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The velocity of an air stream in a duct is not uniform over the cross section of the duct. This is because friction against duct walls causes the air velocity to be lower near the sides than the velocity in the center, creating a parabolic velocity profile. Continue reading “Duct Traversing for Average Air Velocity and Air Volume”
Dwyer Instruments offers a multitude of sensors for monitoring air velocity in HVAC systems. Some of this instrumentation has a simple construction (Pitot tubes, for example) while others are more complex, such as hot-wire anemometers.
The initial term and first “hot-wire anemometer” was developed back in 1914 by Louie Vesso King. He is also accredited for King’s Law, which mathematically describes heat transfer in air flows using a heated wire. As the air moves over the wire, it causes a loss of temperature in the wire and removes some of the wire’s heat energy. Continue reading “The Basics of Air Velocity Sensors”
There are two common ways to measure air velocity: by using pressure-based instrumentation or by using temperature-based instrumentation. Before we discuss the different technologies available for measuring velocity pressure, it is important to understand the basics of air velocity.
Differential pressure measurement can be beneficial in a wide variety of building automation system (BAS) applications.
One of these applications involves determining if a filter in your system needs to be cleaned or replaced. Filter health can be established by monitoring the pressure on the primary and secondary sides of a filter. A clean filter will typically have a baseline differential pressure generated as air flows through it. As the filter becomes dirty, this differential pressure increases.
When using a mechanical flow sensor, such as a pitot tube, differential pressure can also be used to determine flow velocity and volumetric flow. Continue reading “Velocity Measurement and Environmental Effects”
If you’ve ever spent time in a hospital, you may have noticed a device similar to a home thermostat mounted on the wall with a display stating “ACH”. What exactly does “ACH” stand for and why is it important in a building or hospital? Continue reading “What is ACH and Why is It Important?”