out the
electrolyte. Replace the membrane without electrolyte to avoid contamination of the gold and silver
surfaces. When the sensor is stored in this way, the membrane should be replaced and the sensor
refilled before use.
