Teledyne 402REU - Hydrocarbon analyzer - Manual (Page 15)

Total Hydrocarbon Analyzer Operational Theory
2-3 Teledyne Analytical Instruments
2.1.1 Detection Cell
The upper section of the stainless steel flame ionization cell houses
the cylindrical collector electrode, the high voltage (+260 VDC) anode-
igniter coil, and the sensing thermistor of the flame guard circuit (see cell
wiring diagram and Figure 2-1).
WARNING: Dangerous high voltage exists at the anode igniter
coil (+260 VDC). Do not attempt to disconnect the
igniter coil cable or disassemble any of the flame
ionization cell components without first applying
the following procedure in
Section 1 Page VII: "PROCE-
DURE FOR REMOVAL OF INTERNAL INACCESSIBLE
SHOCK HAZARDS"
, then disconnect plug P3 from
receptacle J3.
The collector is interconnected with the electrometer-amplifier PC
board by a coaxial cable. Although the cable and fittings are intended for
coaxial service, the cable is actually being used as a shielded single-con-
ductor connection.
The anode-igniter, as its name implies, serves two functions. When
the selector switch is held in the ignite position, the coil becomes an elec-
trical heating element that glows red hot and ignites the hydrogen fuel.
When the switch is released, the heater circuit is broken, and the coil is
connected to the +272 volt DC terminal of the anode-flame guard power
supply PC board. In this configuration, the necessary potential difference is
established between the coil (anode) and collector to promote ionization of
the burned hydrocarbons. The coil functions as the high voltage anode in
all three range positions of the selector switch.
The thermistor acts as the sensor in the flame guard circuit. Its ambi-
ent temperature resistance is in the 100 K
Ω
region. When the flame is
ignited, its resistance is reduced by a factor of 100. The thermistor is
coupled to a semiconductor control circuit on the anode-flame guard power
supply PC board, which will be described in a following section.
The cell electrodes of both the anode-igniter and flame guard ther-
mistor are connected to the electronics chassis by means of a plug-in cable.
The electrode section of the cell may be removed for inspection by
turning off the power, disconnecting the electrode lead plug, and removing
the screws which retain the electrode assembly in the sensor body.
2.1.2 Electrometer-Amplifier
The collector cable is coupled directly to a coaxial fitting located on
the electrometer-amplifier PC board. The plug-in PC board is located on