HEAD FLOW METERS
The flowpath restriction, such as an orifice, causes a differential pressure across the orifice. This
pressure 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 flow
rate is easily calculated or computed from volumetric flow rate by knowing or sensing
temperature 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 is
compensated for changes in temperature and/or pressure, a true mass flow rate signal can be
obtained. In Thermodynamics it is described that temperature and density are inversely
proportional, while pressure and density are directly proportional. To show the relationship
between temperature or pressure, the mass flow rate equation is often written as either Equation
4-1 or 4-2.
= mass flow rate (lbm/sec)
A = area (ft2)
DP = differential pressure (lbf/ft2)
= pressure (lbf/ft2)
= temperature (°F)
K = flow coefficient
The flow coefficient is constant for the system based mainly on the construction characteristics
of the pipe and type of fluid flowing through the pipe. The flow coefficient in each equation
contains the appropriate units to balance the equation and provide the proper units for the
resulting mass flow rate. The area of the pipe and differential pressure are used to calculate
volumetric flow rate. As stated above, this volumetric flow rate is converted to mass flow rate
by compensating for system temperature or pressure.