The humidity levels of C

4

F

7

N/CO

2

mixed insulating gas in electrical equipment affect the insulation perfo

rmance of the gas. Regularly checking this humidity is one of the daily operation and maintenance tasks. In this paper

different humidity C

4

F

7

N/CO

2

mixed insulating gas is detected by chilled-mirror dew point meter and capacity-resistance hygrometer commonly used in the daily operation and maintenance of substation

and the experiments indicate that conventional humidity monitors mentioned above are not able to be used for C

4

F

7

N/CO

2

mixture humidity detection directly. The chilled-mirror dew point meter above is modified technically and the accuracy tests are did using standard humidity C

4

F

7

N/CO

2

mixture in this paper. The results show that the dew point measurement error of the modified chilled-mirror dew point meter is less than ±0.2 ℃

and it can accurately measure humidity of no less than 150 μL/L when the C

4

F

7

N proportion is not more than 12%

which meet the operation and maintenance requirements of the electrical equipment insulated with C

4

F

7

N/CO

2

mixture. The capacity-resistance hygrometer above after recalibration using C

4

F

7

N/CO

2

mixture humidity generator can be used for C

4

F

7

N/CO

2

mixture humidity detection

with a dew point detection error of no more than ±2.0 ℃.