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What Is Being Measured?
DO probes respond to the partial pressure of oxygen in liquid or gas
being measured. The probes measure the “pressure” of the oxygen rather
than the 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 + 4e
-
4 OH
–
Since all of the 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 make an adjustment for the temperature and
salinity of the water to obtain corrected readings in these units.
When DO is reported in terms of partial pressure or % Saturation, then
a temperature and/or salinity compensation for oxygen solubility is not
necessary. Most probes are temperature compensated—i.e. they convert
the “partial pressure measurement” to mg/L of DO at the given water
temperature, salinity, and barometric pressure.
Air Calibration
Understanding the principle of air calibration is easy, once it is
understood that the probe is responding to partial pressure. 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 the air. Therefore, all that must be known is
the temperature of the air in which the probe is placed. By consulting
solubility tables for oxygen at the specif c 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 give correct readings in mg/L
even though the actual water temperature may be different than the air
calibration temperature. Note: The closer the air calibration temperature
is to the water temperature, the more accurate the calibration.
