Pitot Tubes

11/29/17

“How can we measure the velocity of a fluid without using any moving parts?”

 

Measuring the velocity of a fluid is one of the most useful things we can do. With this, we can find out how much mass is flowing within a system, and adjust all calculations accordingly. But since fluids lack any form of defined shaped, measuring their average velocity can be very difficult. So how can we use our engineering mindset to solve this problem? Well, to begin, let’s look at how pressure moves within a system. A fluid’s total pressure is made of up both static pressure (the default, inert pressure) and velocity pressure (the pressure associated with the momentum of the fluid).

 

Since it is rather simple to obtain the total and static pressures and use their values to find the final velocity, let’s build a machine to do exactly that. Since fluids move steadily through a pipe, let’s start with that. And since we want to find the total velocity of a fluid, let’s also put the fluid through the hole. Then let’s also have holes perpendicular to the main tube to measure the static pressure. Then let’s subtract the difference to get the velocity pressure, and divide by the fluid’s density to obtain the fluid velocity. This machine is known as a pilot tube and is used widely in airplanes to measure the airspeed and HVAC systems to find the refrigerant flow rate.