DigitalFlow™ GC868 Startup Guide (1 and 2-Channel)
1-30
Chapter 1. Installation
Figure 1-38: Conventional and Fail-Safe Operation
1.8.6
Wiring a 0/4-20
mA Analog Inputs
Option Card
To calculate the standard volumetric flow rate of a gas, the Model GC868 requires
accurate
temperature
and
pressure data
from the measurement site. Transmitters
installed in the flowcell can provide this information via an optional 0/4-20 mA
analog input card. This option card includes two isolated 0/4-20 mA analog inputs
(designated as A and B), each of which includes a 24 VDC power supply for
loop-powered transmitters. Either input may be used to process the temperature
signal, while the other input is used to process the pressure signal.
Note:
To enter programming data during operation of the meter, it will be
necessary to know which input is assigned to which process parameter. This
information should have been entered in Appendix B,
Data Records
.
The analog inputs, which have an impedance of 118 ohms, should be connected with
standard twisted-pair wiring. Power to the transmitters may be supplied either by
the integral 24 VDC power supply on the analog input card or by an external power
supply. Figure 1-39 below shows typical wiring diagrams, with and without an
external power supply, for one of the analog inputs.
C
NO
NC
NC
C
NO
NC
NC
C
NO
C
NO
Monitoring
Alarm
Device
Monitoring
Alarm
Device
Monitoring
Alarm
Device
Monitoring
Alarm
Device
not triggered
Conventional,
not triggered
Fail-Safe,
triggered
Conventional,
triggered or power failure
Fail-Safe,
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