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Measure Air Velocity With Precision Using Professional Flow Meters!

Calculating average flow rates through ducts, pipes, hoods and stacks is simple. The cross-sectional area of the duct is multiplied by the average air velocity to find the volume per time or flow rate. On the other hand, gathering data accurately and precisely to measure air velocity across ducts, has been more difficult due to differing degrees of friction across a duct.

Poor data collection produces the errors in duct balancing. The challenge is; “How to acquire the average flow rate measurement across differing cross-sections of duct – and how can using professional flow meters help?”

Before addressing the problem lets clarify what we mean by “air velocity” and “professional flow meters”.

Air velocity in a duct, pipe, hood or stack is the distance travelled by air per unit of time and is commonly expressed as m/s or FPM.

A professional flow meter is a scientific instrument that allows the user to determine air flow in a volumetric measurement. These devices enable a technician to quickly and accurately determine the air volume flowing past a specific point in a duct per unit of time. This is commonly expressed in l/s (Litres per second) or CFM (cubic feet per minute).

Common Applications for “Professional Flow Meters”

Flow meters are most commonly used by HVAC professionals – that is, technicians who work in the heating, ventilation and air conditioning (HVAC) services.

Applications where more accurate measurements of air velocity through a duct matter using flow meters include:

  • Balancing air conditioning ducts
  • Using flow meters to measure the efficiency of fume hoods, exhaust fans and rangehoods
  • Measuring air flow using flow meters through any system that has a duct, such ventilation shafts in a commercial building.

The Physics of Air Flowing Trough a Duct

The measurement challenge can be summarised as follows:

  • Air is affected by friction the closer it is to the edge or surface of the duct
  • The greatest air velocity is in the frictionless middle of the cross-section of the duct
  • The velocity profile of the duct is dependent on the shape of the duct and the force pushing the air
  • Minimizing perimeter walls to achieve greater cross-sectional area will increase the frictionless area in a duct - therefore, the most efficient shapes of ducts are round, square, and rectangular in that order.

How to Make Measurements for a Good Database Using Flow Meters - The Log-linear Method

Considering the cross-section at any point of a duct, the grid lines which lay out the duct flow measurement points for flow meters are traverses. The log-linear method provides high accuracy (±3%) in flow totalization by preferentially measuring air flow closest the edges of the duct space. Now, the question is how to measure cubic meters per minute (cmm) in a duct using flow meters? This all depends on the shape of the duct itself.

Air Flow in Round Ducts

The log-linear traverse for round ducts - three diameter approach. Three traverses, diameters, evenly spaced at 60° create six pieces of pie in a round duct. Three air flow measurements using flow meters are taken per radius: on the edge; one-third toward the centre; two thirds toward the centre. A total of eighteen readings accurately describes the air flow rate.

An alternative method, the log-linear traverse for round ducts - two-diameter approach uses twenty data points but will not produce as accurate an average using flow meters as the eighteen points along three traverses.

Air Flow in Rectangular or Square Ducts

Accuracy using this method demands a minimum of twenty-five data points - up to a maximum of forty-nine. A duct side of less than 75 cm requires five traverses. A duct side of greater than 75 cm requires seven traverses.

By gathering the data from these readings when using a flow meter, you can accurately calculate the mean average air flow in a duct. Alternately, let the microprocessor in your flow meter do the work. The automated processing capacity of professional flow meters, as well as error correction features enable HVAC technicians measure air velocity efficiently and with precision.

Examples of Professional Flow Meters

Modern air flow meters like handheld anemometers offer digital read outs in cubic meters per minute - self-contained calculators to save professionals time and frustration. Here’s some examples:

Testo 420 Air Flow Hood - IC-0563-4200

 

This professional flow meter measures air flow velocity, volume flow, absolute pressure, temperature and relative humidity.

For analysis and documentation, measurement data can be saved, and measurement protocols created directly on site using a free app that facilitates access through a PC.

The built features of this professional flow meter drastically reduce measurement errors such as zero function for low pressures and integrated volume flow straightener for wind measurements.

Click here for a detailed description and specifications for this flow meter.

Thermo-anemometer with hotwire - VT110S

 

This flow meter features a wired probe, hold-min-max functions, adjustable automatic shut-off, and adjustable backlight. Settings and calculations include airflow calculation, airflow calculation with cone, automatic average, and detection of flow direction.

Click here for a detailed description and specifications for this flow meter.

Kestrel 5200 Professional Environment Meter - IC-0852HVG

 

This environment meter also has the capacity to operate as an air flow meter. In addition, it features distinct functions for evaporation rate, Delta T, relative air density and air density, measures altitude, pressure trend, barometric pressure, wet bulb temperature, relative humidity in %, temperature, heat stress index, dewpoint, and density altitude. Its in-built data logger has customizable data storage for up to 10,000 data points. The on-board microprocessor of this flow meter calculates minimum, maximum and average values.

Click here for a detailed description and specifications for this flow meter.

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