The CL10 chlorine sensor manual indicates using chlorine free water in the chemical calibration. What is chlorine free water?
Chlorine free water definition.
To get chlorine free water, leave a two or three gallon container of water open to the atmosphere overnight to have the chlorine dissipate. It is best to verify there is no longer chlorine present before performing the zero chemical calibration. This can be done with a handheld unit such as a Pocket Colorimeter II. You can also purchase chlorine free water from a local grocery store.