Stanford Research Systems RGA100, Operating Manual And Programming Reference

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Programming the RGA Head 6-21
SRS Residual Gas Analyzer
electrometer, the noise floor is set to the maximum averaging (best signal-to-noise ratio
and longest measurement time), the zero of the detector is readjusted, and the
measurement is triggered. The single current value returned by this command is the
maximum current in the 28 +/-0.3 amu region of the mass spectrum, and must be
corrected with a sensitivity factor to obtain the actual partial pressure reading at mass 28.
Important : The RF/DC on the QMF are left at 28.3 amu at the end of this measurement,
and the MR0 command can be used to turn them off.
Important : The CA command turns off the RF/DC at the end of its execution.
HV0 High Voltage CDEM = 0 Volts, FC detection enabled.
NF0
Maximum averaging (slowest scan rate) selected.
CA Readjust the zero of the detector for clean baseline and update
the internal scan parameters.
MR28 Single measurement triggered @ 28 amu. A single current value
is returned and the RF/DC are left at 28.3 amu.
Collect the 4 byte current value returned.(Use sensitivity factor to convert it to
pressure value.)
MR0 Turn off the RF/DC on the QMF rods when done measuring.
Single Mass Measurement programming tips
x
Single mass measurements are very commonly used in program loops where several
different masses are monitored in a merry-go-round fashion and their partial
pressures displayed as a function of time.
x
The outputs provided by a set of single mass measurements are often used in process
control programs to control alarms, analog and digital outputs, and relays. For
example:
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.
x
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.
x
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.