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There are a wide variety of methods for measuring with dew point meters (humidity instruments), and their performance and prices vary greatly. This requires us to be careful when selecting such instruments; we need to take into account not only their performance and price, but also the environment in which they will be used, as well as the type of gas being measured and its corrosiveness. The general principles are as follows: 1)**Humidity reference: Given the need for high measurement accuracy and an ideal sample gas, a cold-mirror dew point meter is generally recommended. 2) Enterprise standard or laboratory analysis: If high measurement accuracy is required, cold mirror instruments can be used. 3) On-site testing: If high measurement accuracy is required, cold mirror method instruments can be used (as mentioned above). 4) Continuous online monitoring: If the accuracy requirements are not very high, resistive-capacitive method instruments can be used. 5) Natural gas explosion-proof measurement: In the petrochemical and natural gas industries, explosion protection is required, which necessitates the use of specific intrinsically safe dew point meters. 6) Dew point measurement in the gas air separation industry: As we know, in this industry very low moisture levels are required, with a dew point below -70°C (at the 1ppm level). In principle, cold mirror dew point meters, electrolytic dew point meters, and film dew point meters are not capable of measuring gases at temperatures below -80°C; therefore, it is more appropriate to use dew point meters based on the capacitive principle.