Application

Sequence of Operation
Control Loop - Damper Application
10
Siemens Industry, Inc. Application Note, Apps 2941/2942 140-1319
Restricted 2015-11-04
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.
ODP Face Velocity
FACE VEL = EXH VOL / FACE AREA
The average value of AVE FACE VEL is displayed at the ODP. The following two
points help stabilize the displayed face velocity:
 DISPLAY WT assigns a weighted calculated value to the face velocity. If DISPLAY
WT is set to 100%, the value of face velocity displayed at the ODP is the same as
the value displayed at FACE VEL 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.
 DISPLAY RES is the COV limit for exhaust volume readings.
NOTE:
If you set DISPLAY RES to zero, the ODP continuously displays the value of FVEL
STPT. Resetting DISPLAY RES to a value other than zero displays the face velocity
incrementally. The factory default is 5, fpm values displayed will be in increments of 5
(for example, 80, 85, 90, 95, 100, etc.). If the actual filtered fpm is 84, 85 will be
displayed.
These two points affect the ODP display only; they do not affect the values in the
controller or values sent to the network.
Control Loop - Damper Application
The PID loop controls the damper based on the values of EXH VOL and EXH STPT.
The loop output, DMPR CMD, controls EXTN DO1 and RETC DO2 through a time
modulation scheme. The DMPR CMD ranges of -100 to 100%.
 -100% is the maximum extend which closes the damper at full speed.
 0% holds the damper at its current position.
 100% is the maximum retract which opens the damper at full speed.
For example, if you command the point to -50, the damper will still close the drive but
not as quickly as if it were commanded to -100.