Oil in water monitors are used to detect the presence of hydrocarbons in water. Samples are extracted from a stream or monitored on a continuous basis. Oil in water analysis consists of many different measurement techniques. Fluorescence analysis determines the intensity of radiation (fluorescence) from a sample which absorbs light at a shorter wavelength and emits light at a longer wavelength. Ultraviolet analysis (UV) measures the amount of UV light that a sample absorbs. Infrared (IR) analysis evaporates a solvent from a plate through which IR light is passed, or uses an IR-transparent solvent to read the solvent/oil mix with an IR instrument. Gravimetric analysis measures the dry weight of a contaminant per unit volume of fluid. After the solvent is separated from the water and boiled off in a controlled environment, the residual is weighed on a balance and reported as oil and grease. Colorimetric analysis is based on the principle that the color intensity of a solution is proportional to the concentration of a specific substance. The solvent is separated, but not boiled off. Instead, a colorimeter or spectrophotometer is used to compare the color of the solvent to known standards for the clean solvent and the specific oil to be measured.