# What is a flow meter?: Differential Pressure Flow Meters

In a differential pressure drop device, flow rate is calculated from measurements of the pressure difference created by the flowing fluid. The differential pressure flow meter is based on Bernoulli’s Equation, where the pressure drop is a squared function of the fluid velocity.

Some common types of differential pressure flow meters are: Orifice Plate, Flow Nozzle, Venturi, and the Pitot Tube.

## Orifice Plate

With an orifice plate, the fluid flow is inferred through the difference in pressure from the upstream side to the downstream side of a partially obstructed pipe. The plate obstructing the flow offers a precisely measured obstruction that narrows the pipe and forces the flowing fluid to constrict.

Orifice plate meters are simple, cheap and can be delivered for almost any low viscosity application in any material.
The turndown ratio for orifice plates are less than 5:1. Their accuracy is poor at low flow rates. A high accuracy requires that the orifice plate be in good shape, with a sharp edge to the upstream side. Wear to the plate edges reduces the accuracy.

## Flow Nozzle Flow Meter

Flow nozzles are often used as measuring elements for air and gas flow in industrial applications. The flow nozzle is relative simple and cheap, and available for many applications in many materials. The turndown ratio and accuracy are similar to the orifice plate.

## Venturi Tube Flow Meter

Due to simplicity and dependability, the Venturi tube flow meter is often used in applications that require higher turndown rates, or lower pressure drops, than the orifice plate can provide.

In the Venturi Tube the fluid flow rate is measured by reducing the cross sectional flow area in the flow path, generating a pressure difference. After the constricted area, the fluid passes through a pressure recovery exit section, where up to 80% of the differential pressure generated at the constricted area, is recovered.

With proper instrumentation and flow calibration, the Venturi Tube can measure over a 10:1 turndown.

## Pitot Tube Flow Meter

The Pitot tube was invented by the French engineer Henri Pitot in the early 18th century. It is widely used to determine the airspeed of an aircraft and to measure air and gas velocities in industrial applications. The Pitot tube is used to measure the local velocity at a given point in the flow stream and not the average velocity in the pipe or conduit.

The Pitot tube measures the fluid flow velocity by converting the kinetic energy of the flow into potential energy.

The use of the Pitot tube is restricted to point measuring. With the "annubar", or multi-orifice pitot probe, the dynamic pressure can be measured across the velocity profile, and the annubar obtains an averaging effect. The Pitot tube is a very common (and inexpensive) way to measure fluid flow, especially in air applications for ventilation and HVAC systems.

They are easy to install and have a very low pressure loss. Typical turndown is 10:1.

-Presented by Max Machinery, Inc. the Positive Displacement Meter Experts