3. The water level in a large concrete reservoir is found to drop by 20 mm over a period of 10 days. The average temperature of water is 23C and the average humidity of the bulk air is 0.0135 kg per kg dry air. Permeation through the concrete may be neglected. If loss of vapor occurs through a stagnant film of air, calculate the thickness of this film. The diffusivity of moisture in air under given conditions is 1.0 ft2/h and the vapor pressure of water can be calculated using Antoine's equation