The JJG 499-2021 standard is suitable for the first verification, subsequent verification and in-use inspection of precision dew point meters that use the principle of water vapor pressure equilibrate and use the photoelectric automatic detection of refrigeration mirrors. The dew point meter cools the mirror surface of the dew point Sensors through a thermoelectric cooler, resulting in the condensation of water vapor in the sample gas on the mirror surface. The optical receiver collects the reflected light signal, and the dew condensation on the mirror surface and the water vapor in the gas reach the equilibrate state through an automatic wiping circuit. The platinum resistance temperature sensor accurately measures the dew condensation temperature of the mirror surface, thereby determining the dewpoint temperature of the gas. This standard ensures the accuracy and Reliability of dew point measurement results and is suitable for the accurate measurement of gas dewpoint temperature in industrial and laboratory environments.