ADM-870 05/23/0835
Backpressure compensation requires two readings in immediate sequence. The first reading is the flaps open
reading discussed above. The second reading is done with the flaps closed. The meter automatically
determines the required backpressure compensation calculated from the relationship between the two readings
and displays the compensated result. A backpressure compensated reading must always be immediately
preceded by a nonbackpressure compensated (flaps open) reading at the same location. The meter will
register incorrect readings if the CF: UNCORR reading is taken at another time or any other location. Any
adjustments or changes to the air flow system between the two measurements will result in inaccurate results.
This technique is used for both supply and return outlets. Backpressure compensation readings may be
performed at any flow rate, but are usually unnecessary at flows less than 80 cfm.
First, perform the nonbackpressure compensated portion of the measurement with the flaps open, as described
above. Close the flaps when the meter displays CALC. When the first reading is displayed, press the READ
key to take the second (backpressure compensated) reading. If necessary, the FlowHood may be removed
from the outlet in order to close the flaps. Reposition the FlowHood as before, and wait two or three seconds
for the flow to stabilize before taking the second reading. The FlowHood may be removed from the outlet as
soon as CALC is displayed. The compensated reading will be displayed within two seconds.
11.2.1 ERROR
The meter may display ERROR following a backpressure compensated measurement in which the numerical
ratio of the two parts of the measurement sequence exceeds predetermined limits. Normally this indicates that
the operator has made a procedural error, or that a dynamic change (such as a changed damper setting) has
occurred between the two parts of the backpressure compensated measurement process. ERROR may also
be displayed as the result of measurements taken at extremely low resistance outlets (less than .003 in wc @
500 cfm). These outlets are not suitable for the backpressure compensation method, and may require other
means of air flow measurement.
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