Aalborg DFC Digital - Manual (Page 15)

5.4 Set Point Reference Signal
DFC flow controllers have a built-in solenoid valve and allow the user to set the
flow to any desired flow rate within the range of the particular model installed. This
valve is normally closed when no power is applied.
The set point input in analog mode responds to an analog 0 to 5 VDC reference
voltage or 4-20mA reference current. This voltage is a linear representation of 0
to 100% of the full scale mass flow rate. Response time to set point changes are
1 second to within 2% of the final flow over 25 to 100% of full scale.
A variable 0 to 5VDC analog signal may be applied directly to the SET POINT and
COMMON connections of the DFC transducer (see Figure b-1).
If a potentiometer is used to adjust the set point reference signal its value should
be between 5K to 100K ohm and it should be capable of at least 10-turns or more
for adjustment.
5.5 Valve OFF Control
It may, at times, be desirable to set the flow and maintain that setting while being
able to turn the flow control valve off and on again. This can be accomplished via
pin 5 on the 25-pin "D" connector. When 0 VDC (LOW) signal is applied (connection
via a relay, switch or NPN open collector transistor is permissible), the solenoid
valve is not powered and therefore will remain normally closed. Conversely, when
the pin is disconnected from 0 VDC ("floating”) the solenoid valve will remain active.
The simplest means for utilizing the VALVE OFF control feature, is to connect a
toggle switch between the COMMON and FORCE VALVE CLOSED pins of the
DFC transducer. Toggling the switch on and off will allow for activating and deac-
tivating the solenoid valve.
5.6 Valve Open /Purge
At times, it may be necessary to purge the flow system with a neutralizing gas
such as pure dry nitrogen. The DFC transducer is capable of a full open condition
for the solenoid valve, regardless of set point conditions. Connecting the FORCE
VALVE OPEN pin (pin 4 on 25-pin "D" connector) to ground will fully open the
valve. This connection can be made with a relay, switch or NPN open collector
transistor. Conversely, when the pin is disconnected from 0 VDC ("floating”) the
solenoid valve will remain active. (Note: in digital mode hardware I/O overrides
software command).
The simplest means for utilizing the VALVE OPEN control feature, is to connect a tog-
gle switch between the COMMON and FORCE VALVE OPEN pins of the DFC trans-
ducer. Toggling the switch on will cause the valve to open fully and purge the system.
Toggling the switch off will allow the solenoid valve to resume normal activity.
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Caution: If the valve is left in the AUTO (control) or OPEN mode for an
extended period of time, it may become warm or even hot to the touch.
Use care in avoiding direct contact with the valve during operation. ƽ