Stanford Research Systems
UGA Series Universal Gas Analyzers
Calibration and Input Design
5 –
5.1.4 Single gas measurement example
for details on the automated tuning procedures built into the RGA
Windows program. Also see the Sensitivity and Electron Multiplier
Tuning sections of the RGA Tuning Chapter for more general infor-
mation.
The Table mode of RGA Windows offers scaling factors for all of its
channels eliminating the limitations imposed by the single sensitiv-
ity factor on multiple partial pressure calculations. For example,
the scaling factors can be used to display correct partial pressure
for all the species in a table if the ratios between the partial pres-
sure sensitivities of the different components are known and only
principal mass peaks are used to monitor them. The scaling factors
can also be adjusted to correct against the mass discrimination of
the electron multiplier’s gain.
Important
: Following current industry standards, the partial pres-
sure sensitivity factor stored at the factory corresponds to N2 mea-
sured at 28 amu with ∆m
0%
=1 amu, default ionizer settings and
Faraday Cup Detection.
Monitoring the concentrations of one or few components in a sys-
tem is easy in the absence of severe spectral interference.
Suppose a system where argon is measured at 40 amu (principal
mass), in the absence of any other gases that contribute a signal at
that mass value. The sensitivity to argon was previously measured
at S
Ar
=0
-4
amps/Torr, and the electron multiplier is biased and its
gain at mass 40 was previously measured at g
CDEM
=.02x0
3
relative
to the FC signal.
The partial pressure of argon, P
Ar
, is easily calculated measuring the
intensity (i.e. peak height) of the ion current at mass 40, I
40
.
P
Ar
= I
40
/ (g
CDEM
•S
Ar
), units of Torr
(4)
