Measurement of air humidity

Transcription

1 Measurement of air humidity The humidity of air is an important environment's property in many industrial processes and technologies, as well as it affects a men's health. There is always some moisture in the air. Absolute humidity of air refers to water vapor in m 3 volume of air m p =, () V where m p is the mass of water that is contained in the air of volume V. As it follows from equation [], the SI unit of absolute humidity is kg/m 3. Amount of water vapor contained in the air is delimited by the point of vapor saturation at each temperature. The humidity reaches its maximal value max at this point. As can be seen from the table bellow, maximal value of humidity as well as vapor pressure that corresponds to vapor saturation point increases with the temperature. When the temperature of air with maximal value of humidity decreases, excess amount of water vapor condenses (transforms its phase) into liquid water such that air humidity is equal to its maximal value. Relative humidity of air ϕ is the ratio of absolute humidity to its maximal value max ϕ = (2) max Relative humidity is typically expressed in percents as ϕ % = 00 (3) max Minimal value of relative humidity (0 %) corresponds to the dry air and maximal value (00 %) corresponds to vapor saturated air. Relative humidity of air that falls between 50 and 70 % represents most pleasant environment for men.

2 Air humidity measured by psychrometer We will learn how to use Assmann's psychrometer (Fig. ) to measure the air humidity. It consists of two mercury thermometers; first of them is called dry thermometer and it is used to measure the temperature of surrounding environment t. The bulb of second thermometer, called the wet thermometer, is covered with a wet fabric. Evaporation from the wet thermometer lowers the temperature of surrounding air t 2 (i.e. t 2 < t ) by the latent heat of liquid-to-vapor phase transition. Difference between the two temperatures (t - t 2 ), psychrometric difference, therefore depends on the air humidity. Evaporation increases with decreasing humidity, which causes an increase in temperature difference. For precise measurement, Assman's psychrometer includes a fan that prevents water vapor from the formation of saturated vapor environment. dry thermometer fan wet thermometer temperature difference flowing air wet fabric nasávaný air intake vzduch Fig.. Assmann's psychrometer 2

3 Water vapor contributes to atmospheric pressure by the partial pressure. It is defined as the pressure of a vapor in dynamic equilibrium with its liquid form. Partial pressure of saturated vapor is constant for each particular temperature. Tab. 2 shows its values for several different temperatures. Relative humidity of air determined by the Assmann's psychrometer can be then calculated from the partial pressure of water vapor p (at the temperature t ) and partial pressure p 2 (at the temperature t 2 ) via relation p Ab p ( t t ) 2 2 ϕ = (4) where b is an atmospheric pressure in pascals (Pa) and A is a psychrometric constant. Its value for psychrometer used in our laboratory is A=0,00066 K -. Equipment: Assmann's psychrometer, barometer, test-tube with distilled water. Experimental procedure:. Read the temperature t of the dry thermometer. 2. Wet the fabric that covers the bulb of wet thermometer by placing the test-tube with distilled water into the opening underneath the thermometer for about 0 seconds. Spin the string of fan in top part of psychrometer such that it will rotate during the entire measurement. Watch the continuous decrease of temperature of the wet thermometer and write down the value t 2 after it becomes steady. 3. Wait until fan stops moving and temperature of wet thermometer increases again. Repeat the measurement two more times and record all the values into the table. 4. Read the atmospheric pressure b of the barometer. 5. Calculate relative humidity ϕ according to the equation (4) and express it in percents (ϕ % ). Calculate absolute humidity using the equation (2) and values from the Tab. 2. Determine average values of ϕ % and using all three measurements. 3

4 Tab.. Temperature of dry thermometer t, temperature of wet thermometer t 2, partial pressure of water vapor p at temperature t and partial pressure p 2 at temperature t 2, relative humidity ϕ, maximal humidity max and absolute humidity of air at the laboratory temperature t. measurement t [ C] t 2 [ C] p [Pa] p 2 [Pa] ϕ 2 3 max [kg/m 3 ] [kg/m 3 ] Tab. 2. Temperature dependence of the partial pressure of water vapor p and maximal humidity of air max. t [ C] p [Pa] max [kg/m 3 ]

5 References:. Kopecký, F.: Physics for Students of Pharmacy I. Bratislava, UK s. (in Slovak). 2. Edition of Department of Physical Chemistry: Laboratory Practice in Physics for Students of Pharmacy. Faculty of Pharmacy, Comenius University, Bratislava, UK Oremusová J., Vojteková M.: Determination of air humidity by psychrometer. Manual for laboratory practice. (in Slovak) Manual written by RNDr.J.Gallová, CSc. English version prepared by N. Kučerka, PhD. 5