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Figure 8-1 provides a linear graph:8.4 Taking a Dew Point Measurement of the Main Tank
After the leak test is complete and at the end of vacuum hold time of four to six hours, the next
steps should be followed:
• Break the vacuum with dry air or nitrogen and apply a positive pressure of two to three
pounds. The dry air or nitrogen should have a dew point of -50ºC (-70ºF) or lower.
• Allow the transformer to sit idle for 24 hours in order to allow the gas and the surface
moisture of the insulation to approach equilibrium.
• Measure the dew point after a 24-hour hold time. Determine if the dew point measurement is
acceptable. The graph in Figure 8-1 may be used as a guide.
In order to use the graph that follows, the insulation temperature must be estimated. The best
estimate would be to use the temperature of the oil from the oil temperature gauge. The reading
should be made when the oil temperature is steady and should be taken early in the day before
the sun has had an effect on the oil and tank temperatures.
If the dew point is found acceptable, the transformer can be vacuum oil filled. If the dew point
reading is found unacceptable, a dry-out procedure must be followed before proceeding with oil
filling.