What is Differential Pressure Flow Meter?

Differential pressure meters work on the principle of partially obstructing the flow in a pipe. This creates a difference in static pressure between the upstream and downstream sides of the machine. This difference in static pressure is measured and used to determine the flow rate. DP flow meters are widely used in industry to measure flow and are popular due to their simple design and low cost.

Differential Pressure (Δp) Flow Meter

Manometer tubes measure the difference between the static pressure upstream and downstream of the restriction. When a fluid flows through a restriction, it accelerates to a higher velocity (ie V2 > V1 ) to conserve mass flux and as a result its static pressure decreases. This differential pressure (Δp) is then a measure of the flow rate through the device. Simply put, for a given size of restriction, the higher the Δp, the higher the flow rate.

The relationship between differential pressure and flow rate is derived from Bernoulli's equation. Using Bernoulli's equation, and conservation of mass, it can be shown that the differential pressure generated is proportional to the square of the mass flow rate, Qm (kg/s).

Most of the available Δp meters work on this principle of measuring the difference in pressure between upstream and downstream but there are some meters that use differential pressure in other ways, for example, variable area meters.

Types of Differential Pressure Flow Meter

Common types of differential pressure meters are

Orifice plates
Venturi tubes
Cone meter (eg V-cone)
nozzles
Low loss meter (eg Dall tube)
Variable area meter
Inlet flow meter
Venturi cone
Venturi nozzles
Drag the plates

Advantages of DP Meter

They are easy to make, with no moving parts
They are cheaper than other meters, especially in larger pipes
They can be used in any orientation
They can be used for most gases and liquids
Some types do not require calibration for specific applications

Disadvantages of DP Meter

Rangeability (turndown) is low compared to other types of flow meters
A significant loss of pressure can occur
The output signal is non-linear with flow Discharge coefficient and accuracy can be affected by pipe layout or flow characteristics
They may suffer from the effects of aging