Yokogawa GC1000 Mark II Process Gas Chromatograph - Manual (Page 18)

<Toc> <Ind> <1. Principle of Gas Chromatograph >
1-3
IM 11B03A03-01E 3rd Edition : Aug. 23. 2006-00
1.3 Detector
The components separated in the column are led to the detector where the concentration
of each component is measured.
The GC1000 Process Gas Chromatographs can be fitted with thermal conductivity detec-
tors (TCD), flame ionization detectors (FID) or flame photometric detectors (FPD). The
thermal conductivity detector can measure almost all non-corrosive components but
sensitivity is relatively low. On the other hand, the hydrogen flame ionization detector can
measure hydrocarbon and the flame photometric detector can measure sulfur compounds,
respectively with high sensitivity.
(1)Thermal Conductivity Detector (TCD)
The TCD utilizes the difference in the thermal conductivity between the measured gas and
the carrier gas and detects the unbalanced voltage produced in a bridge circuit as a mea-
sure of concentration.
Figure 1.3 shows the fundamental principle of the TCD. As shown, there are two streams,
each having two filaments. One stream passes the carrier gas only and the other, con-
nected to the column outlet, allows the measured gas to pass during analysis. The fila-
ments in the two streams form a bridge circuit such that the filament in one stream is
adjacent to the filament in the other stream. The unbalanced voltage in the bridge is propor-
tional to the concentration of the measured gas (liquid) component.
The TCD is frequently used to measure the component concentration of the measured gas
(liquid).
F0103.EPS
Comparison
filament
Comparison
filament
Measurment
filament
Measurment
filament
Output
Z1 Z2
Z4 Z3
Carrier gas
Carrier gas
+
Sampling gas
Constant voltage
Figure 1.3 Fundamental Principle of Thermal Conductivity Detector