Calibration - Total Chlorine; Introduction; Determination Of Total Chlorine; Sensor Current As A Function Of Total 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 — TOTAL CHLORINE

9.1 INTRODUCTION

Total chlorine is the sum of free and combined chlorine. The continuous determination of total chlorine requires two

steps. See Figure 11-1. First, the sample flows into a conditioning system (SCS 921) where a pump continuously

adds acetic acid and potassium iodide to the sample. The acid lowers the pH, which allows total chlorine in the

sample to quantitatively oxidize the iodide in the reagent to iodine. In the second step, the treated sample flows to

the sensor. The sensor is a membrane-covered amperometric sensor, whose output is proportional to the con-

centration of iodine. Because the concentration of iodine is proportional to the concentration of total chlorine, the

analyzer can be calibrated to read total chlorine.

Figure 11-2 shows a typical calibration curve for a total chlorine sensor. Because the sensor really measures

iodine, calibrating the sensor requires exposing it to a solution containing no iodine (zero standard) and to a solu-

tion containing a known amount of iodine (full-scale standard).

The zero standard is necessary because the sensor, even when no iodine is present, generates a small current

called the residual current. The transmitter compensates for the residual current by subtracting it from the meas-

ured current before converting the result to a total chlorine value. New sensors require zeroing before being

placed in service, and sensors should be zeroed whenever the electrolyte solution is replaced. The best zero stan-

dard is sample without reagent added.

The purpose of the full-scale standard is to

establish the slope of the calibration curve.

Because stable total 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

precautions when taking and testing the grab

sample.

•

Take the grab sample from a point as close

as possible to the inlet of the SCS921 sam-

ple conditioning system. Be sure that taking

the sample does not alter the flow through

the SCS921. Sample flow must remain

between 80 and 100 mL/min.

•

Chlorine solutions are unstable. Run the

test immediately after taking the sample.

Try to calibrate the sensor when the chlo-

rine concentration is at the upper end of

the normal operating range.

The Model 499ACL-02 (total chlorine) sensor

loses sensitivity at high concentrations of chlo-

rine. The 5081-A transmitter has a dual slope

feature that allows the user to compensate for

the non-linearity of the sensor. However, for

the vast majority of applications, dual slope

calibration is unnecessary.

SECTION 11.0

FIGURE 11-1. Determination of Total Chlorine

FIGURE 11-2. Sensor Current as a Function of Total