Specifications

Electrometer 4-9

SRS Residual Gas Analyzer

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

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.