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MODEL 54eA | SECTION 8.0 |
| CALIBRATION - FREE CHLORINE |
SECTION 8.0
CALIBRATION - FREE CHLORINE (499ACL-01)
8.1 INTRODUCTION
This section covers the calibration of the
As Figure
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
•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
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
FIGURE | 55 |
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