SRS Labs QMS 100 Series, User Manual

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RGA Command Set 3-49
Technical Reference
received to the time it actually starts. Using the SC1 command and waiting
until the whole scan data stream is transmitted back to the host computer will
minimize the problems that are associated to this feature.
•
Perform a complete Peak Tuning procedure on the RGA ECU if the peaks
in the spectrum do not appear at their correct mass values (See Peak Tuning
section in the RGA Tuning chapter).
TP?, TP0, TP1
Description : Total Pressure Measurement .
Echo: Measured Ion Current.
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 :
•
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.
•
TP_Flag is cleared whenever the CDEM is turned on to protect the multiplier
from large ion currents. TP1 can be used to reset the flag to one after making
sure the total pressure is in a “safe” range for the CDEM.
•
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