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Jun 2001
Detectors
Agilent 6890 Gas Chromatograph Service Manual
Thermal Conductivity Detector (TCD) 330
Theory of operation
Delta PRT
TCDs manufactured after April 1997 use a second temperature sensor in the
detector block to increase filament life.
The temperature sensor installed with the TCD heater sends data only to the
main board, which shuts down the detector if the amount of voltage applied
to the heater does not correspond to the actual temperature reading from the
sensor. In TCDs without a ∆PRT, the filament temperature is set at one of two
setpoints, 310°C or 440°C, depending on the whether the detector block
setpoint is above or below 200°C. In TCDs with a ∆PRT, the ∆PRT sends block
temperature data directly to the detector interface card, which then
automatically adjusts the filament voltage. Filament temperature is
maintained at a relatively constant difference above detector operating
temperature. See Figure 330-3 below. This adjustment increases filament life
and minimizes changes in detector sensitivity based on the filament/detector
block temperature difference.
Figure 330-3 Filament temperature versus detector block temperature
50°C
135
°C
∆ T
100
200
300
400
500
0
0
100
200
300
400
Detector temperature (body temp) °C
Filament temp (°C)
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