SEP 2005 MODEL 1000
DESCRIPTION
1-12
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
bridge can be balanced by the fine and coarse adjustment potentiometers located on the preamplifier
circuit board.)
The analysis begins when a fixed volume of sample is injected into the column by operation of the
sample valve. The sample is moved through the column by the continuous flow of carrier gas. 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.
Figure 1-3. Detector output during component elution
1. Detector bridge balanced.
2. First component begins to elute from column and to be sensed by the measurement
thermistor.
3. Peak concentration of first component.
4. Second component begins to elute from column and to be sensed by the measurement
thermistor.
5. Peak concentration of second component.
In addition to amplifying the differential signal developed between the detector's two thermistors,
the preamplifier also supplies drive current to the detector bridge. The preamplifier also supplies
drive current to the detector bridge. The voltage signal is converted to a 4 to 20-milliamp (mA)
current loop for transmission to the GC Controller. The signal is proportional to the concentration
of a component detected in the gas sample. The preamplifier provides four different gain channels
as well as compensation for baseline drift. The signals from the preamplifier are sent to the GC
Controller for computation, recording on a printer, or viewing on a PC monitor or LCD.
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