As the path arcs over, failure occurs. Water deposited on horizontal insulators and on the bottom surface of horizontal runs of waveguide causes increased reflected power. Design considerations Condensation in transmission lines depends on the amount of moisture in the contained air, expressed as dew point or relative humidity, and the outside ambient air temperature. Dew point is the temperature at which water droplets or ice crystals condense on a colder surface. In other words, when the partial pressure of the water vapor equals the saturation partial pressure of the water, moisture condenses.
Relative humidity is the ratio of the actual vapor pressure to the saturated vapor pressure.
Both dew point and relative humidity are affected by temperature and atmospheric pressure. Dehydrators remove specific amounts of water at a specified maximum relative humidity and temperature to provide air at a specific minimum dew point. Outside ambient temperature changes require special consideration. The dry gas supply system must be sized to maintain transmission line pressure when the outside temperature drops at rates as high as 50 F per hour. Likewise, because the transmission line pressure increases when the ambient temperature and solar load on the transmission line increase faster than the leak rate, the transmission line must be protected with a high pressure relief valve.