MicroScribe arms have two types of probes: the Master probe, with which the device was calibrated, and Working probes, which a include any other probe you use with your MicroScribe arm.

Master probes are identified by a red dot on the base of the probe.  In some older units, the Master probe will have a red line around the base instead.

The Master probe is used for validating working probes and for testing the device in MUS.  If the Master probe is damaged, your MicroScribe will need to be returned to Revware for calibration to create a new Master probe.  

Working probes can be replaced without returning your MicroScribe for calibration.