Appendix A – DO Measurement
83
Appendix A – DO Measurement
Sensor Interface
The output signal from a Dissolved Oxygen probe is in the form of a constant DC
current which is proportional to the partial pressure of the liquid being measured.
In a 100% saturated solution at room temperature and pressure, the output from a
Galvanic probe will be of the order of hundreds of micro-amps (10
-6
Amps),
whereas the output from a Polargraphic probe will be of the order of hundreds of
nano-amps (10
-9
Amps).
In addition, Polargraphic probes require a bias voltage to be applied between the
cathode an anode of the DO cell to excite an output.
The equation for converting current input to % saturation is as follows :
% Saturation = (I/I
o
) x P
c
x M x 100
Where : I = Measured Input Current
I
o
= 100% Saturation Current
P
c
= Pressure Correction Term
M = Membrane Correction Term
The pressure correction term compensates for the effect that pressure has on the
solubility of oxygen in water. This is almost directly proportional, i.e. a 10%
variation in pressure will lead to a 10% variation in the solubility and therefore
saturation of the liquid.
The pressure correction term is defined as follows :
P
o
- P
vapor(To)
P
c
= -------------------
P - P
vapor(T)
Where : Po = Pressure at 100% Calibration
P
vapor(T
= Saturation Vapour Pressure at T
P = Pressure
T = Temperature
To = Temperature at 100% Calibration
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