Abstract: When sensors are employed to monitor the evaporator refrigerant pressure and the leaving chilled liquid temperature in an air conditioning system of the type having a liquid chiller, the sensor outputs will normally have a prescribed relationship with respect to each other as long as the sensors are functioning properly and regardless of the operating condition of the air conditioning system. By effectively comparing the output of one sensor relative to that of the other sensor, a faulty condition of either sensor may be detected. This is achieved by calculating the equivalent evaporator temperature, from the evaporator refrigerant pressure, and subtracting the equivalent temperature from the leaving chilled liquid temperature to obtain a difference temperature which is then compared to a predetermined known temperature range representing normal functioning of the two sensors. When one of the sensors is defective the difference temperature will fall outside of the range.

Abstract: When sensors are employed to monitor different operating variables or parameters in a refrigeration system, the sensor outputs will normally have predetermined known relationships with respect to each other as long as the sensors are functioning properly and regardless of the operating condition of the refrigeration system. By comparing the output of one sensor relative to that of another sensor, a faulty condition of either of those two sensors may be detected. For example, during stabilized system operation the output of a condenser pressure sensor should always indicate a higher pressure than that reflected by the output of an evaporator pressure sensor. By effectively subtracting the evaporator pressure from the condenser pressure, a faulty sensor may be discovered. If the result of the subtraction is zero or negative, at least one of the pressure sensors is defective. When a faulty sensor is detected, a warning message is displayed to operating personnel.

Abstract: A programmable maintenance timer system suitable for use with industrial equipment such as a centrifugal water chilling system and a telemetry system including a microprocessor-based computer with a programming device for presetting selectively a maintenance time interval representing a number of hours before a maintenance service is to be performed, for counting down the time interval to zero, and for generating an output signal after the time interval has reached zero to the telemetry system so as to alert service personnel that the industrial equipment is due for a scheduled maintenance. The timer system also includes a non-volatile memory device to record permanently the total elapsed operating hours in the preset maintenance time interval so as to avoid loss of such information upon a power failure and/or back-up battery failure.