shown in table 1-1. The purity meter indications will vary 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 circuit. 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 outlet of the demineralize. Typical operating requirements for the demineralize are conductivity 1 micromno/cm at 77°F (resistivity 1 megohm/cm  at  77°F),  oxygen  content  0.1  ppm  by weight, and mechanical filtration 0.5 microns absolute. When water has been circulated through the system 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 demineralize is maintaining a high degree of purity. When a system is filled with a fresh charge of water, it should be allowed 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 demineralize 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/conductivity reading required for a specific installation must be maintained for optimum operation of the cooling water system. Your  first  indication  of  a  problem  in  the demineralize is usually indicated by abnormal purity meter readings (too low or too high), an abnormal flowmeter reading, and/or a light and audible warning from  the  purity  monitor.  Some  purity  monitors can be tested for accuracy by a built-in test function on Table 1-1.—Distilled Water Resistivity Versus Conductivity 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  return  the  demineralize  to  its  normal operating condition. Maintenance   of   the   demineralize   consists primarily 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 external 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 arty case, should be as particle-free as possible and should not exceed 0.065 ppm chloride. OXYGEN ANALYZER In some secondary cooling systems, an oxygen analyzer is installed 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 oxygen sensor installed in the supply side of the secondary cooling system. The sensor is an electrolytic cell in an electrolyte solution or gel. A thin membrane covers the end of the sensor, which is inserted in the coolant. This membrane is gas permeable to the dissolved oxygen in the secondary coolant. This allows the oxygen to pass through the membrane.    The oxygen reacts with the electrolyte, which causes a proportional change in the amount of current flow in the sensor. The sensor’s electrical output is measured and displayed on the oxygen analyzer’s meter. ‘The meter is calibrated to read the oxygen content in parts per million or billion. Because  of  solid-state  electronics  and  the  few components used, the oxygen analyzer requires very little maintenance other than the cleaning and changing of the electrolyte in the sensor. When the meter on the analyzer requires frequent calibration because the meter readings are drifting or changing sharply, you should recognize that the analyzer has a bad sensor. When you clean and recharge the sensor, use caution to prevent contamination of the membrane from the oil on your fingers. Data 1-22