SRS Labs QMS 100 Series, Руководство пользователя

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3-18 Programming the RGA ECU
SRS QMS Gas Analyzer
• The output of a D/A converter can be linearly related to the readings obtained at a
certain mass.
• A relay switch can be closed whenever another mass concentration goes above a
certain level.
• A digital output can be set high when a third mass goes under a minimum acceptable
partial pressure value.
•
For best accuracy of results, it is best to perform all consecutive mass measurements in a set
with the same type of detector and at the same noise floor (NF) setting. Fixed detector
settings eliminate settling time problems in the electrometer and in the CDEM’s HV power
supply.
•
It is good practice to perform an analog scan before triggering a long set of measurements
to assure the correct tuning (i.e. correct peak locations and widths) of the quadrupole mass
filter. Perform a complete Peak Tuning Procedure as described in the RGA Tuning chapter
of this manual if shifts in the peak locations are observed.
•
The RF/DC voltages are not turned off at the end of a measurement. Use the MR0
command at the end of a set of measurements and before quitting a program to
make sure the RF/DC voltages are deactivated completely.
Total Pressure Measurements
The RGA might be thought of as a Total Pressure Ionization gauge with a mass analyzer interposed
between the ionizer and the detector. Thus, by disabling the mass-discriminating action of the analyzer
section, it is possible to use the RGA as a total pressure ionization gauge.
Total pressure measurements are automatically requested at the end of each analog and histogram scan,
and can also be triggered directly by the user with the TP? command. The response of the RGA to a
Total Pressure measurement request depends on the status of
TP_Flag at the time the measurement is
requested:
TP_Flag : 1 => The measurement is performed and the total ion current is transmitted.
TP_Flag : 0 => No actual measurement is performed but a null current is still sent out over RS232.
TP_Flag is set upon a power-on reset and cleared while the CDEM is activated. It can also be toggled
on/off by the user with the TP command.
The total ion current measured by the electrometer is transmitted in the standard format : Integer in units
of 10
-16
A, 4 Bytes long, 2's complement format, with the Least significant byte transmitted first.
Note : While performing a total pressure measurement the RF in the quadrupole mass filter is set to 1
amu and the DC bias is zeroed. All ionic species have stable trajectories down the filter under those
conditions and reach the ion current detector with different efficiencies.
Important: The sensitivity factor for the TP measurements is highly mass dependent.
The additional mass discrimination that takes place in the filter results in the mass
dependence of the RGA readings being very different from that of the Bayard Alpert