# Relationship between pressure and flow velocity measurements

### Fluid Velocity Measurement Using a Pitot Tube (Pitot Static Tube)

This difference, called the dynamic pressure, can be used to calculate the fluid velocity at the point of measurement. The fluid density is also needed in order to. The v-dp relation of a proximal stenosis was not influenced by a distal stenosis. The diameter of the segment where flow velocity was measured influenced the. Air velocity can be measured by sensing the pressure produced by the movement of the air. This application note will describe the basic relationships between.

### Bernoulli's Equation

Anyone who has put their hand out the window of a moving car has experienced the force applied by moving air. The figure below shows how this might look.

The tube that is placed into the air stream is called a pitot tube after Henri Pitot, the French engineer of the early 18th century who invented it. As the velocity of the air increases the pressure also increases inside the pitot tube with respect to the ambient atmosphere. Note that the pitot must be pointed directly into the flow — if the tube is mounted at some angle to the direction of flow, the transducer will not sense the full pressure developed by the air velocity.

The pressure developed by the air velocity is called the velocity head, and it is affected by the density of the air. The density of the air, in turn, is a function of the local atmospheric pressure and the temperature.

The equations that relate all these factors are: Note that to determine the air velocity the density must first be known.

## Fluid Velocity Measurement Using a Pitot Tube (Pitot Static Tube)

This is the second equation and relates ambient atmospheric pressure and temperature to density. Assuming average conditions of 70 F and a barometer of If, for example, we measure a differential pressure from the pitot tube of 2. Air velocity is a function of air density and differential pressure, but determining air flow requires that the geometry of the piping be taken into account.

Its measurement is illustrated with the third U-tube in the diagram at the left. The stagnation pressure is then given by the following equation: Velocity Measurement with a Pitot Tube For pitot tube measurements and calculations, the reference plane is taken to be at the height of the pitot tube measurements, so the equation for stagnation pressure becomes: This is a concentric pitot tube.

The inner tube has a stagnation pressure opening perpendicular to the fluid flow and the outer tube has a static pressure opening parallel with the fluid flow.

Example Calculation Consider a pitot tube being used to measure air velocity in a heating duct. The air is at 85 oF and 16 psia.

**Fluids - Lecture 3.3 - Flow Rate Measurement**

The pitot tube registers a pressure difference of 0. Calculate the velocity of the air at that point in the duct?

Convert the pressure difference of 0. Now V can be calculated: