Unlike the LabQuest version of the Conductivity Probe which has three ranges that you change manually with a switch, the Go Direct Conductivity automatically changes range based on the solution you are in. Because of this, the best way to calibrate it is to use a one-point calibration. A two-point calibration, essentially performing a linear fit, would actually worsen the accuracy of the sensor.

This is also true for the Go Direct Platinum-Cell Conductivity Sensor. The cabled version of the Platinum-Cell Conductivity sensor had a much smaller range and was able to be calibrated linearly quite well. When we increased the range on the Go Direct Platinum-Cell Conductivity Sensor, it became nonlinear and auto-ranging. Therefore, a one-point calibration is the most accurate way to calibrate it.

See also Where do I get Conductivity Standards?