Cloud Point of Oil Review
Cloud point: The cloud point is the
temperature at which dissolved solids in the oil, such as
paraffin wax, begin to form and separate from the oil. As
the temperature drops, wax crystallizes and becomes visible.
Certain oils must be maintained at temperatures above the
cloud point to prevent clogging of filters.
Measuring cloud point of petroleum products
The test oil is required to be transparent in layers 40mm in thickness (in accordance with ASTM D2500). The wax crystals typically first form at the lower circumferential wall with the appearance of a whitish or milky cloud. The cloud point is the temperature at which these crystals first appear.
The test sample is first poured into a test jar to a level approximately half full. A cork carrying the test thermometer is used to close the jar. The thermometer bulb is positioned to rest at the bottom of the jar. The entire test subject is then placed in a constant temperature cooling bath on top of a gasket to prevent excessive cooling.
At every 1 °C, the sample is taken out and inspected for cloud then quickly replaced. Successively lower temperature cooling baths may be used depending on the cloud point. Lower temperature cooling bath must have temperature stability not less than 1.5 K for this test.
ASTM D5773, Standard Test Method of Cloud Point of Petroleum Products (Constant Cooling Rate Method) is an alternative to the manual test procedure. It uses automatic apparatus and has been found to be equivalent to test method D2500.
The D5773 test method determines the cloud point in a shorter period of time than manual method D2500. Less operator time is required to run the test using this automatic method. Additionally, no external chiller bath or refrigeration unit is needed. D5773 is capable of determining cloud point within a temperature range of -60 °C to +49 °C. Results are reported with a temperature resolution of 0.1 °C.
Under ASTM D5773, the test sample is cooled by a Peltier device at a constant rate of 1.5 +/- 0.1 °C/min. During this period, the sample is continuously illuminated by a light source. An array of optical detectors continuously monitor the sample for the first appearance of a cloud of wax crystals. The temperature at which the first appearance of wax crystals is detected in the sample is determined to be the cloud point.