A concept of the laminar flowmeter is based on the well-known relationship between laminar flow rate of fluid Q through a measuring element of length L and pressure drop ΔP. Usually the measuring element is a capillary tube, capillary tube bundle, parallel plate bundle, or concentric annular duct (Fig. 1).

The pressure drop in the measuring element (“long duct”) consists of two components:
where: C is a constant (depends on a duct cross-section shape), A is the duct cross-sectional area, Dh is the hydraulic diameter of the duct, K is the incremental pressure drop number in a fully developed flow region (depends both on the duct cross-section shape and on the inflow conditions), L is the distance between the flowmeter pressure...
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