Configuring Measurement - Emerson Rosemount FCL-01 Reference Manual

Reference Manual
00809-0100-3412
4.5

Configuring measurement

4.5.1 Purpose of configuring measurement
This section explains how to do the following:
1. Program the transmitter to measure free chlorine (and pH). This step is necessary,
2. Set automatic or manual pH correction for the free chlorine measurement.
3. Set the level of electronic filtering of the raw signals from the chlorine and pH
4. Make various pH measurement settings. The transmitter supplied with the
4.5.2 Chlorine definitions
Chlorine
oxidants
Filter
pH
correction
Rosemount FCL 1056
because the transmitter can be used with other sensors to measure other chlorine
oxidants.
sensors.
Rosemount FCL is designed to be as versatile as possible. The pH settings below
are needed in some applications but are not used when pH is measured for the
purpose of correcting free chlorine readings.
a. Solution temperature correction
b. Transmitter isopotential point
Although the Rosemount FCL is used to measure free chlorine only, the
transmitter used in the FCL can be used to measure other chlorine oxidants,
for example, monochloramine and total chlorine.
The transmitter applies a software filter to the raw sensor current. The
filter reduces noise but increases the response time. The available filter(s)
depend on the time setting. If the filter is between 0 and 10 seconds,
the 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 percent of the final value following
a step change.
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
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
Programming the transmitter
October 2023
47