Teledyne 4040 - Methane / Nonmethane analyzer - Manual (Page 22)

Operational Theory
22 Teledyne Analytical Instruments
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.
2.3.1 Electrometer-Amplifier
The collector cable is coupled directly to a coaxial fitting located on
the electrometer-amplifier PC board. The PC board is located on the side
panel next to but outside of the isothermal chamber. See Figure 2-1 and
consists of an electrometer amplifier and an operational amplifier. This
circuit is a very high-gain, current-to-voltage converter circuit, having
an input impedance measuring in the billions of ohms. It is static
sensitive and highly susceptible to contamination. Special care must be
taken in handling this PC board.
Refer to Section 5.5.4: Electrometer-Amplifier PC Board for more
information concerning the electrometer-amplifier and the other printed
circuits that follow.
2.3.2 Anode Power Supply
The high voltage anode power supply components are mounted on
the anode power supply printed circuit board. High voltage regulation is
achieved through the use of series-connected zener diodes. The
simplicity of this circuit’s design can be attributed to the extremely low
current demand of the anode circuit. The positive output voltage is
nominally 125 volts. Output tolerance is ±10 volts from the specified
125 volts.
2.3.3 Flame Guard Circuit
A thermistor-controlled, transistorized switching circuit is employed
to operate a relay in the event of a flame-out condition. A panel indicator
light and fuel shut-off solenoid valve are operated by the relay to alarm
personnel that a flame-out condition has occurred. The fuel shut-off
solenoid valve stops the hydrogen flow.
2.3.4 Flame Ignition Circuit
The flame ignition circuit includes the anode-igniter electrode (in the
detector cell), a transformer, and processor controlled relay. The circuit
is automatically energized when the FID cools due to lack of flame.