Emerson Rosemount 700XA Reference Manual page 17

July 2020
Liquid sample injection valve (LSIV)
The optional LSIV converts a liquid sample into a gas sample for analysis.
Figure 11: Liquid Sample Injector Valve (LSIV)
Methanator
After all other components have been separated from the sample, carbon monoxide and carbon dioxide, which are normally
present in quantities too small to be detected by the gas chromatograph (GC), can be sent through the optional methanator,
where the two gases are combined with hydrogen to make methane in a heat-generated catalytic reaction. The methanator is also
known as a methanizer or a catalytic converter.
Data acquisition
Every second, the controller assembly takes exactly 50 equally spaced data samples (i.e., one data sample every 20 milliseconds).
As a part of the data acquisition process, groups of incoming data samples are averaged together before the result is stored for
processing. Non-overlapping groups of 50 samples are averaged and stored, and thus reduce the effective incoming data rate to
50/10 samples per second. For example, if N = 5, then a total of 40/5 or 8 (averaged) data samples are stored every second.
The value for the variable N is determined by the selection of a peak width parameter (PW). The relationship is
N = PW
where PW is given in seconds. Allowable values of N are 1 to 63; this range corresponds to PW values of 2 to 63 seconds.
The variable N is known as the integration factor. This term is used because N determines how many points are averaged, or
integrated, to form a single value. The integration of data upon input, before storing, serves two purposes:
■ The statistical noise on the input signal is reduced by the square root of N. In the case of N = 4, a noise reduction of 2 would be
realized.
■ The integration factor controls the bandwidth of the chromatograph signal. It is necessary to match the bandwidth of the input
signal to that of the analysis algorithms in the controller assembly. This prevents small, short-duration perturbations from being
recognized as true peaks by the program. It is therefore important to choose a peak width that corresponds to the narrowest
peak in the group under consideration.
Peak detection
For normal area or peak height concentration evaluation, the determination of a peak's start point and end point is automatic.
The manual determination of start and end points is used only for area calculations in the Forced Integration mode. Automatic
determination of peak onset or start is initiated whenever Integrate Inhibit is turned off. Analysis is started in a region of signal
quiescence and stability, such that the signal level and activity can be considered as baseline values.
Rosemount 700XA 17