Emerson Rosemount FCL, Reference Manual

Page: 54 / 108

transmitter applies a window filter. The window filter averages the
measured value within the filter time. For example, if the filter is 5 seconds
and a step increase is applied to the input, the displayed value increases
linearly, reaching the final value after 5 seconds. If the filter is set to greater
than 10 seconds, the transmitter applies either an adaptive filter or a
continuous filter. An adaptive filter discriminates between noise and real
process change. It filters changes below a fixed threshold value but does
not filter changes that exceed the threshold. It is best used in situations
where the noise is relatively low. A continuous filter dampens all changes.
The filter timer setting is approximately equal to the time constant, the
amount of time required for the reading to reach 63% of the final value
following a step change.
pH
correction Free chlorine is the sum of hypochlorous acid (HOCl) and hypochlorite ion
(OCl¯). The relative amount of each depends on pH. As pH increases, the
concentration of HOCl decreases, and the concentration of OCl¯ increases.
Because the sensor responds only to HOCl, a pH correction is necessary to
properly convert the sensor current into a free chlorine reading. The
Rosemount FCL uses either continuous (live) or manual pH correction. In
continuous (live) correction, the transmitter continuously monitors the pH
of the sample and corrects the free chlorine readings for changes in pH. In
manual pH correction, the transmitter uses the pH you enter for the pH
correction. Generally, if the pH changes more than about 0.2 units over
short periods of time, Emerson recommends continuous (live) pH
correction. If the pH is relatively steady or subject to only seasonal
changes, manual pH correction is adequate.
Resolution If the chlorine concentration is less than 1.00 ppm (mg/L), the display
resolution can be set to
0.XX
or
0.XXX
.
5.5.3 Definitions - pH/ORP
ORP ORP is oxidation-reduction potential. It is the voltage difference
between a noble metal indicator electrode (like platinum) and a silver/
silver chloride reference electrode.
Redox Redox is redox potential. It has the opposite sign from the ORP.
Preamplifier The pH signal has a high impedance. Before it can be used, it must be
converted into a low impedance signal. The pre-amplifier accomplishes
this task, and it can be located in either the transmitter or the sensor.In
the Rosemount
™
FCL-02, the preamplifier is located in the transmitter.
Solution
temperature
correction The pH of a solution, particularly an alkaline one, is a function of
temperature. If the temperature changes, so will the pH, even though
the concentration of the acid or base remains constant. Solution
temperature compensation converts the pH at the measurement
temperature to the pH at a reference temperature (77 ° F [25 °C]).
Generally, solution temperature compensation is used only in the
determination of pH in condensate, feedwater, and boiler water in
steam electric power plants.
Resolution The pH display resolution is user selectable: XX.X or XX.XX.
Programming the transmitter Reference Manual
May 2019 00809-0100-3412
54 Emerson.com/Rosemount