Page 366 - Instrumentation Reference Book 3E

Electrical conductivity 349
be rinsed several times in distilled water and then account should the central electrode be disturbed,
viewed. Looking through the bore towards a as this will impair the accuracy of the electrical
source of illumination, the surface should be constant of the cell. In cases where metal cells have
evenly wetted with no dry patches where the become contaminated with adherent particulate
water has peeled away. If dry patches appear matter, such as traces of magnetite or other metal
rapidly, indicating that a thin film of grease is oxides, ultrasonic cleaning in the detergent solution
present. the surface is not clean. has been shown to be effective.
In all cleaning processes care should be taken
to keep the external electrical contact, cable
Stainless steel and Monel A feature of many entries, and plugs dry.
stainless steel cells is the frosted appearance of
the electrodes which is essential to reduce polar-
ization. It is most important that this frosting is Instruments for conventional a. c. measurement
not polished away by the regular use of abrasive The conductance of a cell may be measured
(a) by Wheatstone bridge methods or (b) by
direct measurement of the current through the
Oscillator cell when a fixed voltage is applied.

Wheatstone bridge methods The actual conduct-
ance of the cell is usually measured by means of a
self-balancing Wheatstone bridge of the form
shown in Figure 17.4 and described in detail in
Part 3.

Direct measurement of cell conductance The
conductance of a cell may be measured directly
:tivity by the method indicated in Figure 17.5. The cur-
rent is directly proportional to the conductance so
the output from the current amplifier is applied to
(a) (bl the indicator and recorder. Temperature compen-
Figure 17.4 Measurement of conductance using sation is achieved by connecting a manual tem-
Wheatstone bridge: (a) simple circuit, (b) thermistor perature compensator in the amplifier circuit, or
temperature-corrected circuit. a resistance bulb may be used to achieve auto-
matic compensation.
cleaners. This type of cell may be cleaned with a
50 percent water detergent solution and a bottle
brush. Multiple-electrode cells From the foregoing dis-
In the case of screw-in cells, the outer electrode cussion on errors introduced by polarization
may be removed to facilitate cleaning, but on no together with the importance of constancy of

120/240 V
50/60 H z
source

To alarm

Stable ax.
source

Figure 17.5 Direct measurement Temperature r$ T0
of cell conductance. compensator recorder