The purity meter indication varies with ionized
salt concentration and the temperature of the coolant
flowing through the cell. The temperature effect is
canceled by a built-in temperature compensation cir-
cuit.
The inlet conductivity is compared to a preset
value of cell conductance to actuate an alarm circuit
when the purity of the water drops below the preset
level. In addition, the purity meter provides direct
readings of the water purity at the inlet and the outlet
of the demineralizer Typical operating requirements
for the demineralizer are conductivity 1 micromho/cm
at 77°F (resistivity 1 megohm/cm at 77°F), oxygen
content 0.1 parts per million (ppm) by weight, and
mechanical filtration 0.5 microns absolute.
When water has been circulated through the sys-
tem for extended periods of time, a high-resistivity or
low-conductivity reading may be indicated on both
input and output samples. This condition is highly
desirable and indicates that all ionizable material has
been properly treated and that the demineralizer is
maintaining a high degree of purity. When a system is
filled with a fresh charge of water, it should be al-
lowed to circulate for approximately 2 hours before
comparing the input and output readings. During the
initial circulation period, the resistivity readings vary
because of the mixing action of water that has been
treated by the demineralizer with the fresh charge of
water.
A properly operating system can supply water of
acceptable purity in 4 to 8 hours. Water in a system
that has been secured for any length of time should be
of acceptable purity within 2 hours. The resistivity or
conductivity reading required for a specific installa-
tion must be maintained for optimum operation of the
cooling water system.
The first indication of a problem in the demineral-
izer is usually indicated by abnormal purity meter
readings (too low/high), an abnormal flowmeter read-
ing, and/or alight and audible warning from the purity
monitor. Some purity monitors can be tested for
accuracy by a built-in test function on the meter to
establish if the problem is in the purity monitor. If the
purity monitor does not have a test feature, then use
the calibration plug in place of one of the conductivity
cells to test the operation of the purity meter. Most of
the time, only routine maintenance is required to re-
turn the demineralizer to its normal operating condi-
tion.
Maintenance of the demineralizer consists pri-
marily of the scheduled replacement of cartridges
(before they are exhausted) and clogged filters.
Obtaining satisfactory service life from the cartridges
and filters is largely dependent on minimizing ex-
ternal contamination. Replacement cartridges must be
kept sealed and stored in a cool, dry place until used.
The circulating system must be kept tight to reduce
the need for makeup water. Makeup water, in any
case, should be as particle-free as possible and should
not exceed 0.065 ppm chloride.
OXYGEN ANALYZERS
Oxygen analyzers are installed in some secondary
cooling systems to measure the amount of dissolved
oxygen in the liquid coolant. The presence of oxygen
causes oxidation that leads to the formation of scale in
the cooling system. An oxygen analyzer has an oxy-
gen sensor installed in the supply side of the second-
ary cooling system.
The sensor is an electrolytic cell in an electrolyte
solution or gel. The oxygen reacts with the electrolyte,
causing a proportional change in the amount of cur-
rent flow in the sensor. The sensor’s electrical output
is measured and displayed on the oxygen analyzer’s
meter, which is calibrated to read the oxygen content
in parts per million or billion.
Because of the solid-state electronics and the few
components used, the oxygen analyzer requires very
little maintenance other than cleaning and changing
the electrolyte in the sensor. When the meter on the
analyzer requires frequent calibration because the
meter readings are drifting or changing sharply, the
analyzer has a bad sensor.
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