Emerson Process Management 53eA instruction manual Section Calibration Free Chlorine 499ACL-01

MODEL 54eA	SECTION 8.0
	CALIBRATION - FREE CHLORINE (499ACL-01)

SECTION 8.0

CALIBRATION - FREE CHLORINE (499ACL-01)

8.1 INTRODUCTION

This section covers the calibration of the 499ACL-01 free chlorine sensor. See section 9.0 for calibration of the 498CL-01 pH-independent free chlorine sensor.

As Figure 8-1 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 controller 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 fol- lowing 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 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 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 controller uses both automatic and manual pH correction. In automatic pH correction, the controller continu- ously monitors the pH of the solution and corrects the free chlorine reading for changes in pH. In manual pH cor- rection, the controller 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 controller must know the pH of the sample. If the controller is using automatic pH correc- tion, the pH sensor (properly cali- brated) must be in the process liq- uid before starting the calibration. If the controller is using manual pH correction, be sure to enter the pH value before starting the calibration.

The Model 499ACL-01 free chlorine sensor loses sensitivity at high con- centrations of chlorine. The 54eA controller 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.

FIGURE 8-1. Sensor Current as a Function of Free Chlorine Concentration 55