Explain how to take a velocity pressure reading on a duct system. What units are used to measure air velocity?
What tools are used to measure air velocity? What units are used to measure air volume?
The Correct Answer and Explanation is :
To take a velocity pressure reading on a duct system, you need to use an instrument called a “manometer” or a “velometer” to measure the difference in air pressure across a specific point in the duct. Here’s a step-by-step guide:
Steps to Measure Velocity Pressure:
- Select the Measurement Point: Choose a location on the duct where airflow is steady, typically in the middle of the duct, away from bends, dampers, or other obstructions.
- Insert the Probe: Insert the manometer probe into the duct. The probe should be placed at a right angle to the airflow direction. You may need to drill a small hole in the duct to insert the probe.
- Measure the Pressure: The manometer measures the pressure difference between the high-pressure and low-pressure areas at the measurement point. This is known as the velocity pressure. The velocity pressure is proportional to the square of the air velocity.
Calculating Air Velocity:
Once you have the velocity pressure (measured in inches of water column, “WC”), you can use the following formula to calculate the air velocity (in feet per minute, FPM):
[
V = 4005 \times \sqrt{P_v}
]
Where:
- ( V ) = Air velocity in FPM
- ( P_v ) = Velocity pressure in inches of water column
Tools Used to Measure Air Velocity:
- Manometer: Measures pressure differences and calculates velocity pressure.
- Velometer: A type of anemometer specifically designed to measure airflow velocity directly by detecting changes in the airflow speed.
- Thermistor Anemometer: This type measures the heat loss from a heated sensor in the airflow and provides velocity readings.
Units Used to Measure Air Velocity and Volume:
- Air Velocity: Measured in feet per minute (FPM), meters per second (m/s), or miles per hour (mph).
- Air Volume: Measured in cubic feet per minute (CFM), cubic meters per hour (m³/h), or liters per second (L/s).
The units used depend on the system’s preference, but CFM is most commonly used in the United States for measuring air volume.