# Hygrometry

Dew point;

The dew point temperature is the temperature at which the air can no longer "hold" all of the water vapor which is mixed with it, and some of the water vapor must condense into liquid water. The dew point is always lower than (or equal to) the air temperature.

Supersaturated air:

If the partial pressure of water vapor in air exceeds the saturated vapor pressure of water, the air is said to be super saturated.

Triple point:

The triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in equilibrium.

PV diagram of triple point

Difference between saturated and unsaturated vapor:

 saturated vapor unsaturated vapor 1. A saturated vapor is one which is in contact with its liquid in a closed space.   2. The pressure exerted by a saturated vapor is always constant for a given temperature of the liquid, even though the vapor undergoes changes in volume. 1. An unsaturated vapor at a given temperature is one which exerts less than the saturation vapor pressure        for that temperature and is obtained when no liquid is in contact with it in a closed space.   2. The    pressure of a    given mass of unsaturated vapor depends on its volume as well as its temperature.

Relative humidity;

The amount of water vapor present in air expressed as a percentage of the amount needed for saturation at the same temperature. The relative humidity of an air-water mixture is defined as the ratio of the partial pressure of water vapor (H2O) in the mixture to the saturated vapor pressure of water at a given temperature. Thus the relative humidity of air is a function of both water content and temperature.

Relative humidity $= \frac{{\rm{m}}}{{\rm{M}}}{\rm{*}}100{\rm{\% }}$

Hygrometer is used to find out relative humidity at a place. Hygrometer is an instrument used for measuring the moisture content in the atmosphere. Humidity measurement instruments usually rely on measurements of some other quantity such as temperature, pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed.

Dry bulb and wet bulb hygrometer

The difference in temperature between the dry bulb and wet bulb is used to calculate the relative humidity of the air. The temperatures of both thermometers are noted after every one minute. Finally the constant temperature of wet bulb thermometer is noted and same for the dry bulb thermometer also. The different in the reading of wet bulb thermometer is measure of relative humidity of air of that place.

Triple point;

The triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in equilibrium. The triple point for solid, liquid, and gas phases, a triple point may involve more than one solid phase, for substances with multiple polymorphs. Helium-4 is a special case that presents a triple point involving two different fluid phases. Two dimensional P-T diagrams are also sufficient to discuss about phase of matter. Such P-T diagram is called phase diagram. The phase diagram will be complete by drawing following three curves namely;

1. Vaporization curve: it is a graph between pressure and boiling point of the substance in the liquid state.

2. Fusion curve; it is a graph between pressure and melting point of the substance in the solid state.

3. Sublimation curve: it is a graph between pressure and temperature at which a solid directly changes to vapor state.

PV diagram of triple point

Dry and wet bulb hydrometer:

Hygrometer is used to find out relative humidity at a place. Hygrometer is an instrument used for measuring the moisture content in the atmosphere. Humidity measurement instruments usually rely on measurements of some other quantity such as temperature, pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed.

Dry bulb and wet bulb hygrometer

An instrument that uses the difference in readings between two thermometers, one having awet bulb and the other having a dry bulb, to measure the moisture content or relative humidity of air.The difference in temperature between the dry bulb and wet bulb is used to calculate the relative humidity of the air. The temperatures of both thermometers are noted after every one minute. Finally the constant temperature of wet bulb thermometer is noted and same for the dry bulb thermometer also. The different in the reading of wet bulb thermometer is measure of relative humidity of air of that place.

Distinguish between saturated and unsaturated vapor:

 saturated vapor unsaturated vapor 1. A saturated vapor is one which is in contact with its liquid in a closed space.   2. The pressure exerted by a saturated vapor is always constant for a given temperature of the liquid, even though the vapor undergoes changes in volume. 1. An unsaturated vapor at a given temperature is one which exerts less than the saturation vapor pressure        for that temperature and is obtained when no liquid is in contact with it in a closed space.   2. The    pressure of a    given mass of unsaturated vapor depends on its volume as well as its temperature.

An experiment to study the variation of saturated vapor pressure with temperature:

Figure21_5; An experiment to study the variation of saturated vapor pressure with temperature

Consider two barometric tubes X and y as shown in the figure. X has toricellian vacuum and y has unsaturated vapor. Both tubes are enclosed by wider glass tube containing water which can be heated with a heating coil and temperature can be noted with thermometer.

At the beginning water is heated to a certain temperature about 80oC and then heating is stopped. As temperature farther falls the saturated vapor pressure varies with temperature as shown in the figure. It may be noted that S.V.P varies rapidly at higher temperature and slowly at lower temperature.

Effect of pressure on boiling point;

The higher the vapor pressures of a liquid at a given temperature, the lower the normal boiling point (i.e., the boiling point at atmospheric pressure) of the liquid.

Clothes dry faster when wind is blowing:

The faster the wind blows, the more quickly the micro-layer of air against the clothes is swept away and replaced by drier air. Or it increases the rate of evaporation by driving molecules of wet clothes away from it.

Damp clothes dry more quickly on winter day;

In sunny winter the temperature is not as high as in summer, relative humidity of air is low, water evaporates rapidly and it takes less time to evaporate water from the wet clothes.

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