![]() A duct side of greater than thirty-six requires seven traverses. A duct side of less than thirty inches requires five traverses. Rectangular or Square DuctsĮxample of a 25-point log linear-traverse for rectangular ducts.Īccuracy demands a minimum of twenty-five data points to a maximum of forty-nine. These twenty data points will not produce as accurate an average as the eighteen with three traverses, but the results are acceptable. The fifth point is two-thirds toward the center. The first four evenly spread over the first half of the radius starting at the edge and moving toward the center. In the case where only two traverses can be measure, set them at 90 degrees and take five samples on each radius. Note the air most affected by friction is seemingly over-represented.Ī total of eighteen readings accurately describes the air flow velocity. Three measurements are taken per radius: on the edge one-third toward the center twothirds toward the center. Three traverses, diameters, evenly spaced at 60° create six pieces of pie in a round duct. Log linear-traverse for round ducts, threediameter approach. Log-linear traverse for round ducts, two-diameter approach. The log-linear method provides high accuracy (☓%) in flow totalization by measuring air flow closest the edges of the duct space preferentially. The grid lines which lay out the duct flow measurement points are traverses. Given these facts, how many measurements make a good database? The preferential shapes of ducts then are round, square, and rectangular in that order of efficiency. Industrial Air Velocity/Temperature Transmitter The velocity profile of the duct is dependent on the shape of the duct (minimizing perimeter walls to achieve cross-sectional area) and the force pushing the air The greatest velocity is achieved under laminar flow conditions in the frictionless middle of the cross-section The more precise question is how to measure the average velocity of air across differing cross-sections of duct.Īir is slowed by friction with contact to the edge of the duct The newest microprocessor-based anemometers complete duct air flow measurement data collection accurately even before the patience of the HVAC tech wears out. Air flow measurement instruments, anemometers, have been limited in the past by time. And, poor data collection procedures produce the errors in duct balancing. Simple.ĭata collection, accurately and precisely measuring air velocity across ducts, has been a difficult task. ![]() The cross-sectional area of the duct is multiplied by the average velocity of the fluid to find the volume per time or flow rate. Air Flow Measurement Instruments: How Precise and Accurate?Ĭalculating flow rates through ducts, pipes, hoods and stacks (collectively called ducts for our purposes), has never been difficult.
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