Temperature sensor calibration is done by a comparison test of the sensor with a reference standard like a dry-block calibrator.  See the How to Do a Temperature Sensor Comparison Calibration blog article for more on the comparison calibration process.
 
Generally speaking, the reference standard needs to be at least four times more accurate than the sensor being calibrated (4:1 test uncertainty ratio, or TUR).  For example, if the process tolerance for a temperature sensor is ±0.8 ºC, then the dry block being used as the measurement standard must have an accuracy of ±0.2 ºC or better.
 
Dry blocks use an internal control sensor for their display accuracy specification.  There are two options to improve dry-block accuracy by using an external reference PRT (platinum resistance thermometer):