MKS 937B - Page 48

MKS 937B Operation Manual
48
5. Slowly and methodically probe with a small amount of leak checking gas such as He or Ar (it
must be different from the gas inside the chamber). Flooding the leak with gas or moving the
gas quickly past the leak can confuse the search since system time lags may be significant.
6. A 24-segment, centered-zero bar graph shows pressure changes in the system with greater
sensitivity than the numerical display. The more the bar shows on the screen, the higher the
leak exists in the system. On this graphic display, 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 reference pressure.
8. To exit the leak checking mode, just press the
Leak
Check
key.
The bar graph resolution is non-linear. The first segment offset from the center is
highly sensitive with subsequent segments decreasing in sensitivity.
Since set points remain active in the Leak Test function, the probe gas may change
the indicated pressure enough to switch the relay state. Disable any process control while
probing the vacuum system.
As with any leak testing, many factors can influence the sensitivity of the test. Described in greater
detail below, these include the chamber volume; system pressure; probe gas; type of vacuum pump;
location of the Sensor, leak, and pump; and others such as pumping speed and system tube size.
 Reducing the search area by minimizing the chamber volume will increase the efficiency of
the test.
 Sensitivity to gas leaks is also pressure dependent. In general, leak test sensitivity is
greater for lower system pressures.
 The Pirani Sensor is sensitive to any probe gas leak lighter or heavier than the gas in the
system. For optimal sensitivity, select a probe gas with the largest difference between its
molecular weight and that of the system gas.
The type of vacuum pump used can also affect the accuracy of the leak test. For moderate size
leaks, pump down the system with a high vacuum pump such as a diffusion or turbo pump, if possible
(ion and cryo pumps are not recommended). Leak testing can be done with a mechanical pump;
however, they may cause cyclical variations in pressure with the rotation of the vanes. This shows up
as a large background noise signal possibly masking the leak signal.
Place the pump away from the suspected leak source and place the sensor between the leak and
the pump to reduce the sensor response time. Vacuum tubing between the suspected leak and the
sensor should be as short and wide as possible to shorten the time required for the probe gas to
reach the sensor.
If the above leak detection method fails to indicate the location of a leak, unexpected high pressures
may be caused by a virtual leak, i.e., outgassing of a system component. You can locate outgassing
parts, or “virtual leaks,” as well as true gas leaks using the rate-of-pressure-rise method below.
1. With the Controller on, pump down the system to a base pressure.
2. Close a valve to isolate the pump.
3. Measure the rise of the pressure over a time interval. A very fast rise indicates a leak.