Emerson Rosemount 370XA, System Reference Manual

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370XA Gas Chromatograph System Reference Manual
APRIL 2014 7P00370-H01
4 - 37
NOTICE
Step 7 of 10 : If the certificate states an uncertainty percentage for each component,
enter them here. If the certificate does not show the uncertainty percentages, then
the default value of 2% should be used. The uncertainty values can be used to
confirm that the analysis of the calibration gas matches the certificate values before
the standard is used to calibrate the GC.
NOTICE
Step 8 of 10 : If the certificate includes the energy content, enter it on this screen. If
the certificate does not include an energy content, use the calculated energy
content shown.
After you have followed all of the assistant’s prompts, it will analyze
the calibration gas and repeat the analysis until the nitrogen value
repeats within the uncertainty value entered on a previous screen.
Since air is 78% nitrogen and the calibration gas contains significantly
less nitrogen than this, the analyzed nitrogen content should typically
start high and then decrease as the system is purged with the new
gas. The screen displays the nitrogen value for each run on the table
to the left of the chromatogram. Once the nitrogen value has
stabilized, the last analysis will be compared to the entered certificate
values to ensure that the GC is analyzing accurately.
4.4.3 Auto valve timing
The analytical valves in the oven are switched on or off during the
analysis cycle to change the analytical flow path to separate certain
components through particular columns. For the C6+ analysis,
column one separates the hexane and heavier components (C6+)
from the pentane and lighter components, column two separates the
propane to pentane components, and column three separates the
nitrogen, methane, carbon dioxide, and the ethane. The timing of the
valve switching is critical to the accurate performance of the analysis.
Over time, the resistance to flow and the performance of the columns
will change (typically, the retention times get slower) resulting in the
switching of the valves "cutting" into some of the component, rather
than switching between the peaks as they elute from a column.