Teledyne GC-Pro/FID - Gas Chromatograph - Manual (Page 110)

Maintenance GC-Pro FID
Teledyne Analytical Instruments 96
The circuit consists of an electrometer amplifier and an operational
amplifier. It 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, and special handling
precautions must be taken.
Because of its high impedance, the input circuitry to the electrometer
has had careful design consideration. The resistors (R2 and R3) in the
input gain circuit (see schematic) are installed on Teflon-insulated
standoffs, instead of directly to the printed circuit, to eliminate the
possibility of leakage currents.
To eliminate any possibility of contamination of the insulating
materials employed, the completed PC board is ultrasonically cleaned in
laboratory grade alcohol. Under no circumstances should the parts
described be handled with bare fingers. A freshly-scrubbed finger,
stroked along one of the glass resistors, would deposit enough skin oil to
completely upset the range division of the attenuator circuit.
Resistor R3 is a 1000 MΩ resistor used in the feedback circuit of the
amplifier. R2 has a resistance of 10,000 MΩ 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 zero and span calibration, so that the meter
and/or recorder indicate the hydrocarbon concentration of the gas being used.
The positive and negative operating voltage required by the
electrometer amplifier is furnished by a switching power supply circuit,
mounting at the back panel of the Analyzer
The stability of the electrometer circuit can be tested as follows:
1. Disconnect the collector cable.
2. Place the analyzer in the auto range.
3. Adjust zero offset value so that the recorder reads at some
point upscale, and record a 24 hour chart.