# Differential Pressure Flow Meters

Differential pressure flow meters use laminar plates, an orifice, nozzle, or Venturi tube to create an artificial constriction then measure the pressure loss of fluids as they pass that constriction. According to Bernoulli's principle, the pressure drop across the constriction is proportional to the square of the flow rate. The higher the pressure drop, the higher the flow rate. These rugged, accurate meters are ideal for a wide range of clean liquids and gases.

## What is a differential Pressure (Î”p) meter?

Differential pressure meters work on the principle of partially obstructing the flow in a pipe. This creates a difference in the static pressure between the upstream and downstream side of the device. This difference in the static pressure (referred to as the differential pressure) is measured and used to determine the flow rate.
Differential-pressure meters are hugely popular and it is estimated that at least 40% of industrial flow meters in use at present are differential-pressure devices, with the orifice plate being the most popular. Differential-pressure devices have been used to meter a wide variety of different fluids from gases to highly viscous liquids.
The popularity of differential-pressure flow meters is in part due to their simple design and low cost. By reading this guide you will have a much clearer idea of the benefits, viable metering options and applications for using differential-pressure meters.

## How Differential Pressure Flowmeters Work

Differential pressure flowmeters use Bernoulli’s equation to measure the flow of fluid in a pipe. Differential pressure flowmeters introduce a constriction in the pipe that creates a pressure drop across the flowmeter. When the flow increases, more pressure drop is created. Impulse piping routes the upstream and downstream pressures of the flowmeter to the transmitter that measures the differential pressure to determine the fluid flow. This technology accounts for about 21% of the world market for flowmeters.
Bernoulli’s equation states that the pressure drop across the constriction is proportional to the square of the flow rate. Using this relationship, 10 percent of full scale flow produces only 1 percent of the full scale differential pressure. At 10 percent of full scale flow, the differential pressure flowmeter accuracy is dependent upon the transmitter being accurate over a 100:1 range of differential pressure. Differential pressure transmitter accuracy is typically degraded at low differential pressures in its range, so flowmeter accuracy can be similarly degraded. Therefore, this non-linear relationship can have a detrimental effect on the accuracy and turndown of differential pressure flowmeters. Remember that of interest is the accuracy of the flow measurement system --- not the accuracy of the differential pressure transmitter.

## Types of Differential Pressure Flow Meter

• Orifice plates
• Venturi tubes
• Cone meters (e.g. V-cones)
• Nozzles
• Low loss meters (e.g. Dall tubes)
• Variable area meters
• Inlet flow meters
• Venturi cones
• Venturi nozzles
• Drag plates

## Advantages of DP Flow Measurement

• Simple devices with no moving parts
• Less expensive compared to other flow technologies
• Installation can be in any orientation
• Can be used for most gases and liquids
• Maintenance is similar to that of pressure transmitters

## Disadvantages of  DP Flow Measurement

• Rangeability (turndown) is less than for most other types of flow meter
• Significant pressure losses may occur
• The output signal is non-linear with flow
• The discharge coefficient and accuracy may be affected by pipe layout or nature of flow
• They may suffer from ageing effects, e.g. the build-up of deposits or erosion of sharp edges