Calibration - Monochloramine; Purpose - Emerson Rosemount MCL-220 Manual

7.3

Calibration - monochloramine

7.3.1

Purpose

As Figure 7-1
the concentration of monochloramine in the sample. Calibrating the sensor requires
exposing it to a solution containing no monochloramine (zero standard) and to a solution
containing a known amount of monochloramine (full-scale standard).
Figure 7-1:
The zero standard is necessary, because monochloramine sensors, even when no
monochloramine 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 monochloramine value. New
sensors require zeroing before being placed in service, and sensors should be zeroed
whenever the fill solutions is replaced. Deionized water makes a good zero standard.
The purpose of the full-scale standard is to establish the slope of the calibration curve.
Because stable monochloramine 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.
•
•
Instruction Manual
shows, a monochloramine sensor generates a current directly proportional to
Sensor Current as a Function of Monochloramine Concentration
Take the grab sample from a point as close to the MCL 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 MCL.
Monochloramine solutions are moderately unstable. Run the test immediately after
taking the sample. Try to calibrate the sensor when the monochloramine
concentration is at the upper end of the normal operating range.
Calibration
49