Nova-Tech DO 700 - Manual (Page 26)

20
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.
The “mg/L” unit is equivalent to ppm (a liter of water weighs 1 million
milligrams—one part in a million is similar to one milligram in a liter).
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.
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.