Teledyne 402REU - Hydrocarbon analyzer - Manual (Page 50)

Chapter 5 Model 402 R-EU
Teledyne Analytical Instruments 5-16
one of the glass resistors, would deposit enough skin oil to completely
upset the range division of the attenuator circuit.
Resistor R3 is a 1000M
Ω
resistor used in the feedback circuit of the
amplifier. R2 has a resistance of 100,000M
Ω
and is used in series with the
zero potentiometer slider. This circuit is used to nullify any offset signal
introduced by the signal electrode. Trimmer P1 is used to nullify the offset
signal generated by the electrometer amplifier.
The output of the circuit is standardized against gases with known
hydrocarbon concentrations by setting the zero and span control potentiom-
eters, located on the door, so that the meter and/or recorder indicates the
hydrocarbon concentration of the gas being used.
The positive and negative operating voltage required by the electrom-
eter amplifier is furnished by a regulated differential power supply circuit,
a plug-in printed circuit board which plugs into a connector on the
motherboard.
The stability of the electrometer circuit can be tested as follows:
1. Disconnect the collector cable.
2. Turn the span control about midway between clockwise and
counter-clockwise.
3. Place the selector switch in the low range position.
4. Offset the zero control so that the recorder reads at some
point upscale, and record a 24 hour chart.
Electrometer-Amplifier PC Board Adjustments:
1. Input Zero Adjustment: Connect DMM (millivolt range)
between ground and the wiper of the zero potentiometer. Adjust
the zero pot until the DMM reads zero.
2. Absolute Zero Adjustment: Disconnect the input cable
from the tower (sensor assembly). With zero millivolts input to
the circuit, open the plug of P1 trimmer on the electrometer-
amplifier cover. Using a DMM, measure the the signal between
the clockwise terminal of the span pot and ground, and adjust
P1 in the electrometer-amplifier board to obtain zero reading on
the DMM.
The electrometer-amplifier circuit is now nulled.