HEAD FLOW METERSFlow DetectorsThe flowpath restriction, such as an orifice, causes a differential pressure across the orifice. Thispressure differential is measured by a mercury manometer or a differential pressure detector.From this measurement, flow rate is determined from known physical laws.The head flow meter actually measures volume flow rate rather than mass flow rate. Mass flowrate is easily calculated or computed from volumetric flow rate by knowing or sensingtemperature and/or pressure. Temperature and pressure affect the density of the fluid and,therefore, the mass of fluid flowing past a certain point. If the volumetric flow rate signal iscompensated for changes in temperature and/or pressure, a true mass flow rate signal can beobtained. In Thermodynamics it is described that temperature and density are inverselyproportional, while pressure and density are directly proportional. To show the relationshipbetween temperature or pressure, the mass flow rate equation is often written as either Equation4-1 or 4-2.(4-1)mKA DP(P)(4-2)mKA DP(1/T)where= mass flow rate (lbm/sec)mA = area (ft2)DP = differential pressure (lbf/ft2)P = pressure (lbf/ft2)T = temperature (°F)K = flow coefficientThe flow coefficient is constant for the system based mainly on the construction characteristicsof the pipe and type of fluid flowing through the pipe. The flow coefficient in each equationcontains the appropriate units to balance the equation and provide the proper units for theresulting mass flow rate. The area of the pipe and differential pressure are used to calculatevolumetric flow rate. As stated above, this volumetric flow rate is converted to mass flow rateby compensating for system temperature or pressure.IC-04 Page 2 Rev. 0
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