Calibrating a thermocouple or a glass thermometer only means to determine its output compared to that of a standard. After calibration you will have a chart of error values versus that of a standard for the temperatures of interest e.g. +0.42 deg C @ -100 deg C, -0.6 deg C @ 0 deg C, and -1.2 deg C @ +100 deg C. There is no way to calibrate a specific thermocouple to make it perfectly accurate although you can buy premium grade thermocoouples that offer higher accuracy (see Omega).