SRS Labs QMS 100 Series, Руководство пользователя

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2-6 Electrometer
SRS QMS Gas Analyzer
When using the RGA Windows program to operate the RGA, the Scan Speed parameter setting
available in the Scan Parameter Setups of the Scan menu is used to set the NF parameter value in the
RGA Head according to the equation: NF = ScanSpeed - 1.
The following table summarizes the performance of the RGA electrometer during mass measurements as
a function of the Scan Speed and NF settings. Please refer to this table to estimate minimum detectable
partial pressures and scan rates for different scanning conditions.
Scan Speed
parameter
NF
parameter
Scan rate
(msec/amu)
Single mass
meas. time
(msec)
Baseline noise
(a)
(std.dev.)(Amps)
1 0 2000 2200 7e-15
2 1 1000 1100 1e-14
3 2 400 440 1.5e-14
4 3 200 220 2e-14
5 4 126 139 4e-14
6 5 45 50 1.2e-13
7 6 30 33 2.5e-13
8 7 15 16.5 5e-13
(a) The values in this column are the result of averaging a large number of data points from
several different units. Use this values
for reference only . Variations are to be expected
from unit-to-unit, as a function of operating conditions and even as a function of time.
Adjusting the Zero of the Ion Detector
The zero of the ion detector is automatically readjusted at the beginning of each analog and histogram
scan so that the baseline is always centered around zero. The zero can also be readjusted at any time
with the command CA. In both zeroing procedures, the output of the electrometer is measured in the
absence of input ion current and stored, as a current value, in the RGA’s memory. The current value
(called offset correction factor) is then automatically used by the firmware to offset-correct all ion
currents measured (i.e. including those from single mass and total pressure measurements) under the
same detector settings. The detector settings are the electrometer’s noise floor parameter value and the
type of detector (i.e. FC or CDEM) in use at the time the zeroing is performed. Offset correction
factors for all the possible combinations of detector settings can be generated and accumulated in the
RGA’s memory. However, all offset correction factors are cleared after a recalibration (CL) of the
electrometer is performed, and when the unit is turned off.
Please consult the RGA Command Set section for details on the CL and CA commands.