Frequently Asked Questions

Temperature compensation: Gas density meter measure the gas density of SF6 gas by measuring its pressure and converting it to a standardized pressure as if at ambient temperature of 20℃ (also known as P20). This conversion process is known as temperature compensation. Through temperature compensation, the dial on a gas density meter/monitor gives directly a P20 pressure reading, which is equivalent to gas density.

Temperature compensation is typically achieved through the use of a bimetal chip. The bimetal chip consists of thin layers of two different metals with different thermal expansion constants, bent in a U-shape. When temperature changes, the different thermal expansion ratio causes the two thin layers to expand at different rate, and, and thus cause the U-shaped chip to expand outward (when temperature rises above 20℃) or shrink inward (when temperature drops below 20℃). By joining the bimetal chip to the bourdon tube of the pressure gauge, this would effectively cancel out the SF6 gas pressure change due to temperature change. The result is a constant P20 reading regardless of the temperature variations, giving reliable and accurate reading to gas density.

Hermetic sealing is a term used to define high airtightness level (or low air leakage rate) of a gas density meter. In gas density meter/monitors, hermetic sealing is required at all levels.

SF6 is a greenhouse gas with significant environmental impact, therefore its leakage is controlled at all stage. Besides the environmental impact, the significance of airtightness in SF6 meters is to ensure the stability and reliability of SF6 meters under various environmental conditions, thereby ensuring the accuracy of their measurement results. The sealing performance of SF6 meters can ensure normal operation in extreme environments such as high pressure and low temperature, avoiding measurement errors caused by gas leakage and affecting the safe operation of power systems.

Besides the airtightness of the airway, the casing of the gas density meter is also hermetically sealed. This is a feature known as Altitude Compensation.

As altitude increases, atmospheric pressure gradually decreases, which could affects the measurement accuracy of SF6 meters by skewing reference pressure in the gauge. Therefore, to compensate for this effect, it is necessary to design the casing of the SF6 meter to be hermetically sealed, which encloses the standard atmospheric pressure gas inside the meter to avoid the influence of external atmospheric pressure and ensure the accuracy and reliability of SF6 meter measurements in high-altitude areas.

The reason why the pointer of SF6 density meter/monitor is not at zero position is because of the built-in temperature compensation mechanism. When the ambient temperature changes, the bimetal chip deforms due to the temperature change, which drives the pointer to deflect. However, the temperature compensation is meant to target SF6 gas, not atmospheric air. Therefore, when the ambient temperature exceeds 20 ℃, the pointer will tend to deflect downward; when the ambient temperature is below 20 ℃, the pointer will deflect upward. This accuracy of the gauge will perform normally when installed onto SF6 gas tanks.