MKS 20340-341-T1, Инструкция по эксплуатации

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34
Series 340 Vacuum Gauge Controller
Instruction Manual - 340005 - Rev. D
The amount of ion current for a given emission current and pressure depends on the ion gauge
design. This gives rise to the definition of ion gauge
sensitivity , frequently denoted by S :
Bayard-Alpert type gauges typically have sensitivities of 10/Torr when used with nitrogen or air.
Refer to Section 5.3.5 on page 48 for more information on gas sensitivities for other gases.
The ion gauge controller varies the heating current to the filament to maintain a constant electron
emission, and measures the ion current to the collector. The pressure is then calculated from these
data.
Ion gauge degas is accomplished by resistance heating (I
2
R). During I
2
R degas, a large current is
passed through the grid structure, raising its temperature and driving off contaminants.
NOTE: Some ionization gauge designs do not allow I 2 R degas.
4.2.2 Convectron Gauge Theory of Operation
The Convectron transducer is represented in Figure 4-4 as R1, R2, R3, and R4. These four
resistances form the legs of a bridge circuit, with R1 designating the sensor wire of the transducer.
R2 is a resistive network in the tube which compensates for changes in the ambient temperature. At
bridge null, R1 = R2 x R3/R4. If there are no changes in ambient temperature, the value of R1 is a
constant and the bridge is balanced.
As the vacuum system pressure is decreased, there are fewer molecules in the system to conduct the
heat away from the sensor wire causing the temperature and resistance of R1 to increase. The
increased resistance of R1 causes the bridge to unbalance and a voltage is developed across the null
terminals. The bridge control circuit senses the null voltage and decreases the voltage across the
bridge until the null voltage is again zero.
When the bridge voltage is decreased, the power dissipated in the sensor wire is decreased causing
the resistance of R1 to decrease to its previous value. The opposite events happen for a pressure
increase. The bridge voltage is a non-linear function of pressure.
Figure 4-4 Convectron Gauge Schematic