Stanford Research Systems RGA100, Operating Manual And Programming Reference

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Scan and Measurement Commands 6-53
SRS Residual Gas Analyzer
Perform a Total Pressure measurement or toggle the TP_Flag on/off.
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.
The total ion current measured by the elctrometer 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.
Notes :
x
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.
x
TP_Flag is cleared whenever the CDEM is turned on to protect the
multiplier from large ion currents. TP1 can be used to reset
TP_Flag to one
after making sure the total pressure is in a ““safe”” range for the CDEM.
x
The sensitivity of the RGA for total pressure measurements will usually be
affected after mass axis recalibrations. Check the sensitivity calibration after
any peak tuning procedure.
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.
Parameters :
TP0: TP_Flag is cleared. Total Pressure measurement is disallowed and a null
current value is returned as a response to a total Pressure Measurement request
(note that this includes the total pressure measurement requests at the end of
scans!).
TP1: TP_Flag is set. Total pressure measurement is fully enabled.