300781172_002_C0 - 012/2019 - © Leybold 42
Cautions in operation
Furthermore, ion current cannot be detected unless it is 1 uA or more because the
direction of leakage is the reverse of ion current.
9.5.3 Faraday Cup sensitivity calibration
Faraday Cup (FC), sensitivity is specified by inputting the ion current of the specified
gas type measured under constant pressure condition and the pressure value at
that time. For details, refer to "FC sensitivity calibration" in the software instruction
manual.
9.5.4 Operating environment temperature and humidity
Resolution varies depending on operating environment temperature and humidity. If
the resolution changes, adjust it while the temperature and humidity are constant.
Mass numbers shift depending on operating temperature and humidity. In that
event, calibrate the mass number while temperature and humidity are constant.
9.6 Converting to partial pressure
Peaks of mass numbers that are measured by a mass spectrometer are detected
as ion current values (A). You can use various methods to convert them into partial
pressure measurements.
The software includes the following methods:
1. Using the correlation between the ion current set in the sensitivity calibration
mode and the pressure
For example, if sensitivity is calibrated when the vacuum gauge for the equipment
to measure under measurement indicates 1 x 10
-1
Pa and the ion current is set at
5 x 10
-7
A, the partial pressure conversion factor will be:
1 x 10
-1
(Pa) / 5 x 10
-7
(A) = 2 x 10
5
(Pa/A)
Convert ion current values (A) to a partial pressures (Pa) by multiplying the ion
current value by this partial pressure conversion factor.
2. Using the total pressure measuring function
Given that PT (Pa) is the value measured by the total pressure measuring function
and IN (A) is the ion current value of each mass number and partial pressure is PN
(Pa), pressure is converted to partial pressure (Pa) by this formula.
P
N
= ( I
N
/ ΣI
N
) x P
T
The numerical value obtained by this formula is recorded as partial pressure (Pa).
In the software, to apply this conversion select "total pressure" as the specified value
of the partial pressure conversion reference value. However, if the main gas
component is removed with the recipe setting of the software in the atmosphere to
be measured, deviation may occur in the partial pressure calculation result.
With a pumping system which can reach 1 x 10
-5
Pa or less, select method 1) or 3)
because the total pressure measurement range of LEYSPEC view models is from 1
x 10
-2
to 1 x 10
-5
Pa.
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