SRS Labs QMS 100 Series, Руководство пользователя

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3-8 Calibration
SRS QMS Gas Analyzer
Calibration for Multiple Operating Conditions
The QMS capable of being used over a variety of operating conditions, which in turn require different
overall sensitivity factors. Examples are:
•
one QMS system used with multiple capillaries
•
measurements of gas streams at different total pressure, temperature, or composition
•
measurements at multiple ionizer conditions
The RGA sensitivity factor is not meant to be directly adjusted by the user. The software only allows
this value to be changed via the Sensitivity Tuning dialog box using a reference gauge reading. Users
cannot type a new value into the Sensitivity Factor text box. Instead, the Pressure Reduction Factor
feature is provided to account for widely varying operating conditions.
The Pressure Reduction Factor is stored in the .RGA files. When using the QMS with various
operating conditions, one .RGA file can be made for each set of conditions. Each of these files will
contain a different pressure reduction factor. The procedure to determine the pressure reduction factor
is the same as used in the Initial Calibration section above. Briefly: disable the pressure reduction factor,
compare the measured value with a reference, and calculate a new pressure reduction factor. When
determining the pressure reduction factor for each set of conditions, make sure that the RGA sensitivity
factor has not been changed.
To use the QMS at one of the multiple conditions, simply open the appropriate .RGA file and connect
the window to the ECU (if the ECU is already connected to another window, disconnect from that
window first). The software will now be ready to make measurements.
It is worth restating that the pressure reduction factor is only accurate when used with the matching
RGA sensitivity factor. The value in the RGA electronics can be changed by other users, so the RGA
sensitivity factor should be recorded or locked using the security feature of the software.
Calibration with Fixed Reservoir
Air is a convenient calibrant gas for the QMS, but only provides nitrogen as a useful reference. The
other major components,
e . g . oxygen, water, and carbon dioxide, are not present at reliable
concentrations. For more precise calibrations a reservoir and pressure gauge can be used as a
calibrant. When using this method, be aware that the QMS continuously draws 1-5 milliliter per minute
of gas, depending on the capillary. The reservoir should be large or the total pressure will change
quickly.
Peak Tuning Procedure
When analyzing a sample, you expect the peaks of the different gases to be displayed at their correct
mass-to-charge ratio values and the peak widths to be less or equal than 1 amu at 10% of peak height.
The correct location of the peaks is essential for accurate qualitative analysis, and unity resolution
(
∆
m
10%
=1 amu) minimizes the overlap between adjacent peaks. Both the calibration of the mass scale
and the widths of the peaks can vary with time due to aging of the head. Changes in resolution are
particularly serious since they affect the sensitivity of the RGA and introduce errors in the partial
pressure measurements.