urn on the ultra low temperature freezer, when the internal temperature is stable, choose a calibrated thermometer that can measure -80 degrees. Open the ultra low temperature freezer door, we can clearly see an aluminum block behind the freezer and there is a hole under the aluminum block, then put the sensor of the thermometer into the aluminum block through the hole,  and observe the temperature on the thermometer and the temperature on the display of the ultra low temperature freezer. If the difference is within 1 degree, it is not necessary to calibrate the display temperature of the freezer.