LaMotte DO 6 Dissolved Oxygen Meter - Manual (Page 51)

electrochemically reduced to hydroxyl ions producing an electrical current
within the probe:
O
2
+ 2 H
2
O + 4 e - à 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.
14.1.4 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 partial pressure of oxygen in the water will be the same as it is in air.
Therefore, all you need to know is the temperature of the air in which the
probe is placed. By consulting solubility tables for oxygen at the particular
barometric pressure and salinity of the water being measured, the
corresponding concentration (mg/L or ppm) can be found for air saturated
water at the air calibration temperature, and the meter can be set accordingly.
Because most meters are temperature compensated, they will still give correct
readings in mg/L even though the actual water temperature may be different to
the air calibration temperature.
Note: The closer the air calibration temperature to the water temperature, the
more accurate is the calibration.
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