The test determines the hardness of Bitumen by measuring the depth ( in tenths of a mm) to which a standard, and loaded needle will vertically penetrate in 5 seconds, a sample of Bitumen maintained at a temperature of 25 deg C ( 77deg F). Hence the softer the bitumen, the greater will be its number of penetration units.
This test is carried out by the Ring and Ball method, which consists of suspending a brass ring containing the test sample of Bitumen in water at a given temperature. A steel ball is placed upon the bituminous material, the water is then heated at the rate of 5 deg C increase per minute. The temperature at which the softened bituminous material first touches a metal plate at a specified distance below the ring is recorded as the Softening point of the sample.
In the in terest of safety, legislation has been introduced in most countries fixing minimum flash point limits to prevent the inclusion of highly inflammable volatile fractions in kerosene distillates. According to Controller of Explosives classification it falls in the category of Class B Petroleum Products. Its flash point (Abel) is stipulated as Min. 35 deg C in the IS specification.