In many processes, mass žow rate, rather than volume žow rate, is the desired parameter because volumetric žow measurements are less reliable than are mass žow measurements. Since changes in gas temperature and pressure will a¥ect the density of a Ÿxed volume of gas, thermal mass žow meters and controllers, unlike rotameters, turbine meters, and other volumetric žow devices, are relatively immune to changes in inlet temperature and pressure. Because these instruments directly measure molecular žow, they provide the most reliable, repeatable, and accurate method for delivering material from a supply volume to a process at a desired rate. Features, such as direct electronic readout, fast response, exceptional sensitivity at low žow rate, negligible pressure drop, no moving parts, simple installation, unobstructed straight through žow path, absence of temperature or pressure corrections, and superior retain accuracy over a wide range of žow rates, make thermal mass žow meters a superior alternative to conventional žow meters in many important applications. Some other advantages of this type of žow meter are accuracy in a 2% range, low žow sensitivity, reliability, and long life with advances in electronic technology; in a sense, these žow meters are getting smarter and more capable with microprocessor computing power, thus expanding the range of their usage.