With everyday sampling, the cell cup may build up a residue or film on the cell walls that may cause the readings to become erratic.
Use a 50/50 mixture of a common household cleaner (i.e. Lime-A-Way, CLR, Tilex, etc) and DI water. Pour into conductivity cell cup and scrub with a q-tip. Be sure to get around all the electrodes and the thermistor probe. On the DS handheld unit, use an acid brush to scrub the cell cup.
Let it set for about 10 minutes. Rinse the cell cup thoroughly with tap water, then a final rinse with DI water.

Source: Myron L FAQ