Application

Sequence of Operation
AO2 Flow Signal
8
Siemens Industry, Inc. 140-1341
Restricted 2015-11-09
AO2 Flow Signal
An analog signal of the exhaust flow is available at AO2 of the controller board and is
displayed at EXH SIG AO2. To get an output from EXH SIG AO2, you must command
AO2 RANGE to the maximum expected flow for the fume hood. Then AO2 is scaled
such that 1 Vdc is equal to 0 cfm, and 10 Vdc is equal to AO2 RANGE. If the output
drops below 1 Vdc, this indicates a GENERAL FAILURE or loss of power.
EXH SIG AO2 = (EXH VOL/AO2 RANGE x 9 volts) + 1 volt
AO2 V MIN allows the minimum AO2 output voltage to be modified.
NOTE:
If the AO2 V MIN is set to 0, failure detection is lost.
The value of AO2 is determined by the following conditions:
Operating Condition
AO2 FLOW SIG Value
Normal/warning condition inside the range of
AO2 DEADBAND
EXH STPT
Normal/warning condition outside the range of
AO2 DEADBAND
EXH VOL
Alarm and emergency condition
AO2 DEADBAND defaults to 5.2%. If EXH STPT is 1000 cfm, AO2 FLOW SIG = EXH
STPT if the actual flow is between 948 and 1052 cfm (these values are approximate
and will vary based on duct area). If the actual flow is outside these values AO2 FLOW
SIG = EXH VOL.
For stable pressure reading, lower the AO2 DEADBAND. For unstable pressure
readings, raise the AO2 DEADBAND until the output signal stabilizes.
AO2 DEADBAND can be set from 0 to 102% in 0.4 % increments. 0% will give the
actual flow all the time. This signal may be too unstable to give a stable output and will
cause short-term room instability during fume hood sash movements. 5.2% removes
most of the signal bounce, maintains tighter control of actual flow changes and
maintains room stability during fume hood sash movements. A 10% deadband is equal
to a ± 5% of the flow. Any value over 100% will turn the feature off and revert to
standard control.
The average value of AVE EXH VOL is displayed at the ODP. The following two points
help stabilize the displayed face velocity:
 DISPLAY WT assigns a weighted calculated value to the exhaust volume. If
DISPLAY WT is set to 100%, the value of exhaust volume displayed at the ODP is
the same as the value displayed at EXH VOL and may fluctuate rapidly. When
DISPLAY WT is set to a value less than 100%, the displayed value updates less
frequently.
NOTE:
If you set DISPLAY WT to zero, the ODP “freezes” at the last read value. Resetting
DISPLAY WT to a value other than zero allows the Operator Display Panel to update
the displayed value.