User Guide

Instruction Manual DO 100
49
12.1.2 Measurement Units
One measure of DO in water is parts per million (ppm) which is the number of oxygen
molecules (O
2
) per million total molecules in a sample. Calculating the % Saturation is
another way to analyze 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.
12.1.3 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
-
Æ 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.