A variety of techniques are used to measure flow, including the Coriolis Effect, mass air flow, vortex, magnetic, thermal, ultrasonic, turbine, and positive displacement meters as well as pressure sensors. With the high volume, low cost, and reliability of microelectromechanical system (MEMS) pressure sensors, these products are a frequent choice for measuring both liquid and gas flow.
In fact, many applications use pressure sensor measurements to calculate other key parameters such as flow, altitude, water column height, fluid depth, weight and more.
For fluid flow measurements, orifice plates, venturi tubes and nozzles simplify the use of differential pressure (ΔP) sensors to determine the flow rate. In these cases, the flow is related to ΔP (P1-P2) by the equation:
q = cD π/4 D22 [2(P1-P2) / ρ(1 – d4) ]1/2
Where:
q is the flow in m3/s
cD is the discharge coefficient, and is the area ratio = A2/A1
P1 and P2 are in N/m2
ρ is the fluid density in kg/m3
D2 is the orifice, venturi or nozzle inside diameter (in m)
D1 is the upstream and downstream pipe diameter (in m)
and d = D2/D1 diameter ratio
In industrial applications, the most common use of pressure sensors to calculate another parameter is flow. This technique is also used to measure air flow of blowers, air flow through filters, vent hoods, gas boilers, or in heating ventilation and air conditioning (HVAC) variable air volume (VAV) controllers. In medical applications, drug delivery (liquid flow) uses differential pressure sensors to measure flow rates of 0.5-10.0 microliters/min. Many gas flow measurements are also made with pressure sensors.
