990 CLD Autoline™ Leak Detector
2-6
DRAFT 2/2/04
Sensitivity
During the process of installing the 990 CLD leak detector and building the associated
vacuum system, determining the sensitivity of the system is necessary. The detector is able
to measure four decades of leak rate signal. For example, in a typical system with the
V70D MacroTorr pump running at high speed, the four decades of leak rate might be
10
−
7
, 10
−
6
' 10
−
5
, and 10
−
4
. Running the MacroTorr pump at slow speed or using a
V70 Turbomolecular pump in place of the MacroTorr pump would yield 10
−
8
, 10
−
7
,
10
−
6
, and 10
−
5
decades of leak rate. Table 2-2 indicates the effects of pump type and
speed on the four decades of leak rate.
The pumping speed of the backing pump and the conductance of the vacuum system also
play an important role in the determination of system sensitivity. A calibrated helium leak
on the as-configured system must be used to characterize the leak rate measurements.
Set the leak detector to display the desired for decades of leak rate measurement using the
following parameters:
SENSITIVITY
Using the SENSITIVITY-SWITCH, set the speed of the high vacuum
pump to high or low, depending on the sensitivity requirement of
the system. The highest tolerable testing pressure is achieved with
the switch in low sensitivity position.
RANGE SWITCH
Using the RANGE switch, set the exponent of the least sensitive
range of the leak detector depending on the sensitivity of the
vacuum system.
Table 2-2
Leak Rate Range for Pump Type and Speed
Fast Speed
Slow Speed
Turbo Pump
10
−
8
, 10
−
7
, 10
−
6
, 10
−
5
10
−
9
, 10
−
8
, 10
−
7
, 10
−
6
MacroTorr Pump
10
−
7
, 10
−
6
, 10
−
5
, 10
−
4
10
−
8
, 10
−
7
, 10
−
6
, 10
−
5
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