Granville-Phillips Series 330, Instruction Manual, Page: 29 / 76

3 Operation
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Series 330 Vacuum Gauge Controller
Instruction Manual - 330004 - Rev. 02
3.2.2 Convectron Gauge Theory of Operation
The Convectron Gauge is a heat-loss transducer. The response of the gauge is gas type dependant
and non-linear. The Convectron Gauge uses a feedback amplifier circuit to maintain the sensor at a
constant temperature. See Section 5.3 and Section 5.5 for more detailed information on the
Convectron Gauge Operation.
3.2.3 Thermocouple Gauge Theory of Operation
The thermocouple gauge is a heat loss transducer. A constant AC voltage is applied to the sensor
wire causing the wire to heat up. Molecules in the surrounding gas conduct heat from the wire so
that at some constant concentration of molecules a particular temperature is achieved at the sensor.
As the concentration of gas molecules impinging on the wire decreases, that is the pressure
decreases, the temperature of the wire must increase. A wire of metal dissimilar from the sensor
wire metal is attached to the sensor. The junction of the dissimilar metals causes a voltage to be
generated which is non-linearly dependent upon the temperature of the junction. The 330 VGC
measures this voltage and calculates the corresponding pressure in the gauge. Sources of error in
the thermocouple gauge include: contamination of the sensor wire, calibration error at vacuum,
and unknown gas composition.
3.2.4 Microcontrollers and Bus Structure
The Electrometer Module has a dedicated microcontroller with internal ROM, RAM, timing, and
interrupt management functions. The microcontroller is equipped with a watchdog timer, which
automatically generates a reset if the processor fails to fulfill timing
checkpoints within its code.
Inter-processor communication is accomplished via the display bus. These lines carry BCD-format
pressure data which is used to generate the 330 display.