ADM-860C 04/14/0847
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 may be used for either supply or return outlets. Backpressure compensated readings may be
performed at any flow rate, but are usually unnecessary at flows less than 100 cfm.
First, perform the nonbackpressure compensated portion of the measurement with the flaps open, as described
above in Section 11.1 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.
11.2.1 MEMORY DISPLAY IN FLOWHOOD MODE
Backpressure compensated and noncompensated readings are recorded separately in memory. Individual
readings displayed using the RCL keys will have a small ‘sc’ for stored, backpressure compensated readings or
a small ‘su’ for stored, nonbackpressure compensated readings.
The average function will display two different averages. The average of just the backpressure compensated
readings will be displayed as nnc nnnn. The average of just the nonbackpressure compensated readings will
be displayed as nnu nnnn.
The sum function will display two different sums. The sum of just the backpressure compensated readings will
be displayed as
3c nnnn. The sum of the noncompensated readings will be displayed as 3u nnnn.
The maximum and minimum functions will each display two different maximums or minimums. The maximum
backpressure compensated reading will be displayed as xx
¸
c nnnn. The minimum of just the backpressure
compensated readings will be displayed as xx
ºc nnnn. The maximum nonbackpressure compensated reading
will be displayed as xx
¸
u nnnn. The minimum of just the nonbackpressure compensated readings will be
displayed as xx
ºu nnnn.
11.2.2 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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