For most applications, the Conductivity Probe does not need to be calibrated. If you choose to calibrate it, however, you will want accurate standard solutions. The 1000 mS/cm Standard that shipped with the Conductivity Probe will last a long time if you take care not to contaminate it with a wet or dirty probe. This is a good concentration to calibrate your Conductivity Probe in the middle range (0 to 2000 mS). Vernier sells 500ml bottles of three Conductivity Standards, one appropriate for each range of the Conductivity Probe.