AllTCDdetectorsaresusceptible tofilamentdamageordestruction ifoperatedathighcurrent in the
absenceofcarrierand/or referencegas flow. The filamentswill incandesceandburnout if thecarrieror
referencegas flow is interrupteddue toavarietyofpossible factorssuchasacolumnbreak, inadvertent
columndisconnectionduringcolumnchanges,removaloftheseptumnutforseptumreplacement,orwhenthe
carriergascylinderrunsdryduringananalysis. TheSRITCDfilamentprotectioncircuitisacurrent“cut-out”
circuit thatmonitors thecolumnheadpressureduringGCoperation. Undernormalcircumstances, there isno
reasonfor thecolumnheadpressure todropbelow3psi,withmostcolumnsoperatingat8psiorabove. When
theheadpressuresensorlocatedinthecarriergasflowpathdropsbelow3psi,theprotectioncircuitisactivated,
and thecurrent to theTCDfilaments is interrupted immediately. AredLEDon theGC’sfrontcontrolpanel
under“DETECTORPARAMETERS”will light to indicate that theprotectioncircuithasdetectedagas
pressure lossandshutdown the filamentcurrent. Thecauseof theprotectioncircuitactivationshouldbe
immediately investigatedandcorrected. Asanadditionalcaution,useHIGHcurrentonlywithheliumor
hydrogencarriergases. Withnitrogencarrier,useLOWcurrentonly,or thefilamentsmaybedamaged. The
pressureatwhich theprotectioncircuitactivates isuseradjustablewith the trimpoton the topedgeof thefront
controlpanel,above the label reading“TCDPROTECT.”
TCD Filament Protection Circuit
1-Pressing theLOCALSETPOINTbuttondisplays thefilamentcut-offsetpointvalue(factorysetat3psi) in
thebrightredLEDdisplay in theupperrightcornerof theGC’sfrontcontrolpanel. If thecarriergaspressure
reachesorfallsbelowthisvalue, thefilamentcurrentwillimmediatelybeinterrupted.
2-Pressing theTOTALSETPOINTbuttondisplays thecarriergaspressurepresent in theGCsystem. Under
normaloperation, thisvaluewillbewellabove the3psicut-offsetpoint.
3-TheSTATUSLEDglowsbright redonlywhen theTCDprotectioncircuithasbeenactivated.
4-Pressing theACTUALbuttondisplays thevoltagepresentacrossonehalfof theTCDbridge. Avalueof
3.5to4.5voltsistypicalwhenusinghighcurrent;lowcurrentwilldisplay2.5-3.5volts(note:theLEDdisplays
4voltsas“400,”3.5as“350,”etc.). Anyvalue lower than these indicatesapotentialproblem in theTCD
detectorbridge.
DETECTORS
Thermal Conductivity Detector - TCD
TCDprotectioncircuit LED lit onanSRI
model 8610GC front control panel
1-
2-
3-
4-
LEDpanel
displayscontrol data
corresponding to
thebuttonpressed
Bright redLEDdisplay
1-
2-
3-
4-
LOCALSETPOINTbutton
TOTALSETPOINTbutton
ACTUALbutton
STATUSLED
TheDISPLAYSELECT
switchallows theuser
tochoosebetween
displaying thecontrol
zonesusing thebuttons
or thecolumnoven
temperature
