Switchgear Testing

Electron capture detector(ECD) method for sf6 gas switchgear

An electron capture detector is used to detect SF6 with high sensitivity below 1 ppmv. This device makes use of the high electron attachment coefficient of SF6, i.e. the ability of capturing electrons. The free electrons to be attached to the SF6 molecules are created by a radioactive source. The ECD uses a radioactive emitter, typically, a metallic membrane coated with the radionuclide nickel. The emitted electrons are accelerated in an electric field and ionize the background gas, normally ambient air. Ions and electrons are collected at the electrodes resulting in a steady state ionisation current.
The presence of SF6 besides air reduces the number of free electrons, as they are now attached to the SF6 molecules. The decrease in the ionisation current is proportional to the SF6 concentration. However, other molecules also have a certain electron attachment coefficient. the detector is also sensitive to those molecules.
In principle this device is a flow rate detector, since the sensor is pumping the gas at a constant speed through the electric field.By calibration this flow rate is internally converted into SF6 concentrations and recorded as ppmv.
Photo show a ECD.

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