The so-called density refers to the mass per unit volume of a particular substance under specific conditions. SF6 gas in SF6 circuit breaker is sealed in a fixed and unchanging container, and it has a certain density value under the rated pressure at 20°C. Within the various permissible conditions of circuit breaker operation, although the pressure of SF6 gas changes with the temperature, the density value of SF6 gas always remains the same. Because the insulation and interrupting performance of SF6 circuit breaker depends largely on the purity and density of SF6 gas, it is especially important to detect the purity and monitor the density of SF6 gas. If an ordinary pressure gauge is used to monitor the leakage of SF6 gas, it will not be possible to distinguish whether the pressure of SF6 gas changes due to the real existence of leakage or due to the change of ambient temperature. In order to achieve the purpose of monitoring density frequently, the national standard stipulates that SF6 circuit breaker should be equipped with pressure gauge or SF6 gas density meter and density relay. Pressure gauge or SF6 gas density meter is to play a monitoring role, and density relay is to play a control and protection role.
SF6 gas density meter installed on SF6 circuit breaker, with pointer and scale is called density meter; those without pointer and scale are called density pressure switch; some SF6 gas density meter also has electric contact, that is, also used as density relay. They are all special meters used to measure SF6 gas.
SF6 gas density meter is mainly composed of elastic metal curved tube, gear mechanism and pointer, double metal belt and other parts, which is actually composed of double metal belt added to the spring tube type pressure meter mechanism. The hollow flexible metal curved tube is connected with the circuit breaker and its internal space is connected with the SF6 gas in the circuit breaker, and the end of the flexible metal curved tube is hingedly connected with the bimetal belt, which plays the role of temperature compensation, and the double-layer metal is hingedly connected with the gear mechanism and the pointer mechanism.
If for some reasons, such as air leakage or when doing tests, the circuit breaker will reduce the mass of SF6 gas, the pressure becomes smaller, the end of the flexible metal tube moves, which drives the pointer to indicate the direction of reduced value, the result is a smaller density or pressure value indicated by the pointer. Because the density watchband has two pairs of electrical contacts, which are used for sending signal and locking circuit breaker when SF6 gas density decreases. When the pointer drops to a certain position, the air supply signal will be sent or the circuit breaker is closed.
The density meter can only accurately measure the density or pressure value of SF6 gas when the SF6 circuit breaker is out of operation and after the temperature inside and outside the circuit breaker has reached balance; when SF6 circuit breaker is running, the reading error of gas density meter depends on the load current of circuit breaker and the temperature rise caused by loop resistance.