CFM-88L 07/20/0921
First, perform the nonbackpressure compensated portion of the measurement with the flaps open, as described above in
Section 10.1 NONBACKPRESSURE COMPENSATED READINGS. This first reading will be displayed with a small ‘u’ for
uncompensated (nonbackpressure compensated) as CF u nn. Close the flaps after the reading is displayed. Allow the
system to settle, then 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. Position the FlowHood as before, and wait three
seconds for the flow to stabilize before taking the second reading. The compensated reading will be displayed with a ‘c’ for
compensated, as CF c nn.
10.2.1 RATIO ERR DISPLAY
The meter may display RATIO ERR following a backpressure compensated measurement in which the numerical ratio of the
two parts of the measurement sequence exceeds predetermined limits. Normally this indicates, either 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.
RATIO ERR may also be displayed as the result of measurements taken at outlets with extremely low pressure drops (less
than .003 in wc @ 500 cfm). These outlets may not be suitable for the backpressure compensation method, and may require
other means of air flow measurement.
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