937B Controller Instruction Manual
pg. 44
6.6.3 Power (including degas) Control of a Sensor using Serial
Communication Commands
Power to sensors can also be turned ON/OFF via serial communications. Commands for this are given in
Section 9 under each gauge type.
Degas of a HC sensor can also be controlled via serial commands. The commands are given in Section
9.8.
6.7
Leak Testing Using the 937B Controller
6.7.1 Leak test principle applied for the 937B
The Series 937B Controller offers a simple and inexpensive method for locating leaks in high vacuum
systems. It is not intended to replace mass spectrometer leak detectors. Under ideal conditions, a Pirani
sensor can detect leaks as small as 1x10
-4
Torr l/s and the cold/hot cathode sensor can be used to
detect leaks as small as 1x10
-7
Torr l/s.
The principle for detecting a leak with the 937B controller is based on the gas dependency of the
pressure reading for the Pirani/Convection, hot or cold cathode. When different gases (such as helium or
argon) enter into the vacuum system, a change in gas composition will lead to a ch
ange in “indicated
pressure
”, indicating a leak in the system. For heat loss sensors this is due to thermal conductivity
differences between gases and with hot or cold cathode sensors it is due to the difference in the
ionization probability in the gases.
Leak Test Mode will work with all sensors except the capacitance manometer, which is not gas
dependent. The leak test mode and bar graph will not be displayed if the 937B Controller detects
that the requested sensor for leak testing is a capacitance manometer.
6.7.2
Procedures for a leak test with the 937B
1. With the Controller ON, use the UP or DOWN keys to select the desired sensor (
must be a gas
dependent sensor such as a Pirani, Convection Pirani, CC, or HC
) with the assistance of the
Green LED.
2. Pump down the system and make sure the system is stabilized.
3. Press the LEAK CHECK key to display the leak bar graph on the LCD screen. Once the Leak
Check is pressed, the pressure for the corresponding leak checking channel will turn blue.
4. Press BEEPER to provide audible assistance for leak checking.
5. Slowly and methodically probe with a small amount of leak checking gas such as He or Ar (
the
probe gas must be different from the gas inside the chamber
). Flooding the leak with gas or
moving the gas quickly past the leak can decrease effectiveness of the search since system time
lags may be significant.
6. 24-segment, centered-zero bar graph shows pressure changes in the system with greater
sensitivity than the numerical display. The more the bars showing on the graph, the larger the
leak size. The black bar at the center is zero. The green bar indicates a relatively slow leak, while
the red bar indicates a large leak in the vacuum system.
7. If the pressure has drifted during the leak checking process, press the ZERO key to set the bar
graph and beeper for a new background pressure.
8. To exit the leak checking mode, press the LEAK CHECK key.