892 J006 MNAH | PGC5000 G EN 2 | 85
5.2.2 Thermal conductivity detector (TCD)
The Thermal Conductivity Detector (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.
A multiport version of the Thermal Conductivity Detector (MTCD) is available to achieve more
measurements in a smaller space.
5.2.3 Flame photometric detector (FPD)
The Flame photometric detector (FPD) works on the principle that when sulfur is burned in a
hydrogen-rich atmosphere, luminescence (light) characteristic to sulfur is produced. The FPD cell
consists of a burner chamber equipped with a spark ignition system and a photomultiplier tube which
is thermoelectrically temperature controlled. A narrow bandpass filter optically connects the burner
chamber and the photomultiplier tube. An exponential amplifier conditions and amplifies the
photomultiplier tube output to provide a linear output over a wide dynamic range. Sulfur addition
permits accurate measurement of low-level sulfur compounds.
When a sulfur compound passes through the hydrogen-rich flame, strong luminescence occurs
between 320 and 460 nm. The narrow band-pass filter allows a sulfur spectra-centered transmission
at 394 nm ± 5 nm to achieve a specific ratio of sulfur to non-sulfur compounds between 10,000 and
30,000:1. A photomultiplier tube views the filtered light and outputs a voltage proportional to the
intensity of the filtered luminescence. For maximum sensitivity, the detector is optimized with respect
to temperature, gas flow rates, and bias voltage on the photomultiplier tube.
Because the amount of sulfur in the sample is very small, a sulfur addition module provides a
standard level of sulfur. This keeps the sulfur readings above the noise level within the analyzer.
When the analyzer processes a sulfur compound, the sulfur in the sample adds to the standard sulfur,
providing a level more easily measured by the photomultiplier tube.
The FPD Electrometer PCB, located inside the electronics compartment of the oven, digitizes the
detector output signal for processing. This PCB assembly also provides energy for FPD flame sensing
and ignition.
5.2.4 Optional Dielectric Barrier Discharge Ionization Detector
(DBDID)
The Dielectric Barrier Discharge Ionization Detector (DBDID) consists of a detector unit and a power
supply. (See Table 5-1 for parts information).
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