ASTM D5773 Standard Test Method for Cloud Point of Petroleum Products and Liquid Fuels (Constant Cooling Rate Method)
4. Summary of Test Method
4.1 A prescribed specimen (11.5) is cooled by a Peltier device (A1.1) at a constant rate of 1.5 °C/min +/- 0.1 °C/min while continuously being illuminated by a light source (A1.1.4). The specimen is continuously monitored by an array of optical detectors (A1.1.5, Fig. A1.1) for the first appearance of a cloud of wax crystals. The detectors are sufficient in number to ensure that any solid-phase hydrocarbon crystals that may form are detected. The temperature at which the appearance of a cloud of wax crystals is first detected in the specimen is recorded to 0.1 °C resolution. When the recorded temperature is rounded to the next lower integer temperature, it is designated as the D2500/IP 219 equivalent cloud point per Test Method D5773.

5. Significance and Use
5.1 The cloud point of petroleum products and biodiesel fuels is an index of the lowest temperature of their utility for certain applications. Wax crystals of sufficient quantity can plug filters used in some fuel systems.

5.2 Petroleum blending operations require a precise measurement of the cloud point.

5.3 This test method can determine the temperature of the test specimen at which wax crystals have formed sufficiently to be observed as a cloud with a resolution of 0.1 °C.

5.4 This test method provides results that are equivalent to Test Method D2500.

NOTE 1 - This is based on the Test Method D2500 equivalent cloud point in which the 0.1 °C result is rounded to the next lower integer.

5.5 This test method determines the cloud point in a shorter period of time than Test Method D2500.

NOTE 2 - In cases of samples with cloud points near ambient temperatures, time savings may not be realized.

5.6 This test method eliminates most of the operator time required of Test Method D2500.

5.7 This test method does not require the use of a mechanical refrigeration apparatus.

NOTE 3 - In certain cases of high ambient temperature, a source of cooling water may be required to measure low-temperature cloud points (see 7.1).