MKS 946 Series, Operation And Maintenance Manual, Page: 120 / 130

946 Controller Instruction Manual pg. 120
Pin Numbers Resistance (Ohm) for a
good sensor
Resistance (Ohm) for a
bad sensor
Between F1 pins 0 - 5 Open (>100 Ohm)
Between F2 pins 0 - 5 Open (>100 Ohm)
Any pin to ground/shell >10
6
Ohm <10
6
Ohm
Table 11-1 Resistance Readings of a Normal HC Sensor
F1 and F2 are identified on the Low Power Nude sensor (HC). For a Mini BA, use the drawing in
Figure 11-6 to locate F1 and F2.
F1
F2
Locator
Key Slot
Figure 11-6 Filament Pin Locator for LPN and Mini BA Hot Cathodes
11.4 Maintenance of a Pirani Sensor
11.4.1 Theory of a Pirani Pressure Sensor
Measurement of pressure with any type Pirani sensor is based on the thermal conductivity of the gas type
and amount in the vacuum environment. A wire, sometimes referred to as a filament, suspended from
supports (see Figure 11-7) is heated and maintained at a constant temperature during the measurement
process.
Figure 11-7 Schematic of a Pirani Thermal Conductivity Sensor
The amount of heat exchanged between the hot wire and a colder environment (wall of the sensor) is a
function of the pressure when the distance between the hot wire and the cold (cooler) environment is
comparable to, or less than, the mean free path of the gas molecules. When gas pressure is higher, the
gas thermal conductivity becomes pressure insensitive and natural convective heat transfer must be
employed to improve the sensitivity. This demands the horizontal placement of the sensor tube (see
Figure 11-8) for a convention enhanced Pirani used to measure pressures greater than 100 Torr.
To Vacuum
System
Electrical
Leads
Heated Wire