In chemistry, the burn rate is a measure of the linear combustion rate of a compound or substance such as a candle or a propellant. The industry recognized nomenclatures are mm/s or in/s for millimeters per second or inches per second respectively. Burn rate (or burning rate) applies to a substance and how that substance reacts to combustion. It can be measured at ambient pressure (i.e. at sea level) or at a modified pressure either in a vacuum chamber or in a pressure chamber or in a device (article containing a substance) to determine the burn rate of the substance in a variety of conditions. Burning rate typically varies (upward) with pressure and temperature. Some substances have an inverse relationship of burning rate to pressure, while others (such as black powder in particular) have a nearly neutral burning rate versus pressure. A substance is characterized in terms of burn rate by a burning rate vs pressure chart and a burning rate at each of several pressures vs temperature chart. One apparatus for measuring burning rate is a V shaped metal channel about 1-2 feet long wherein a sample is placed, with a cross-sectional dimension of approximately 6mm or 1/4". The sample is ignited on one end and time is measured until the flame front gets to the other end. The length of the sample as recorded is divided by the time it takes for the sample to burn from one end to another. The resulting figure is the burning rate and it is typically converted to or calculated as mm/s or in/s.