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the output may go to zero and become unresponsive. If this occurs, adjust
your injection site and conditions to avoid over range conditions.) To adjust
for this increase in resistance, set the Filament Temperature Limit to 490
°
C,
even for filament temperature setpoints below 390
°
C.
The TCD filament is automatically protected in the GC. The GC turns off the
filament current after four minutes if it detects that the carrier gas flow has
stopped or that the filament current is low. Also, the GC turns off filament
current when it detects that carrier gas is flowing through one side of the
detector cell but not through the other (e.g., a septum or column is not
installed, or a leak exists in one cell and not the other).
If you select Helium or H
2
as the carrier gas, the GC operates in the standard
way to protect the TCD filament. However, because the filament protection
feature in the GC operates through the detection of air (or N
2
) in the detector
cells, the filament protection feature is disabled when you use nitrogen or
argon as the carrier gas.
Set the Filament Temperature Limit to 390 °C or 490 °C, according to the
highest filament temperature setpoint required. With the Limit set to 390
°
C,
the peak sample filament temperature is limited to approximately 450
°
C.
The temperature limit is approximately 550
°
C at the 490
°
C setting. The
450
°
C limit protects filaments from oxidation when exposed to air over an
extended period of time. However, at the 550
°
C temperature limit the
filament can degrade after only a few minutes.
CAUTION
If you select N
2
/Ar as the carrier gas, the filament protection feature is
disabled. Because the GC does not turn off the filament current, the
TCD filaments can rapidly oxidize if you operate the TCD filament
temperature above 390
°
C without carrier gas flowing through the
TCD cells. If you plan to operate the TCD after the carrier gas has
been turned off, purge the air from the TCD cells with carrier gas for
five minutes before operating the TCD.
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