We devised calibration procedure for industrial thermometers by a comparison method at the boiling point of nitrogen (${sim}-196^{circ}C$). The uncertainty of the calibration was 4 mK (k = 2). As experimentally demonstrated in this work, the effect of the atmospheric pressure on the boiling point of nitrogen can be easily detected by the thermometer. Therefore, when the boiling point of nitrogen is used for calibration of thermometer by comparison, either a reference thermometer must be used to provide the reference temperature or the effect of atmospheric pressure should be carefully considered. The use of a copper block with a large thermal mass soaked into the liquid nitrogen was proven to be more reliable, and the stability of the temperature immersed into the copper block was 1.4 mK. The temperatures at the thermometer wells, evaluated by the crossed-measurement method to compensate for the inaccuracy of the thermometers and the linear drift of the temperature of the copper block, were equivalent within 0.23 mK of standard uncertainty.