FAS622en_pH measurement

2 Measurement
Pressure
Pressure has an effect, firstly on the reference system of the reference electrode, and secondly on
the pH value. The effect on the reference system can easily be taken into account by choosing a
suitably pressure-resistant electrode. Interpretation difficulties with comparison measurements occur
in certain cases. In waters that contain pH-active gases like ammonia, carbon dioxide or hydrogen
sulfide, the pressure changes the pH value. In basic solutions, the pH value increases with rising
pressure (e. g. NH 3 in water), and in acidic solutions it reduces (e. g. SO 2 in water). If a comparison
measurement is made under normal pressure conditions, the measurement result is correspondingly
higher or lower. At lower pressures, the effect is often increased as the dissolved gases
effervesce.
Flow
Continuous measurements almost always take place in flowing water. Each electrode reacts more
or less strongly to the water movement. Although brand new electrodes are relatively insensitive to
changes in flow, the effect can cause considerable deviations in measured values with spent electrodes.
Regular checks on the sensitivity to flow provide information on the status of the electrode. If the
sensitivity to flow is too high, the electrode should be replaced.
2.2.3 Installation
Installing the individual components in a systematic manner gives an initial indication of the correct
operation of the electrode.
First of all the transmitter is installed. Testing with a voltage source (pH simulator) demonstrates
that it is working correctly.
The next step is to connect the pH sensor (sensor with fitting). A comparison measurement between
the water in a bucket (or some other ungrounded container) and the intended measurement
point can give an indication that the sensor is in good condition. The two values must agree, taking
into account the variations with time (decomposition process). The diaphragm and the membrane
must be fully immersed in the water for both measurements.
If this test indicates that there is no fault, connection of the peripheral equipment (recorder, dosing
device, controller, etc.) can start. After each item of equipment is connected, a check should be
made on whether the value indicated by the transmitter has changed significantly.
22 2 Measurement JUMO, FAS 622, Edition 04.07