Stanford Research Systems RGA100, Operating Manual And Programming Reference

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6-22 Programming the RGA Head
SRS Residual Gas Analyzer
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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 when the CDEM is activated. It can
also be toggled on/off by the user with the TP command. For example, total pressure
measurement can be re-enabled after the CDEM is turned on by using the TP1 command
(see Tips below).
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
gauge readings.
Expect to see deviations between the two gauges as the composition
of a residual gas changes.
Total Pressure measurement example
The following list of commands is an example of a total pressure measurement
setup. The first step sets up the FC as the detector, and automatically sets the
TP_Flag = 1. The second step triggers a total pressure measurement in the RGA
Head. The total current returned to the computer can be converted into a pressure
using a mass dependent sensitivity factor.
HV0
High Voltage CDEM = 0 Volts, FC detection activated, TP_Flag = 1.
TP? Total pressure measurement trigger. Total ion current is returned.