In order to verify the accuracy of the built-in relative-humidity conversion results of the precision dew point hygrometer,the internationally recognized Sonntag's saturated water vapor pressure formula and Hardy's wet air enhancement factor formula were applied to the dew point and relative humidity conversion formula, which was used as a standard conversion method to verify the built-in relative humidity conversion results of a variety of commonly used precision dew point hygrometers. The results show that the deviation between the displayed value and the calculated value of relative humidity is no more than ±0.03 %RH for the instrument with resolution of 0.01 ℃, and the relative humidity deviation of individual instruments with resolution of 0.1 ℃ reaches ±0.5 %RH. So it is difficult for an instrument with resolution of 0.1 ℃ to meet the requirements of a relative humidity standard. For the instrument without dew and frost point discrimination function, the results show that the relative humidity obtained by dew point and frost point conversion is very different, and it is also very different from the actual relative humidity, so it cannot be used for the measurement of relative humidity at negative dew point. The formula also results in the following conclusions: the enhancement factor has little effect on the results of the dew point and relative humidity conversion under normal temperature and pressure, which is negligible, but has a certain effect at high temperature and low pressure, which cannot be ignored.