Objective: To measure dew point.
Equipment: Two steel glasses preferably new ones with
shining surfaces, Cold water in a separate bottle, laboratory
thermometer measuring up to 500C.
Introduction: Air in our surrounding contains water vapour.
In general the amount of vapour in a given volume of air is less
than that required to saturate it. That is why wet cloths dry,
water kept in a shallow plate evaporates and so on. The amount
of vapour needed to saturate the air depends on temperature. At
higher temperature, larger amount of vapour in needed to
saturate the air than that needed at lower temperature. If the
temperature of the air in your room is decreased sufficiently,
the existing amount of vapour will become sufficient to saturate
it. If the air is further cooled the existing amount of vapour
will become more than that needed to saturate the air and then
some vapour will condense to liquid water. This is what happens
during the winter nights in open fields. The water that appears
on grass or other objects is called Dew. The temperature at
which the existing vapour in air will saturate it is called the
“Dew point”. Evidently, the dew point will vary with place and
time. During rainy season there is lot of vapour in air and the
due point will be high. During dry seasons the dew point will be
low as there will be only small amount of vapour present in air.
The amount of vapour present in air is often represented by the
pressure exterted by the vapour molecules. This is known as
vapour pressure. If the air is saturated with the vapour the
vapour pressure is called saturation vapour pressure. The
saturation vapour pressure at different temperatures have been
measured carefully and tables are available to give these data.
If you know the dew point, you can immediately calculate the
relative humidity by the relation
Relative
humidity = (saturation vapour pressure at dew point)/(
saturation vapour pressure at the existing air temperature). Procedure: Put two steel glasses side by side. Keep
cold water, taken from a refrigerator or water cooler in a
bottle or a jug. It should be cold enough so that you can see
large water drops on the outer surface of the bottle. Fill one
of the steel gasses with normal tap water up to say one-fourth
of its height. Put the thermometer in the water. Ask some one to
keep looking at the outer surfaces of the two glasses.
Pour a small amount of cold water in the water, stir with the
thermometer and ask the person looking at the glasses if she/he
can see a difference in the shining of the two glasses. If not
pour some more cold water and repeat the observation. When the
difference in shining is observed read the thermometer. This is
the dew point.
Steel is a good conductor of heat and the thickness of the wall
is also quite small. So the temperature of the outer surface of
the glass, up to the height of the water, quickly becomes the
same as that of the water inside. The air in contact with this
part of the steel surface also cools down and attains this
temperature. When the temperature falls to the dew point and
goes slightly below it, vapour in this part of the air starts
condensing and small droplets collect on the glass surface. By
comparing the two glasses kept side by side, you can easily make
out when this condensation has just started. So the dew point
can be easily obtained.
Using the saturation vapour pressure table, find the relative
humidity.
Discussion: The dew point can be measured very accurately
using this procedure. If your thermometer reads up to half a
degree and you do the experiment carefully, you can get the dew
point within the accuracy of half degree. Make sure that your
initial water in cold enough. The test is that water drops are
very clearly seen on the outer surface of the vessel containing
the cold water. You can first do a rough experiment to get an
estimate of the dew point and then do a fine experiment by
varyuing the mixture temperature in that vicinity to get more
accurate value.
Instrument cost and availability: Laboratory thermometer
available in any scientific store.
