3. OPERATION
Page 41 © 1998 DH Instruments, Inc.
3.3.1 CONTROLREF
PURPOSE
To specify whether a vacuum source or atmospheric pressure is connected to the PPC2 AF
EXHAUST port.
PRINCIPLE
The PPC2 AF exhaust (down) control valves and the output of the exhaust pressure regulator
are connected to the PPC2 AF EXHAUST port (see Section 1.2.7).
In order for PPC2 AF to set pressures under atmospheric pressure, the EXHAUST port must
be connected to vacuum. In addition, the pneumatic module pressure regulators assure a
constant differential across the control valves of 3 psi (25 kPa) assuring predictable operation
of the control valves. For the regulators to operate properly, there must be a minimum
difference of 3 psi (25 kPa) between the pressure in the controlled volume and the supply
pressure (in the case of the inlet regulator) or the exhaust pressure (in the case of the
exhaust regulator). Applying a vacuum to the exhaust port lowers the pressure on the down
side of the regulator to achieve the desired differential pressure.
Connecting a vacuum pump to the PPC2 AF EXHAUST port lowers the pressure on the down
side of the exhaust pressure regulator. The ControlRef function is used to inform the
PPC2 AF that a vacuum has been applied to the EXHAUST port so it can take this into
consideration in its control routines.
OPERATION
To access the ControlRef function, press the SETUP
menu key and then 1ControlRef. The display is:
Control reference:
1Vacuum 2Atm
Make the appropriate selection, 1Vacuum if a vacuum supply is connected to the EXHAUST
port and 2ATM if it is open to atmospheric pressure. The selection will be maintained until it
is changed.
Pressure control near and under atmospheric pressure will not operate properly if
ControlRef is not set correctly.
The ControlRef function setting is not range or control mode specific. The setting made
in ControlRef setting will remain the same and apply across transducers and ranges as
they are changed.
See Section 2.3.4 for information on vacuum pump requirements and connection.
3.3.2 RESLTN
PURPOSE
To set the resolution with which the measured pressure is displayed on the main run screen.
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