Emerson Rosemount 700XA, Reference Manual

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Figure 1-2: Analyzer assembly with TCD bridge
A. Detector block (in heated oven section of analyzer)
B. Reference flow (carrier gas)
C. Measurement flow ("MV")
D. Signal out
E. Preamplifier (in analyzer electronics housing)
F. Detector bridge
G. DC power
H. Valves, columns, etc.
In the quiescent condition, prior to injecting a sample, both legs of the bridge are exposed
to pure carrier gas. In this condition, the bridge is balanced, and the bridge output is
electrically nulled.
The analysis begins when the sample valve injects a fixed volume of sample into the
column. The continuous flow of carrier gas moves the sample through the column. As
successive components elute from the column, the temperature of the measurement
element changes.
The temperature change unbalances the bridge and produces an electrical output
proportional to the component concentration.
The differential signal developed between the two thermistors is amplified by the
preamplifier.
Figure 1-3 illustrates the change in detector electrical output during elution
of a component.
Reference Manual Overview
2-3-9000-744 June 2022
Rosemount 700XA 15