Application

Sequence of Operation
AVS Calibration
7
Siemens Industry, Inc. 140-1341
Restricted 2015-11-09
Sequence of Operation
The following paragraphs present the sequence of operation for Fume Hood Controller
Application 6740, 2-Position Constant Volume Controller with Damper.
The Fume Hood Controller can operate at two different setpoints, described as HI flow
and LOW flow setpoints. The controller can be set to an OFF mode. In this mode the
damper is closed and the controller enters into a standby mode of operation.
The Operators Display Panel II (ODP II) can display HI/LOW setpoint or output exhaust
flow in CFM. The Fume Hood Controller maintains the duct air flow at the flow setpoint,
EXH STPT. This is accomplished as follows:
EXH STPT — The EXH STPT will be equal to EXH LO STPT or EXH HI STPT, both of
which are set to field requirements. The LOW or HI set point is determined by the state
of DI2 STPT SW (DI 2) and ODP STPT SW (left ODP switch). The priority sequence of
operation is shown in the following table.
FLOW STPT Priority Schedule
IF
DI2 STPT SW
AND
ODP STPT SW
THEN
FLOW STPT
LO LO LO
LO HI HI
HI LO HI
HI HI HI
DO5 HI/LOW Output Functionality — HI.LO DO5 is ON while the controller is in LOW
flow operation and OFF when the controller is in HI flow operation.
DO7 Alarm Output Functionality —ALARM DO7 is turned ON when the ODP displays
an alarm condition and OFF during normal operation.
Air Velocity Sensing
Primary sensing of the exhaust air flow is through the OAVS. Optionally, AI3 may be
used to input a Differential pressure transmitter or a Linear Flow input signal.
AVS Calibration
Calibration of the air velocity transducer(s) is periodically required to maintain accurate
air velocity readings. Depending on the value of CAL SETUP, calibration takes place
either at fixed time intervals or whenever the application goes into unoccupied mode.
When calibration is in progress, CAL AIR equals YES. After calibration, CAL AIR
returns to NO.
The application uses Autozero Modules connected to AUTOZERO DO8. This means
that the exhaust flow control device does not close during calibration of the
transducers.
NOTE:
The FHC does not monitor Fume Hood flow changes for 3 seconds during AVS
calibration.