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MANUAL P/N 316005 REV: B
1.
Allow the system pressure to rise to atmospheric pressure.
2.
Remove the top of the control unit.
3.
Adjust the atmosphere potentiometer(s) (see Figure 3-5) for the thermocouple gauge(s), until the
pressure display is 9.99 x 10
+9
. Note that when adjusted beyond this point the decimal point of the
display will disappear. "Perfect" calibration is achieved when the decimal point just disappears.
The resolution of the gauge at this point is very poor, and the appearance or disappearance of the
decimal point during normal operation when the gauge is at atmosphere is no cause for concern.
Figure 3-5 Thermocouple Gauge Modules, Top View
(Controller Cover Removed)
3.6 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 316 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.
