MODEL | SECTION 6.0 |
| CALIBRATION |
6.3 CALIBRATION — FREE CHLORINE
6.3.1 Purpose
As Figure
The zero standard is necessary because chlorine sen- sors, even when no chlorine is in the sample, generate a small current called the residual current or zero cur- rent. The analyzer compensates for the residual current by subtracting it from the measured current before con- verting 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:
FIGURE 6-1. Sensor Current as a Function of Free Chlorine Concentration
•Deionized water containing about 500 ppm sodium chloride. Dissolve about 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 uS/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 FCL 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 FCL.
•Chlorine solutions are unstable. Run the test immediately after taking the sample. Try to calibrate the sensor when the chlorine concentration is at the upper end of the normal operating range.
Free chlorine measurements also require a pH correction. Free chlorine is the sum of hypochlorous acid (HOCl) and hypochlorite ion
The analyzer uses either continuous (live) or manual pH correction. In continuous (live) correction the analyzer continuously monitors the pH of the sample and corrects the free chlorine reading for changes in pH. In manual pH correction, the analyzer uses the pH entered by the user for the pH correction. Generally, if the pH changes more than about 0.2 units over short periods of time, continuous (live) pH correction is recommended. If the pH is relatively steady or subject only to seasonal changes, manual pH correction is adequate.
During calibration, the analyzer must know the pH of the solution. If the analyzer is using automatic pH correction, the pH sensor (properly calibrated) must be in the process liquid before starting the calibration. If the analyzer is using manual pH correction, be sure to enter the pH value before starting the calibration.
6.3.2 Procedure-Zeroing the Sensor
1.Place the sensor in the zero standard. See Section 6.3.1 for suggested zero standards. Be sure no air bubbles are trapped against the membrane. The sensor current will drop rapidly at first and then gradually reach a stable zero value. To monitor the sensor current, press the DIAG key. Choose Sensor 1 (chlorine). The input current is the first line in the display. Note the units: nA is nanoamps, uA is microamps. Typical zero current for a new sensor is between
A new sensor or a sensor in which the electrolyte solution has been replaced may require several hours (occa- sionally as long as overnight) to reach a minimum zero current. DO NOT START THE ZERO ROUTINE UNTIL THE SENSOR HAS BEEN IN THE ZERO SOLUTION FOR AT LEAST TWO HOURS.
2. Press MENU. The main menu screen appears. The cursor will be on Calibrate. Press ENTER.
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