Calibration - Free Chlorine; Introduction; Sensor Current As A Function Of Free Chlorine Concentration - Emerson Rosemount Analytical HART 5081-A-HT Instruction Manual

Two-wire chlorine, dissolved oxygen, and ozone transmitter

Table of Contents

MODEL 5081-A

CALIBRATION — FREE CHLORINE

10.1 INTRODUCTION

As Figure 10-1 shows, a free chlorine sensor generates a current directly proportional to the concentration of free

chlorine in the sample. Calibrating the sensor requires exposing it to a solution containing no chlorine (zero stan-

dard) and to a solution containing a known amount of chlorine (full-scale standard).

The zero standard is necessary because chlorine sensors, even when no chlorine is in the sample, generate a

small current called the residual 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 0.5 grams (1/8 teaspoonful) of table

salt in 1 liter of water. DO NOT USE DEIONIZED WATER ALONE FOR ZEROING THE SENSOR. THE

CONDUCTIVITY OF THE ZERO WATER MUST BE GREATER THAN 50 mS/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 liq-

uid. Several manufacturers offer portable test kits for this purpose. Observe the following precautions when tak-

ing and testing the grab sample.

•

Take the grab sample from a point as close to the sensor as possible. Be sure that taking the sample does

not alter the flow of the sample to the sensor. It is best to install the sample tap just downstream from the

sensor.

•

Chlorine solutions are unstable. Run the test immediately after taking the sample. Try to calibrate the sen-

sor when the chlorine concentration is at the upper end of the normal operating range.

Free chlorine measurements made with the 499ACL-01 sensor also require a pH correction. Free chlorine is the sum of

hypochlorous acid (HOCl) and hyprochlorite ion (OCl - ). The relative amount of each depends on the pH. As pH increas-

es, 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 transmitter uses both automatic and manual pH correction. In automatic pH correction, the transmitter con-

tinuously monitors the pH of the solution and corrects the free chlorine reading for changes in pH. In manual pH

correction, the transmitter uses a fixed pH value entered by the user to make the correction. Generally, if the pH

changes more than about 0.2 units over short periods of time, automatic pH correction is best. If the pH is rela-

tively steady or subject only to seasonal changes, manual pH correction is adequate.

During calibration, the transmitter must know the pH of the sample. If the transmitter is using automatic pH cor-

rection, 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.

FIGURE 10-1. Sensor Current as a Function of Free Chlorine Concentration

SECTION 10.0