There are currently four main methods for measuring the humidity of SF6 gas: mass method, electrolysis method, cold mirror dew point method, and resistance capacitance method. The first three methods are recommended by IEC and national standards. See IEC60376 "Technical Specification for Sulfur Hexafluoride in Electrical Equipment", GB/T5832.1-2003 "Electrolytic Method for the Determination of Moisture in Gases", and GB/T 5832.2-2008 "Determination of Trace Moisture in Gases - Part 2: Dew Point Method". The mass method is only suitable for humidity arbitration, while the resistance capacitance method is a newly developed test method.
1. The mass method measures gas humidity.
The mass method for measuring the humidity of SF6 gas is to use magnesium perchlorate to absorb moisture in the gas, and after passing through a certain amount of SF6 gas, weigh the magnesium perchlorate increment after constant weight. Calculate the humidity mass ratio of SF6 gas.
2. Cold mirror dew point measurement of humidity.
According to the definition of dew point temperature, the water vapor in the gas is cooled to the appearance of condensed phase by means of isobaric cooling, or the temperature of the cold surface is controlled to maintain a thermodynamic phase equilibrium between the water vapor in the gas and the flat surface of water (or ice). Accurately measuring the temperature at this time is the dew point temperature of the gas. The instrument for measuring the dew point temperature of the gas is called a cold mirror dew point meter (abbreviated as a dew point meter), and the main detection component of the dew point meter is a condensing mirror (measuring mirror).
3. Electrolytic method for measuring gas humidity.
The electrolysis method uses the Coulomb electrolysis principle to measure the micro amount of water in the gas (0~1000PPmV). The principle is that the water contained in the measured gas is absorbed by the P2O5 film layer as a diluent in the electrolytic cell through the flow of the measured gas through a special structure, and all of it is electrolyzed. When the absorption and electrolysis processes reach a balance, the electrolysis current is proportional to the water content in the gas.
4. Resistance and capacitance of alumina measure humidity.
The resistance capacitance dew point meter for aluminum oxide is designed based on the capacitance variation relationship between water vapor and aluminum oxide. The aluminum oxide sensor consists of an aluminum substrate, aluminum oxide, and a gold film. The aluminum wire or aluminum sheet is placed in an acidic aqueous solution and subjected to alternating current oxidation to form an aluminum oxide film with a humidity dependent property. The humidity varies linearly with the resistance capacity of the aluminum oxide. A humidity testing instrument that measures resistance and capacitance changes is called a resistance and capacitance dew point meter.
Precautions for more accurate measurement of SF6 gas humidity:
1. Measuring pipes, joints, and other components are of good quality and qualified after treatment. Stainless steel pipes are recommended, as well as measuring pipes made of polytetrafluoroethylene. In addition, keep all components clean and dry to reduce test errors.
2. Correctly select measuring instruments to ensure that they are in good condition. In addition to properly storing the instrument, it should be calibrated regularly, usually once a year.
3. The pipeline connection between the SF6 comprehensive analyzer and the SF6 equipment should be sealed without leakage.
4. Suitable measurement methods should be used. For example, instruments using the dew point method. It is recommended to measure the sampling gas at a pressure of 0.1 MPa.
5. The measurement should be conducted in sunny weather to avoid excessive water vapor in the air affecting the measurement accuracy.
6. Ambient temperature also has a significant impact on measuring gas humidity. It is recommended that the temperature be similar for each measurement to facilitate comparison. The ambient temperature for measurement should be around 20 ℃, and the test temperature should be noted in the report.
7. Generally, the circuit breaker should be stabilized for 48 hours after being inflated before measurement.