Emerson Rosemount FCL, Reference Manual

Page: 65 / 108

Figure 6-1: Sensor Current as a Function of Free Chlorine Concentration
The zero standard is necessary, because chlorine sensors, even when no chlorine is in the
sample, generate a small current called the residual current or zero current. The
transmitter compensates for the residual current by subtracting it from the measured
current before converting the result to a chlorine value. New sensors require zeroing
before being placed in service, and sensors should be zeroed whenever the electrolyte
solution is replaced. Either of the following makes a good zero standard:
• Deionized water containing about 500 ppm sodium chloride. Dissolve about 0.5 grams
(1/8 teaspoonful) of table salt in 1 liter of water.
Important
Do not use deionized water alone for zeroing the sensor. The conductivity of the zero
water must be greater than 50 µS/cm.
• Tap water known to contain no chlorine. Expose tap water to bright sunlight for at least
24 hours.
The purpose of the full-scale standard is to establish the slope of the calibration curve.
Because stable chlorine standards do not exist, the sensor must be calibrated against a
test run on a grab sample of the process liquid. Several manufacturers offer portable test
kits for this purpose. Observe the following standards when taking and testing the grab
sample.
• Take the grab sample from a point as close to the FCL as possible. Be sure that taking
the sample does not alter the flow of the sample to the unit. It is best to install the
sample tap just downstream from the tap for the FCL.
• Chlorine solutions are unstable. Run the test immediately after taking the sample. Try
to calibrate the sensor when the chlorine concentration is at the upper end of the
normal operating range.
Free chlorine measuremetns also require a 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 HOCl decreases and concentration of OCl¯. Because
the sensor responds only to HOCl, a pH correction isnecessary to properly convert the
sensor current into a free chlorine reading.
The sensor 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 reading 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, continuous (live) pH correction is recommended. If the pH is
relatively steady or subject only to seasonal changes, manual pH correction is adequate.
During calibration, the transmitter must know the pH of the solution. If the transmitter is
using automatic pH correction, the pH sensor (properly calibrated) must be in the process
liquid before starting the calibration. If the transmitter is using manual pH correction, be
sure to enter the pH value before starting the calibration.
Reference Manual Calibrate
00809-0100-3412 May 2019
Rosemount FCL 1056 65