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Dissolved Oxygen Theory
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Measurement Units
One measure of DO in water is parts per million (ppm) which is the number of
oxygen molecules (O2) per million total molecules in a sample. Calculating the %
Saturation is another way to analyse DO levels. % Saturation is the measured DO
level divided by the greatest amount of oxygen that the water could hold under
various temperature and atmospheric pressure conditions multiplied by 100.
What Is Being Measured?
DO probes respond to the partial pressure of oxygen in liquid or gas being measured
- they measure the “pressure” of oxygen rather than concentration. All of the
oxygen entering the probe is consumed at the cathode where it is electrochemically
reduced to hydroxyl ions producing an electrical current within the probe:
O
2
+ 2 H
2
O + 4 e-
➔
4 OH-
Since all oxygen entering the probe is chemically consumed, the partial pressure
of oxygen in the electrolyte is zero. Therefore, a partial pressure gradient exists
across the membrane and the rate at which oxygen enters the probe is a
function of the partial pressure of oxygen in the gas or in liquid being measured.
When a probe is placed in air saturated water, the current it produces will not
be affected by the temperature or salinity of the water. The DO concentration
in the water, however, will vary with temperature and salinity. Because it is
convenient to report DO concentration in mg/L or ppm, it is necessary to adjust
for temperature and salinity of the water to get correct readings in these units.
If DO were to be reported in terms of partial pressure or % Saturation, then
temperature and/or salinity compensation for oxygen solubility would not be
necessary. Most probes are temperature compensated - i.e. they convert the
“partial pressure measurement” to mg/L of DO at whatever temperature the
water happens to be at for a given salinity and barometric pressure.
Air Calibration
Understanding the principle of air calibration is easy, once you know that it is
partial pressure that the probe is responding to. When the probe is in air, it is
measuring the partial pressure of oxygen in air. If water is air saturated, then the
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