Nova-Tech DO 6+ - Manual (Page 44)

Instruction Manual DO 6+
- 38 -
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.
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.