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17.1.4 Maintenance Of The Membrane
The membrane does not require replacement as long as it remains intact. If punctured or suspected
of leaking around the edges, it must be replaced. To replace the membrane, please see the separate
instruction leaflet supplied with your sensor.
17.1.5 Notes On Units Of Dissolved Oxygen
The terms "Oxygen Concentration" and "Oxygen Partial Pressure" frequently give rise to some
confusion.
•
Oxygen Concentration is the absolute quantity of oxygen present per unit mass of the liquid.
•
Oxygen Partial Pressure is the oxygen fraction of the total pressure of all of the gases present.
For any one liquid system, Oxygen Concentration and Oxygen Partial Pressure are proportional.
However, if the solubility of oxygen in the liquid should change owing to increased quantities of
solutes, etc., then the ratio of the Concentration to the Partial Pressure must change. Thus, if one
saturates distilled water and a 25% solution of Sodium Chloride with air at atmospheric pressure
(25
o
C) both solutions will have almost exactly the same Oxygen Partial Pressure, namely 15.5 cm
of mercury. However, the dissolved Oxygen Concentration parts per million (milligrams per litre)
will be 8.2 in the distilled water and 2.01 in the salt solution. This is a rather extreme example, as
ocean water is only 3.6% saline. It does however stress the importance of correct interpretation of
the salinity.
The Clark Electrode measures the partial pressure of oxygen diffusing through a membrane. The
current is a linear measure of this partial pressure, assuming sufficient liquid flow conditions.
With air at sea level, the 20.9% oxygen exerts about 15.5 cm of Mercury pressure. Water in
equilibrium with air and with no oxygen demand (C.O.D., B.O.D. etc.), is saturated and has this
dissolved oxygen partial pressure. If we define 100% Saturation in Partial Pressure terms, then 15.5
cm. Hg = 100% Saturation. This is a practical unit to use. The probe linear readout is then a linear
function of % Saturation. Organic cell walls behave like the probe and pressure units are valuable.
% Saturation is the best unit for industrial control and not ppM, contrary to popular beliefs. The
partial pressure (and consequently the pressure defined % Saturation) varies only slightly with
temperature. (Recall at this stage that the permeability of the membrane has a temperature
coefficient, but the electronics has scaled this out by the operation of the Automatic Membrane
Temperature Compensator Thermistor incorporated in the D.O. probe).
If mass units are used for measurement of Dissolved Oxygen, the temperature problem of relating
the linear partial pressure reading of the probe to the mass (ppM or mg/L) at different temperatures
becomes more involved. As well, there is the mass variation due to dissolved salts (salinity
correction). Therefore, the fully corrected instrument would need 3 correction systems.
(a)
Membrane correction for temperature permeability effects ;
(b)
Solubility correction of Dissolved Oxygen with temperature and ;
(c)
Salinity correction of Dissolved Oxygen by weight (Salinity has no effect on pressure units
readout).
In the
90-FL
instrument,
(a)
Membrane correction is achieved AUTOMATICALLY ;
(b)
To provide the mass units (ppM) readout (so popular due to the Winkler process used in the
past), the
90-FL
Meter has Solubility Correction via an additional temperature sensor in the
electrode ;
(c)
Salinity correction is performed automatically via the Conductivity/TDS sensor.