Laminar Differential Pressure (DP) Flow Meter – Operating Principle
Alicat Scientific pioneered the use of laminar DP technology to read mass flow in 1992. Previous to this, classic DP technology was used to measure flow rate – but with significantly less precision. Today, Alicat laminar DP mass flow meters and controllers use laminar DP measurement technology to accurately measure mass flow rates for over 130 gases.
Four steps to measure mass flow using laminar DP
- Convert turbulent flow into smooth laminar flow
- Measure the pressure drop across a flow channel
- Calculate the volumetric flow rate
- Calculate a standardized mass flow rate
The following video shows how an Alicat laminar DP flow meter calculates mass flow:
Calculating volumetric flow rate
Alicat laminar DP flow meters and controllers contain a flow passage called a laminar flow element (LFE) that converts all turbulent flow into laminar flow. The instrument first measures the differential pressure drop across the LFE, and then uses the Poiseuille Equation to calculate a volumetric flow rate.
Volumetric flow = (P1-P2)πr4/8ηL = K(ΔP)/η
P1 = Static pressure at the inlet; P2 = Static pressure at the outlet; r = Hydraulic radius of the restriction; η = Absolute viscosity of the fluid; L = Length of the restriction
The Poiseuille equation shows the linear relationship between volumetric flow rate, differential pressure (ΔP), and absolute viscosity (η). In the simplified equation, K is a constant that encompasses the geometric factors of the LFE.
Calculating mass flow rate
Laminar DP mass flow instruments use the volumetric flow rate and a series of density correction factors to calculate a mass flow rate.
1. Temperature density correction = Ts/Ta
Ta = Absolute temperature at flow conditions; Ts = Absolute temperature at standard conditions (STP)
2. Pressure density correction= Pa/Ps
Pa = Absolute pressure at flow conditions; Ps = Absolute pressure at standard conditions (STP)
3. Compressibility = Zs/Za
Za = Compressibility at flow conditions; Zs = Compressibility at standard conditions (STP)
When combined, you can produce an equation that uses volumetric flow, temperature, pressure, and gas compressibility to calculate a mass flow rate.
Mass flow = (Volumetric flow)(Ts/Ta)(Pa/Ps)(Zs/Za)
(Ts/Ta)(Pa/Ps)(Zs/Za) = Density correction factors
In an Alicat mass flow device, a discrete absolute pressure sensor is placed in the laminar region of the flow stream. This information is sent to the microprocessor and is combined with the data from the discrete absolute temperature sensor to make the correct mass flow calculations.
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