The zero standard is necessary, because the sensor, even when no iodine is present,
generates a small current called the residual current or zero current. The transmitter
compensates for the residual current by subtracting it from the measured 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. Deionized water makes a good zero standard.
Important
Do not zero the sensor by leaving it in the TCL flow cell and turning off reagent injection.
Even though no iodine is present, the current measured under these conditions is not the
zero current. Instead, it is the slight response of the sensor to total chlorine in the sample.
Important
Always use deionized water for zeroing the sensor.
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 standards when taking and testing the grab
sample.
• Take the grab sample from a point as close as close as possible to the inlet of the TCL
sample conditioning system.
• Total 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.
Calibration
64 Rosemount TCL
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