At normal flows, measurement errors are usually in the 1-2% full scale range. On the other hand, if heat input is constant, the ability to detect very small temperature differences is limited and both precision and rangeability drop off. Thermal flowmeters provide high rangeability (10:1 to 100:1) if they are operated in constant-temperature-difference mode. per minute)-much lower than can be detected with any other device. One of the capabilities of thermal mass flow meters is to accurately measure low gas flow rates or low gas velocities (under 25 ft. In detecting the mass flow of compressible vapors and gases, the measurement is unaffected by changes in pressure and/or temperature. Thermal mass flow meters are often used in monitoring or controlling mass-related processes such as chemical reactions that depend on the relative masses of unreacted ingredients. Volumetric measurements are affected by all ambient and process conditions that influence unit volume or indirectly affect pressure drop, while mass flow measurement is unaffected by changes in viscosity, density, temperature, or pressure. Thermal mass flow meter These flow meters also measure the mass flow rate of gases and liquids directly. These mass flow measurements were not very accurate, because the relationship between process pressure or temperature and density are not always precisely known-each sensor adds its own separate error to the overall measurement error, and the speed of response of such calculations is usually not sufficient to detect step changes in flow. Density was either directly measured, or was calculated using the outputs of process temperature and pressure transmitters. In the past, mass flow was often calculated from the outputs of a volumetric flowmeter and a densitometer. With these being the most critical flow measurements in a processing plant, the reliability and accuracy of mass flow detection is very important. Mass flow measurement is the basis of most recipe formulations, material balance determinations, and billing and custody transfer operations throughout industry. A Thermal mass flow meter measures gas mass flow rate based on the conventive heat transfer of a heated surface to the flowing fluid.
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