I. Overview
Sulfur hexafluoride gas possesses excellent insulation performance and arc-extinguishing capability, and is widely used in gas-insulated fully enclosed combined electrical equipment, circuit breakers, transformers, and other high-voltage electrical devices. The moisture content within the gas directly determines the insulation performance and operational reliability of the equipment. If the moisture content exceeds the limit, it not only reduces the insulation strength of the gas but also generates toxic and corrosive decomposition products under the action of electric arcs, posing a threat to the equipment itself and the safety of the operators. Therefore, regular detection of the moisture content of SF₆ gas is a core detection item in the preventive tests of power equipment, and the SF₆ micro-water tester is a dedicated precision instrument for this detection.
II. Standard Basis and Limit Values
The design, manufacture, and on-site use of SF₆ micro-water testers must strictly follow industry and national relevant norms. The detection limit values are clearly divided for different scenarios such as new gas, operating equipment, and commissioning and acceptance. The core execution standards are as follows:
(1) Execution Standards
1. DL/T 506-2018 “Measurement Method for Insulation Gas Humidity in SF₆ Electrical Equipment”: This is the core specification for on-site measurement operations, clearly stipulating test environmental conditions, instrument technical requirements, pipeline connection methods, and standardized operation steps.
2. DL/T 603-2017 “Operation and Maintenance Regulations for Gas Insulated Metal Enclosed Switchgear”: Specifies the moisture content limit values for equipment in operation. For gas chambers where there is arc decomposition products such as circuit breakers, the allowable value during operation does not exceed 300 μL/L; for gas chambers without arc decomposition products such as busbars and disconnectors, the allowable value during operation does not exceed 500 μL/L.
3. GB/T 12022-2014 “Industrial Sulfur Hexafluoride”: Specifies the moisture content of newly filled gas, requiring the moisture content to be no more than 8 μL/L.
(2) Supplementary Limitations
The moisture content control standard for new gas is the most stringent. The control standard for in-service operating equipment is relatively lenient; the control requirements for equipment during commissioning and acceptance are usually higher than the regular operation standards.
III. Core Technical Principles
Currently, the mainstream portable SF₆ micro-water testers in the industry generally adopt the capacitance-resistance method for detection.
(1) Capacitance-Resistance Method Working Principle
The core component of this detection method is a polymer film capacitive sensor. The polymer film inside the sensor can adsorb and decompose water molecules in the gas. Changes in water molecule content will cause a fluctuation in the dielectric constant of the film, resulting in a change in capacitance value. The moisture content and capacitance value have a fixed functional conversion relationship, and the instrument can convert the gas dew point temperature and volume ratio concentration through precise collection of capacitance data.
(2) Detection Method Characteristics
The capacitance- resistance method has the characteristics of fast response, small equipment size, and simple on-site operation, and is suitable for outdoor and on-site detection scenarios. It is the mainstream technical solution for portable detection instruments. Among other detection methods, the cold mirror method has higher detection accuracy, but the equipment cost is high and the detection response speed is slow, and is mostly used for laboratory arbitration detection; the electrolysis method has a wide detection range, but requires long drying and blowing before detection, and the operation process is cumbersome, and is not suitable for rapid on-site operations.
IV. Standardized Operating Procedures
Based on the capacitance- resistance portable SF₆ micro-water tester, the on-site standardized operation process is divided into six steps, and the specific operation requirements are as follows:
(1) Connect the gas path
Connect the instrument’s intake pipe to the dedicated detection self-sealing valve for SF₆ electrical equipment, and the exhaust pipe to the side downwind direction. Clearly distinguish the inlet and outlet ports and prohibit reverse connection; use tools to moderately tighten each connection joint to ensure the sealing of the gas path and prevent air leakage and external moisture from invading the pipeline.
(2) Power on and self-calibration
Turn on the instrument’s power supply and wait for the equipment to automatically complete the initialization and self-calibration process. For the first startup of the cold machine, the self-calibration process takes approximately 8 to 10 minutes; for multiple continuous detections of the same equipment, there is no need to repeatedly power on and off, which can significantly shorten the calibration waiting time. (3) Adjust gas flow rate
Slowly turn the flow control valve to stabilize the gas flow rate at 0.6L/min. Flow rate is a key indicator for controlling the detection accuracy, and it should be controlled within the range of ±20% of the standard value. If the flow rate is too high or too low, it will cause deviations in the detection reading.
(4) Wait for stable reading
Real-time observe the instrument’s detection curve and status indicator light until the detection data is stable. When the first device is being tested, the pipeline purging takes a longer time, and it takes about 5 to 10 minutes for the data to stabilize; when testing subsequent devices continuously, the pipeline has been purged, and it can stabilize within 3 to 5 minutes. The determination criteria are: the dew point curve is in a straight state, and the equipment status indicator light is constantly on.
(5) Record measurement data
Read and save the dew point value and volume ratio concentration displayed by the equipment. You can use the instrument’s built-in printing device to print the data label on-site and stick it to the tested equipment, which is convenient for later data traceability and historical data comparison and analysis.
(6) Final operation
Follow the operation sequence of closing the valve first, then removing the pipeline, and finally turning off the instrument power supply. Following the standard operation sequence can prevent environmental air from entering the instrument interior and prevent the sensor from being contaminated.
V. Operating precautions
1. Medium requirements: Do not use this instrument to detect corrosive gases to avoid damaging the sensor’s core components with corrosive media.
2. Flow control: When adjusting the gas flow rate, start from zero and gradually increase the flow rate. Do not suddenly open the flow rate to prevent strong airflow from damaging the sensor’s internal structure.
3. Power management: When the remaining battery power of the instrument is below 20%, it is necessary to charge in time; some instruments support charging while working, and it is recommended to fully charge the battery before conducting on-site detection work.
4. Equipment maintenance: After the detection is completed, use a dedicated protection cap to seal the instrument’s air inlet to prevent dust and moisture from entering the sensor cavity; when the instrument is idle for a long time, it should be properly stored in a dry and clean environment.
Post time: May-19-2026