Emerson Rosemount 700XA Reference Manual page 18

Note
The controller assembly software assumes that a region of signal quiescence and stability will exist.
Having initiated a peak search by turning Integrate Inhibit off, the controller assembly performs a point by point examination of
the signal slope. This is achieved by using a digital slope detection filter, a combination low pass filter and differentiator. The output
is continually compared to a user-defined system constant called Slope Sensitivity. A default value of 8 is assumed if no entry is
made. Lower values make peak onset detection more sensitive, and higher values make detection less sensitive. Higher values (20
to 100) would be appropriate for noisy signals, (e.g., high amplifier gain).
Onset is defined where the detector output exceeds the baseline constant, but peak termination is defined where the detector
output is less than the same constant.
Sequences of fused peaks are also automatically handled. This is done by testing each termination point to see if the region
immediately following it satisfies the criteria of a baseline. A baseline region must have a slope detector value less than the
magnitude of the baseline constant for a number of sequential points. When a baseline region is found, this terminates a sequence
of peaks.
A zero reference line for peak height and area determination is established by extending a line from the point of the onset of the
peak sequence to the point of the termination. The values of these two points are found by averaging the four integrated points
just prior to the onset point and just after the termination points, respectively.
The zero reference line will, in general, be non-horizontal, and thus compensates for any linear drift in the system from the time the
peak sequence starts until it ends.
In a single peak situation, peak area is the area of the component peak between the curve and the zero reference line. The peak
height is the distance from the zero reference line to the maximum point on the component curve. The value and location of the
maximum point is determined from quadratic interpolation through the three highest points at the peak of the discrete value curve
stored in the controller assembly.
For fused peak sequences, this interpolation technique is used both for peaks, as well as valleys (minimum points). In the latter
case, lines are dropped from the interpolated valley points to the zero reference line to partition the fused peak areas into individual
peaks.
The use of quadratic interpolation improves both area and height calculation accuracy and eliminates the effects of variations in the
integration factor on these calculations.
For calibration, the controller assembly may average several analyses of the calibration stream.
Basic analysis computations
Two basic analysis algorithms are included in the controller assembly:
Area Analysis
Peak Height Analysis
Note
MON2020 can perform a variety of other calculations. For more information, see the
Reference Manual.
Concentration analysis - response factor
Concentration calculations require a unique response factor for each component in an analysis. These response factors may be
manually entered by an operator or determined automatically by the system through calibration procedures (with a calibration gas
mixture that has known concentrations).
The response factor calculation, using the external standard, is:
where
ARF Area response factor for component n in area per mole percent
n
18
Calculates area under component peak.
Measures height of component peak.
MON2020 Software for Gas Chromatographs
Emerson.com/Rosemount
July 2020