Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Measure Dew Point in Your Room

Introduction

In the winter early morning, you can easily find small droplets of water on the leaves. As the temperatures dip at this time, the amount of water vapour present in the air saturates the air and further cooling results in some of the water vapour to condense as a liquid to form the Dew. The dew point is the temperature at which dew forms. It is the temperature to which air must be cooled at constant pressure and water content to reach saturation.

Once the dew point is known the relative humidity (RH) is calculated by taking ratio of `saturation vapour pressure at dew point' and `saturation vapour pressure at the room temperature'. (Here saturation vapour pressure is the pressure exerted by the vapour when the air is saturated by the vapour. Saturation vapour pressure has been measured for various temperatures and tables are available which can give us its value at the required temperature.)

Apparatus

Two new steel glasses, cold water bottle, thermometer

Procedure

Put the two glasses side by side.

  1. Take out a cold water bottle from the refrigerator. See that water has condensed on the surface of the bottle after you take it out of the refrigerator.
  2. Now fill one fourth of one glass with normal tap water and put a thermometer in the glass.
  3. Gradually pour some cold water in the glass and stir. See if there is a difference in the shine of the surfaces of the two glasses.
  4. If yes, note the temperature of water in the thermometer.
  5. If not, pour some more cold water till you start noticing a difference in the shine of the surface of the glasses. Note the temperature of the water.
  6. This is the ``Dew point'' of your room.

Discussion

Steel is a good conductor of heat. So the temperature of the water inside the glass and the outer surface of the thin steel glass up to the height of water quickly become same. The air in contact with the surface cools down as it also attains the temperature of the surface. When the temperature falls just below the dew point, vapour starts condensing and small droplets of water collect on the glass surface making it look a little foggy. So a careful comparison of the shininess of the two glasses, one with the cold water and the other empty, gives a fair measure of the dew point.