Cloud and Pour Point

When an oil is cooled slowly the temperature at which the oil becomes cloudy in appearance is called its cloud point. The temperature at which the oil ceases to flow or pour is called its pour point.

Most of the petroleum based lubricating oils contain dissolved paraffin wax and asphalt impurities. When the oil is cooled these impurities undergo solidification which cause jamming of the machine. So the cloud and pour points indicate the suitability of the lubricants in cold condition. A good lubricant must have low cloud point and pour point.

An oil is taken in a flat bottomed tube enclosed in an air jacket and it is cooled in a freezing mixture. Thermometers are introduced into the oil and freezing mixture. As the cooling takes place via the air jacket temperature of the oil falls. The temperature at which the cloudiness appears is noted as the cloud point. The cooling is further continued. The temperature at which the oil does not flow in the test tube for 5 seconds on tilting it to the horizontal position is noted as the pour point. Pour point of a lubricating oil can be lowered by dewaxing or by adding a pour point depressant. Poly alkali benzene called para flow is a commonly used pour point depression.