Measurement of moisture (water content) in a gas may be performed by measuring relative humidity or absolute humidity of the gas in the expansion method, dry and wet bulb method, condensation and dew point method, lithium chloride dew point method, resistance method, capacitance method, electrolysis method, and quality method as provided in national relevant standards. As main methods used in on-the-spot measurement of water content (moisture) in the SF6 gas insulated apparatus in the power supply system, they are electrolysis method, condensation and dew point method, and resistance method, capacitance method. Now dew point meters are used in regular patrol inspection within substations on most occasions. There are a few of common characteristics between them, those are, their power supplies have to be cut off during measurement; the instrument must be regularly calibrated, so that online measurement become impossible; and it is required to discharge SF6 gas for 15 to 30 min in each measurement. Taking an experience we have performed using a DP-19 dew point meter as an example, the first ten minutes at least have to be used for drying the sampling pipeline because of moisture in the sampling pipeline. The whole measurement job will take about 45 to 55min, otherwise accurate and stable data cannot be gotten. Figure 1 shows a time vs. the value found curve obtained by DP-19 dew point meter. Therefore SF6 gas in a gas chamber will require to complement after only 2 to 3 measurements usually.
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