2000-OSM, F1
4-4
The cell consists of a polarized jet, a collector, and a chamber in which hydrogen fuel and sample are
burned in air. A current, whose magnitude is proportional to the concentration of the sample, flows
between the jet and the collector. This current goes to the FID Amplifier Assembly.
The FID Amplifier Assembly provides ignition control for the FID cell and amplification of the detector
cell output signal prior to routing the signal to the control section for signal processing. It consists of a
power supply, ignitor assembly, electrometer amplifier, and associated circuitry. The power supply
provides the necessary dc voltages for the FID Amplifier Assembly and FID.
The ignitor assembly provides control for the ignitor in the detector cell. In the event of a flame-out,
the thermocouple senses the flame-out, illuminates the Flame-Out LED, and sends a message to the
ignitor assembly, which re-ignites the flame. The electrometer amplifier takes the FID cell’s output,
amplifies it, and sends it to the control section for signal processing.
Thermal Conductivity Detector (TCD)
Two types of thermal conductivity detector are used in this analyzer, a filament TCD and a thermistor
TCD (see Figure 4-4).
The filament TCD works on the principle that a heated body loses heat dependent on the composition
of the surrounding gas. Filaments sense the rate of heat loss so that any change in filament current
results in a bridge output proportional to the change. The TCD contains a reference filament and a
measurement filament. When the reference carrier gas flows across both filaments, the two filaments
have the same filament temperature and the bridge has a zero output. During a measurement cycle
a sample flows through the measurement filament, changing thermal conductivity and the filament
temperature, thereby causing a corresponding change in electrical resistance. The bridge senses
this change in current and increases or decreases current flow through the bridge to compensate for
the temperature change. The change in current flow sensed by the bridge is proportional to the
component concentration in the sample being measured.
Cable
Measurement
Vent
Reference
Vent
Measurement
In
Reference In
Measure
Thermistor
Reference
Thermistor
Thermistor TCD
Sample
Vent
Reference
Filament
Measure
Filament
Reference
Vent
Sample In
Reference In
Filament TCD
Figure 4-4. THERMAL CONDUCTIVITY DETECTORS
The thermistor TCD works on the principle that thermistor resistance varies inversely with
temperature change. The TCD contains a reference thermistor and a measurement thermistor,
connected to a comparator circuit. When the reference carrier gas flows across both thermistors, the
two thermistors have the same resistance and the comparator circuit generates a zero output. During
a measurement cycle a sample flows across the measurement thermistor, thereby creating a
resistance difference between the two thermistors. This resistance difference sensed by the
comparator circuit creates an output signal representing component concentration in the sample
being measured.