Teledyne 4020 - Hydrocarbon analyzer - Manual (Page 25)

Total Hydrocarbon Analyzer Operational Theory
Teledyne Analytical Instruments 9
located in line with the sample port. See Figure 1-1 for a flow schematic
for instruments equipped with AutoCal valves and Figure 1-2 for
instruments without the AutoCal option.
2.2.4 Flame Ionization Detection Cell
The sample and fuel are combined within a tee fitting located in the
isothermal chamber. The mixed gas is emitted from a burner within the
sensor assembly. Blanket air is introduced into the sensor (or cell) by
means of a separate fitting that is located in the base section of the
assembly. The upper half of the assembly houses the anode-igniter,
collector, and flame guard thermistor. The cell is located at the left hand
front area inside the enclosure for easy access. See Figure 2.1.
2.3 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 cross-section Figure 1-3).
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
TURNING OFF THE POWER AND DISCONNECTING
THE POWER CORD.
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-
conductor connection.
The anode-igniter, as its name implies, serves two functions. When
relay K2 at PCB part number B74671 is energized, the coil becomes an
electrical heating element that glows red-hot and ignites the hydrogen
fuel. When relay K2 at B74671 is de-energized, the coil is connected to
the +260 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.