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The flash point and fire point refer to the flammability characteristics of the fluid being tested. The flash point is defined as the lowest temperature at which the vapor formed above a pool of the liquid ignites in air at a pressure of 1 atmosphere. The physical setup of the apparatus, the rate of heating, and the source of ignition are precisely defined. If the sample is in a cup exposed to the ambient air, this is referred to as the Cleveland Open Cup (COC) method. If the sample is kept closed to the ambient air until the source of ignition is applied, then this is referred to as the closed-cup method.
The fire point is the lowest temperature at which, on further heating beyond the flash point, the sample will support combustion for 5 seconds.
The details of the entire procedure for determining the flash point and fire point of a fluid are given in the ASTM D 92 (open cup) and ASTM D 93 (closed cup) methods. For the open cup method a metal cup containing a specified amount of the fluid is neated at a rate of 5-6ºC/min.(9-11ºF/min.). Starting at a temperature at least 28ºC (50ºF) below the flash point, the source of ignition (a small pilot light) is swept over the surface of the cup. This is done at 2ºC (5ºF) intervals. The flash point is recorded as the temperature at which the material exhibits a flash when the source of ignition is swept over its surface. The sample is heated further continuing the same procedure and the fire point is recorded as the temperature at which the sample continues to burn for a least 5 seconds.
The flash point is just one flammability characteristic that is used to assess the hazadous nature of a material. A low flash point can be indicative of the presence of highly volatile materials in the fluid. The fire point is used to assess the risk of the materials ability to support combustion. These values can also affect how the fluid may be shipped, stored, and discarded. For mineral oils the minimum accepted value for the flash point oil used in outdoor transformers is 145ºC, although this could vary according to local codes. In general the fire points are about 10ºC higher than the flash point.
Another related flammability property of a fluid is its auto-ignition temperature. This is much higher than the flash point and it represents the temperature at which the sample will ignite from only the heat being applied and no external souce of ignition. The procedure for determining this value is covered in the ASTM E 659 mehod.
The values for some non-mineral oils are a minimum flash point of 300ºC and a minimum fire point of 340ºC for silicones. Askarels exhibit no fire point up to their boiling point.