SRS Labs QMS 100 Series, User Manual

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Glossary
SRS QMS Gas Analyzer
Response time - is a measure of the steepness of the response when a step change is presented at the
inlet.
RGA - residual gas analyzer (a class of QMS)
RGA Cover Nipple. CF Nipple that covers the RGA Probe.
Scan Speed (mass spectrometer). The speed at which the RGA scans through a range of successive
mass numbers.
Scanning. The procedure of continuously changing the mass tuning of the quadrupole mass
spectrometer to bring successive mass numbers into tune.
Sensitivity calibration . The act of establishing a correspondence between the change in ion current
and the corresponding change in partial pressure of the gas from which the ion is produced. The
correspondence might be represented graphically or as a table of values. See also Sensitivity.
Sensitivity.
Partial Pressure: The partial pressure sensitivity of the RGA to a gas g,
S
g
, is defined as the ratio
of the change
(H-H
0
)
in principal mass peak height to the corresponding change
(P-P
0
)
in total
pressure due to a change in partial pressure of the particular gas species.
H
0
and
P
0
are
background values:
S
g
= (H-H
0
) / (P-P
0
).
The units of
S
g
are of ion current per unit pressure
(amp/Torr, for example).
Total Pressure: The total pressure sensitivity of the RGA to a gas g, is defined similarly as the ratio
of the change
(H-H
0
)
in total ion current to the corresponding change
(P-P
0
)
in total pressure due
to a change in partial pressure of the particular gas species.
H
0
and
P
0
are background values.
Space charge . The electrical charge carried by a cloud of free electrons or ions. Space charge can
causes serious changes in the potential distributions of the ionizer, and ultimately limits the pressure
operating range of all RGA’s.
STP. Standard temperature, 273.15 K, and pressure, 1.01325 bar, for gasses. Volume alone is
insufficient to describe the flow of gasses. When temperature and pressure are also defined, the
state is completely defined.
Total Pressure . The average normal force per unit area exerted by all the gas molecules impacting on
the internal surfaces of the vacuum chamber. Units are typically Torr, mbar or Pascal. Note: The
SRS RGA is capable of collecting total ion currents that can be turned into total pressure
measurements with the help of gas dependent total pressure sensitivity factors.
UHV . Abbreviation for Ultra High Vacuum. Pressure < 10
-9
Torr.