SF₆ gas is the “blood” of high-voltage electrical equipment such as GIS and circuit breakers, with excellent insulating and arc-extinguishing properties, but it is highly sensitive to moisture. Excessive moisture can lead to catastrophic consequences:
Insulation deterioration: At high temperatures from electric arcs, moisture reacts with SF₆ decomposition products to form strong corrosive substances like hydrofluoric acid and sulfuric acid, severely corroding metals and insulating components, resulting in a permanent decline in insulation strength.
Equipment corrosion: The aforementioned acids can erode the interior of the equipment, damaging seals and mechanical structures.
Condensation risk: At low temperatures, excessive moisture may condense into liquid water or ice, adhering to insulating surfaces and easily causing surface flashovers, leading to grounding or short-circuit explosions.
Impact on arc extinction: Altering the properties of the gas medium, reducing the switch’s breaking capacity.
Therefore, international and domestic standards impose strict “ppm-level” (parts per million) limits on the moisture content in SF₆ gas.
Mainstream measurement principles
The core technologies of SF₆ moisture meters mainly include the following:
1. Electrolytic method (Coulometric method)
This is a classic measurement method. The gas flows through an electrolytic cell coated with phosphorus pentoxide, where moisture is completely absorbed and electrolyzed. The electrolytic current is strictly proportional to the moisture content. Its major advantage is high accuracy and no need for calibration, but it has a slow response and is not suitable for measuring high-humidity gases. The electrolytic cell is also vulnerable to contamination. It is often used for laboratory reference analysis.
2. Capacitive method (High-polymer film capacitive method)
This is currently the most mainstream and commonly used technology for on-site testing. It utilizes a specially designed polymer film capacitive sensor. When water molecules in the gas are absorbed by the film, it changes the dielectric constant, thereby causing a change in capacitance. This method has an extremely fast response, high sensitivity, strong anti-interference ability, and is compact and portable, making it very suitable for on-site rapid detection. The drawback is that it requires regular calibration.
3. Cold mirror method (Dew point method)
This is the internationally recognized reference method. The principle is to allow the gas to flow through a cooled mirror. When the mirror temperature drops to the point where water vapor condenses (or frost forms), the temperature is determined by photoelectric detection, which is the dew point temperature. It has high accuracy and can be directly traced, but the equipment is expensive, operation is complex, and maintenance requirements are high. It is mainly used in laboratories or as advanced calibration equipment.
4. Quartz crystal oscillation method
This method uses a quartz crystal coated with a hygroscopic layer. When water molecules are adsorbed, the crystal mass increases, and the oscillation frequency decreases. This method is highly sensitive to extremely low humidity, but the sensor is very fragile and prone to contamination, and is mostly used in special industrial fields such as semiconductors.
Summary: For on-site work in the power industry, capacitive method instruments have become the recommended choice due to their outstanding comprehensive performance.
Composition and key components of SF₆ trace moisture testers
A typical on-site micro moisture meter usually includes:
Core: Sensor unit (such as a capacitive sensor).
Lifeblood: Gas path system, including quick-connect inlets/outlets, flow control valves and flow meters, filters for oil and particles, and pressure gauges.
Brain: Control and display unit, equipped with a microprocessor, screen, and keys for setting, calculation, and result display.
Support: Some instruments have built-in miniature pumps for automatic sampling and purging. They also have data storage and export functions.
Core parameters and units
Measurement range: Typically covers 0 to 2000 ppmv (parts per million by volume), or the corresponding dew point temperature range (such as -80°C to +20°C).
Accuracy: Usually expressed as a percentage of full scale (e.g., ±2% FS) or error (e.g., ±1.0 ppm).
Response time: The time required for the reading to reach 90% of the true value. Good on-site instruments can stabilize within a few minutes.
Display units: The most commonly used are ppmv and dew point temperature (°C), which can be converted according to formulas.
Standards followed Testing must be carried out in accordance with the procedures, and the main standards are as follows:
New gas standard: According to GB/T 12022, the moisture content of new SF₆ gas must not exceed 5 ppmv.
Operating gas standard: DL/T 596 “Preventive Test Regulations” is the main reference. It distinguishes requirements based on equipment types. For instance, compartments with arcs such as circuit breaker compartments have stricter requirements (e.g., ≤ 300 ppmv), while compartments without arcs such as busbar cylinders have slightly looser requirements (e.g., ≤ 500 ppmv). When implementing, the latest and most effective version must be consulted.
Post time: Dec-16-2025