Varian 959D, Operation Manual

Page: 22 / 82

959D Leak Detector
1-8
DRAFT
9/13/02
1.5.1 Measurement System
The 959D utilizes a Contra-Flow™ vacuum system design that allows testing at high
test point pressures. The vacuum system connects to the unit to be tested at the test port,
maintains the required vacuum in the spectrometer tube, and removes helium after a test.
An optional probe provides fast response, high sensitivity sniffing mode operations.The vac-
uum system includes the following components:
Spectrometer tube The spectrometer tube measures the helium concentration in the
object under test. The information is sent to the leak rate indicator
to provide a visual representation of the helium concentration. The
spectrometer consists of a dual thoriated iridium filament ion
source, a preamplifier, and a magnet assembly.
High vacuum pump The 959D achieves high vacuum for the spectrometer tube by use
of Varian’s V70 Turbo or V70D MacroTorr pumps.
❑
Use the V70 Turbo turbomolecular pump for best helium
sensitivity.
❑
Use the V70D MacroTorr for operation of the leak detector
with high test port pressures up to 200 mTorr.
The helium sensitivity ranges resulting from the use of these pumps
are in Table 1-1. See page 2-4 for information on the HI and LO
sensitivity switches.
Table 1-1 Helium Sensitivity Ranges and MDL
Sensitivity Selected
Helium Sensitivity in atm cc/sec
HI LO
Pump MDL Range MDL* Range
V70 Turbo 2x10
− 10
10
− 9
to 10
− 6
2x10
− 9
10
− 8
to 10
− 5
V70D MacroTorr 2x10
-9
10
− 8
to 10
− 5
2x10
− 8
10
− 7
to 10
− 4
V70D MacroTorr with
optional power probe
[sniffing mode] N/A 10
− 8
to 10
− 5
N/A 10
− 7
to 10
− 4
*MDL - minimum detectable leak