How to Measure Air Velocity

Do you need to know how to measure air velocity for HVAC, Critical Containment, Electronics, or Automotive Applications?

How to measure air velocity correctly in ducts is a frequent question from HVAC professionals.

How to Measure Air Velocity

Many industries call for the measurement of airflow or air velocity – sometimes with additional requirements for determining air temperature, air pressure, or humidity. This is true in electronics and automobile engineering, as well as critical containment applications in laboratories and pharmacies, and HVAC may lead the list with their need to measure airflow in duct work.

Air velocity or distance traveled per unit of time is most often exhibited in feet per minute (FPM). Multiplying air velocity by the area of a duct determines the volume of air flowing past a point in the duct during specified unit of time. Volume flow is typically measured in cubic feet per minute (CFM).

Instrument Used to Measure Air Velocity:
A Hot Wire Anemometer is an instrument for measuring velocity and direction of fluid (in this case air) by measuring heat loss of an electrically-heated wire placed in the air stream. When placed in an air stream the heat is transferred from wire to the air and the temperature of wire reduces. Resistance of the wire measures the flow rate of the air.

We had HVAC professionals in mind when we designed airflow sensors that measure air velocity, temperature, and humidity. Just 4 easy steps make installation a breeze! It only requires a single hole and there’s no need for screws or getting your hands inside the duct. Features include:

  • Conformal-Coated Electronics – robust, UV-tolerant, splash-proof enclosure for use in corrosive or alkaline environments
  • Innovative “Outside the Duct” Installation – single hole for mounting sensor assembly, without the need for screws or hands inside the duct
  • Remote Sensor Head Options – multiple designs are available to support a wide range of application scenarios
  • Configured-to-order – choice of air velocity range, supply voltage, mechanical length, and output communication style
  • Analog & Digital Communication – supports 0-5V, 0-10V, 4-20mA, and digital UART or I²C communication outputs
  • Universal Supply Voltage – suitable for applications where AC or DC supply voltages are encountered
  • Single or Multi-Channel Data Acquisition up to 180 sensors measuring air velocity, air temperature, surface temperature, humidity, & barometric pressure data in real-time
  • Real-Time Data Logging – of air velocity, temperature, humidity, and pressure with compensation for changing humidity and barometric pressure (altitude)
  • Graphing Features – Line Graph and Gradient Graph features allow users to simply drag-and-drop the desired sensors into an easy-to-view graphing section, and export in jpeg format

Air Velocity Measurement with Embedded Sensor in Duct

Degree Controls has the products and experience to meet the many airflow measurement and testing scenarios faced by our clients.

We met application specific requirements to convey average velocity across a duct area to the client’s BMS (Building Management System) as a 0-10V analog output with a set of our F500 series sensors. For this large duct application, the sensors were modified to communicate in a master/slave arrangement over addressed UART communication:

  1. Each slave sensor communicates velocity reading(s) to its neighboring slave, or master.
  2. The master sensor receives velocity readings from all slave sensors.
How to measure air velocity inside a duct using embedded sensors.

The master airflow sensor first records its own velocity reading and then averages all readings from sensors in the network. Next, the master sensor uses its 0-10V analog output to transmit the average velocity to the BMS or monitoring system.

All sensors are powered by universal 24VAC. Sensors are connected with a latching connector and 0.6 m [2 ft] of cable. The master sensor itself has 2 m [6.6 ft] flying leads to connect to the BMS system.

The master sensor can communicate to as many as 15 slave sensors. This flexible solution ships as a kit, and master and slave sensors already addressed.

How to measure air velocity inside circular or rectangular ducts based on the Tchebycheff Rule.

How to Determine Air Velocity based on the Tchebycheff Rule

The °C SPAR Sensor Pole Array from Degree Controls is optimal for in-duct HVAC air velocity analysis. It is a linear array of airflow sensors assembled into a single tube element with USB outputs. The °C SPAR is designed for multi-point experimentation where there are predefined measurement locations, just as shown in the Log-Tchebycheff Rule for calculating volumetric flow within ducts. With the °C SPAR, air velocity, temperature, and humidity can be measured and recorded at multiple points in real-time for building duct performance testing.

How to measure air velocity at multiple points within a duct.

Consult with an Airflow Sensor Specialist

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