K465i Operations and Maintenance
P/N 1212831 Rev C
Page 5-162
Copyright
©
2012
Veeco Instruments, Inc. Confidential
All Rights Reserved
If a new weld, braze, VCR fitting, valve, mass flow controller or pressure controller
has been installed on the system, leak check the new part first.
Note:
Crossover
occurs when, pumping down with the leak detector, the leak detector
switches into
leak check mode
. If the pressure enters a low range and cannot crossover, it
has a gross leak.
If a gross leak is encountered and crossover is not possible, perform the following
procedure:
Procedure 5-25: Gross Leak Without Crossover
1. Listen for the leak. If it is large, you will hear it. Proceed by employing EITHER step 2 or step 3.
2. Apply a small amount of methanol on the suspected area. Note that the vapor pressure of the liquid
will show a pressure burst on the leak detector.
3. Perform a nitrogen pressure test of 1 PSI over atmosphere and then apply soapy water to the
suspected area. A small stream of bubbles will appear, indicating the location of the leak.
For highly sensitive leaks, use a low flow spray of Helium (
He
) into a beaker of water and
turn the
He
down until there are 1 to 2 bubbles per second.
Some leaks are very slow-responding because the
He
must follow a complex path to get to
the leak detector. An example is a leak in a thermocouple. The
He
must first diffuse through
an epoxy membrane. These types of leaks can be very time-consuming and difficult to find.
If you encounter this kind of leak, it is best to take the time to systemically determine
where the leak is, as follows:
Procedure 5-26: Systematic Leak Detection
1. Reduce the flow utilizing the beaker technique outlined above.
2. Simultaneously spray He in the area of the suspected leak and start a stop watch.
3. Record the time it takes for the leak detector to read a leak rate of a predetermined point.
4. Wait for the background to return to a low point, pick another likely spot.
5. Repeat steps 1 - 4. Continue to circle in until the point of the leak is determined.
A plastic bag can be used to cover portions of the vacuum tubing so they are less likely to
see
He
. This is a good technique when the system has multiple leaks.