LABORATORY CLOUD POINT ANALYZER MODEL 1433 Shell Rejining Limited Derign

PRINCIPLE OF OPERATION The HALLIKAINEN CLOUD POINT ANALYZER is an automatic instrument for detecting and indicating the wax precipitation temperature, or cloud point of gas oils. The instrument is based on the differential tem- perature method of cloud point determination. The principle of operation is as follows:- cI small sample cell containing the oil under test is placed in the cool- ing chamber of on electrothermal refrigerating unit, and CI detector assembly, comprising two thermistors and o thermocouple, is lowered into the oil to o pre- determined position. An automatic cycle of events is then initiated by operation of a "START" switch on the instrument panel. It can be seen from figure below that when the cooling unit is energized the oil will circulate through convection falling at the walls of the tube and rising along the axis. The resulting difference in temperature between top and bottom of the sample container is detected by the thermistor elements and causes electrical unbal- ance of the detector bridge circuit. The thermocouple, measuring sample temperature midway between the thermistor levels, is connected to the potentiometric indicator on the front panel of the instrument for cloud point monitoring. Detector bridge circuit output

is fed to o Schmitt trigger circuit biased to be inoperative while the detector is unbalanced. With continued cooling the sample tempera- ture decreases to its cloud point when wolx precipitation takes place, preventing furthel convection. At this point the temperature difference between the thermistors disappears and the detector bridge circuit balances. This condition allows the Schmitt trigger circuit to operate switching off the supply to the cool. ing unit and clamping the potentiometric in- dicator. The cloud point temperature of the sample will then remain displayed until the operator, by lifting the detector assembly out of the sample cell, automatically resets the instrument. In the initial period of the test when the sample is at a uniform temperature a time delay circuit prevents operation of the trigger circuit. The cooling unit incorporates CI therm- al trip to prevent overheating should the water supply foil. The instrument covers a range of cloud points down to -40°C and will complete CI test cycle in about 5 to 15 minutes depending upon the cloud point temperature of the sample. ACCURACY Accuracy of measurement is within ASTM re. quirements.