Averaging grids for velocity measurement in rectangular ducts

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Averaging grids for velocity measurement in
rectangular ducts
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Test holes for fan volumetric flow measurement
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Averaging grids for velocity measurement in
circular ducts upstream and downstream of the fan
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Averaging grids for velocity measurement in
rectangular ducts upstream and downstream of the
fan
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Averaging grid for velocitymeasurement at grilles
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Specifying Flow Rates and Tolerances

• A tolerance is the variation from the design
  flow rate and pressure that the designer will
  accept. The specification of a departure from a
  precise flow rate and pressure makes sense for
  two reasons -
• a). A small variation may make no detectable
  difference to the performance of the system from
  the point of view of occupant comfort or
  operating cost and
• b). The inaccuracy of the commissioning procedure
  will not allow for precise measurements to be
  made to check against the design figure.

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Accuracy of Flow Measurements

• The accuracy of an air flow measurement will be
• influenced by a number of factors -
• Accuracy of the instrument used for velocity
  measurement
• Accuracy of the measured duct cross-sectional
  area
• The effect of flow disturbances close to flow
  measurement points
• The accuracy of the velocity traversing technique
  used
• The accuracy achieved by the commissioning
  engineer.
• Because of these factors, it is not possible to
  achieve a
• high degree of accuracy and much time can be
  wasted
• in trying to meet fine tolerances.

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Tolerances on System Balance
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Checks at sub-branches and main branches

• When an anemometer is used for obtaining flow
  rates from a number of terminals, it is good
  practice to add up the results obtained and
  compare the totals with the flow rate obtained in
  the sub-branch serving the terminals. If the
  agreement is within ?15 then the measurement is
  taken as correct and the appropriate correction
  applied to all the terminals to get agreement
  between the two totals. If the initial comparison
  is ? 20 or larger, then some form of
  investigation is required.