Detection Cell - Teledyne 4060 Series Instruction, Operating And Maintenance Manual

Operational Theory

2.4 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 in Figure 2-3).
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.
The thermistor acts as the sensor in the flame guard circuit. Its
ambient temperature resistance is in the 100 K ohms 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
thermistor 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.
Teledyne Analytical Instruments
24
Series 4060