MKS HPS 919 Series, Руководство по эксплуатации и обслуживанию

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Hot Cathode Ionization High Vacuum Senor System
Theory of a Hot Cathode Ionization Sensor
Hot cathode ionization sensors use thermionic electrons, electrons emitted
from a hot filament, to create ions in a defined volume. In their passage from
the cathode through the gas volume, the electrons collide with gas atoms or
molecules to form ions. The number of gas molecules ionized depends on the
energy of the ionizing electrons, typically about 150 eV, and the ionization
probabilities of the constituent gases. The total amount of ionization is related
to the molecular concentration. The ions are accelerated to a collector
electrode, where they create a current in a circuit, which includes an
electrometer. The measured current is proportional to the gas density, which
in turn is directly related to the pressure, provided that other parameters like
temperature are held constant. The response to pressure changes in such a
device is virtually instantaneous.
The Bayard-Alpert sensor design uses a fine wire collector located in the
center of a grid, as opposed to the previous large area collectors. Because of
its smaller area, fewer x-rays hit the collector, and the sensor can measure
pressures to much lower levels, sometimes to below 10
-10 Torr. This is
typically the most popular sensor design for high vacuum measurement.
Bayard-Alpert sensors are available in tubed glass envelopes or mounted on
elastomer or metal sealed flanges (nude).
A glass envelope sensor tends to be favored because of its low cost and
limited thermal and photo effects on the vacuum system. A nude sensor, on
the other hand, has the advantage of giving more representative pressure
measurements and faster response to pressure changes, because the
sensing portion of the sensor is located within the vacuum system and
experiences its true pressure, without sensor pumping or outgassing effects.
HPS ® Sensors
HPS ® offers low power, nude sensors with either a yttria-coated iridium or a
tungsten filament, mounted on a 2¾" CF or KF 40 flange. Yttria-coated
iridium filaments greatly extend sensor life and minimize damage caused by
high oxygen partial pressures or accidental exposure to the atmosphere. They
have the further advantage of operating at lower temperatures than tungsten
filaments, thereby giving a lower chemical reaction rate and minimizing
thermal interference with other equipment. For operation at pressures below
10
-9 Torr, sensors with tungsten filaments have the advantage of lower
internal outgassing rates.
The accuracy of pressure measurements is dependent upon electrical
leakage currents. The Series 919 Controller is compatible with most
manufacturer's Bayard-Alpert sensors designed for I
2
R degas.
Appendix A: How the Series 919 Hot
Cathode System Works
A.1
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