Flash point in the context of "Autoignition temperature"

Fuel oil is any of various fractions obtained from the distillation of petroleum (crude oil). Such oils include distillates (the lighter fractions) and residues (the heavier fractions). Fuel oils include heavy fuel oil (bunker fuel), marine fuel oil (MFO), furnace oil (FO), gas oil (gasoil), heating oils (such as home heating oil), diesel fuel, and others.

The term fuel oil generally includes any liquid fuel that is burned in a furnace or boiler to generate heat (heating oils), or used in an engine to generate power (as motor fuels). However, it does not usually include other liquid oils, such as those with a flash point of approximately 42 °C (108 °F), or oils burned in cotton- or wool-wick burners. In a stricter sense, fuel oil refers only to the heaviest commercial fuels that crude oil can yield, that is, those fuels heavier than gasoline (petrol) and naphtha.

The fire point, or combustion point, of a fuel is the lowest temperature at which the liquid fuel will continue to burn for at least five seconds after ignition by an open flame of standard dimension. At the flash point, a lower temperature, a substance will ignite briefly, but vapour might not be produced at a rate to sustain the fire. Most tables of material properties will only list material flash points. In general, the fire point can be assumed to be about 10 °C higher than the flash point, although this is no substitute for testing if the fire point is safety critical.

Testing of the fire point is done by open cup apparatus.

The smoke point, also referred to as the burning point, is the temperature at which an oil or fat begins to produce a continuous bluish smoke that becomes clearly visible, dependent upon specific and defined conditions. This happens when one or multiple substances in the oil start to chemically react with oxygen and burn, which can include the oil itself, proteins, sugars, or other organic material. It is distinct from the flash point and fire point, which denote the temperatures at which the oil itself (specifically, vaporized oil, which is distinct from the smoke produced at the smoke point) begins to burn.

Smoke point values can vary greatly. The most important factor determining the smoke point of an oil is the amount of proteins and free fatty acids (FFAs). Higher quantities of these lower the smoke point. The FFA content typically represents less than 1% of the total oil and consequently renders smoke point a poor indicator of the capacity of a fat or oil to withstand heat, in a non-cuisine related sense. Virgin (raw) oils, which contain various flavorful organic compounds, have lower smoke points than refined oils because the organic compounds burn. Animal-based fats and oils tend to have lower smoke points than vegetable-based ones, as well. Oils made of polyunsaturated fats have lower smoke points, those made of monounsaturated fats have middling smoke points, and oils made of saturated fats have even higher smoke points. The level of refinement, seed variety, and climate and weather of growth of the source plants also significantly affect its smoke point.

A flammable liquid is a liquid which can be easily ignited in air at ambient temperatures, i.e. it has a flash point at or below nominal threshold temperatures defined by a number of national and international standards organisations.

The Occupational Safety and Health Administration (OSHA) of the United States Department of Labor defines a liquid as flammable if it has a flash point at or below 93 °C (199.4 °F). Prior to bringing regulations in line with the United Nations Globally Harmonized System of Classification and Labeling of Chemicals (GHS) in 2012, OSHA considered flammable liquids to be those with a flash point below 37.8 °C (100.0 °F). Those with flash points above 37.8 °C/100 °F and below 93.3 °C (199.9 °F) were classified as combustible liquids. Studies show that the actual measure of a liquid's flammability, its flash point, is dependent on the local air pressure, meaning that at higher altitudes where the air pressure is lower, the flash point is also lower.

The Cleveland open-cup method is one of three main methods in chemistry for determining the flash point of a petroleum product using a Cleveland open-cup apparatus, also known as a Cleveland open-cup tester. First, the test cup of the apparatus (usually brass) is filled to a certain level with a portion of the product. Then, the temperature of this chemical is increased rapidly and then at a slow, constant rate as it approaches the theoretical flash point. The increase in temperature will cause the chemical to begin to produce flammable vapor in increasing quantities and density. The lowest temperature at which a small test flame passing over the surface of the liquid causes the vapor to ignite is considered the chemical's flash point. This apparatus may also be used to determine the chemical's fire point which is considered to have been reached when the application of the test flame produces at least five continuous seconds of ignition. Temperature range of this apparatus is 120 to 250 °C (248 to 482 °F).

The other principal methods of establishing chemical flash points are the Pensky–Martens closed-cup test and the Tagliabue cup method (often called simply the "Tag method").