Thermodynamics Lab

Universität Duisburg-Essen 4. Semester Fak. 5 , IVG Fachgebiet Thermodynamik Laborleiter: Dr. Siddiqi Thermodynamics Lab Humidity Measurement 1 Humidity Measurement Introduction Air comprises a mixture of gases of which nitrogen makes up more than 78%, oxygen 21% and carbon dioxide and the inert gases (argon, neon, krypton, helium etc.) the remainder. These are known as the dry gases of the atmosphere. In addition to these dry gases, the air also contains varying amounts of water vapor. At normal temperatures and pressures water is able to exist in both a liquid and a gaseous (or vapor) form, but it cannot be treated in the same way as the other gases of the atmosphere because its quantity, and hence proportion are continually varying. Water vapor is said to be associated with dry air and the more vapor associated with the dry gases, the more humid the air. Humidity measurements are important not only in meteorology but also in pulp and paper industry, drug and food storage and delivery, film industry etc. Hygroscopic materials draw water from their surroundings and can form solution (e.g., salt, sugar). Strongly hygroscopic materials are used directly for the drying of substances (e.g., gases). The humidity can have dramatic effect on our perception of comfort. It can affect the well-being of a person or even cause diseases. A measure of the water vapor content in the air is the absolute humidity (Feuchte). It shows how many kg of water vapor are present in unit volume (1 m3) of the gas mixture. Depending on the state of the gas mixture (dry or wet) taken as reference one defines: a) Related to the unit volume of the wet gas mixture m kg O = f ′ = W H 2 ′ 3 V m wet air (1a) mw = Mass of water vapor V ′ = Volume of wet gas mixture (slash means here that it is the wet gas mixture) ′ f is also called the density of the water vapor. b) Related to the unit volume of the dry gas mixture 2 mw kg H 2 O = f = (1b) V m3 dry air These both humidity values depend on temperature via the temperature dependence of V under a given atmospheric pressure. If one does not relate the water vapor content to the unit volume but to unit mass of dry air, one gets temperature independent value, known as moisture content or humidity ratio. mw kg H 2 O = x = (2) mL kg dry air Dry air can absorb water vapor in a certain state (at certain pressure and temperature) up to a certain amount. The absorption capacity grows with a rising temperature. If the maximum absorption capacity is reached at a certain temperature, it is then „saturated air“. Another way to express the absorption capacity of wet air in a certain state (1) (p1, t1, x1), is the relative humidity ϕ. It is defined as the ratio of the available absolute humidity in state (1) ´ ´ humidity f , to the maximum possible humidity f s at the same temperature t1, i.e. the absolute humidity in the saturation state: ϕ = (f ′ t1 )/ f s (’ t1 ) (3) x . t + s 1 n o ϕ 1 h c = = S ϕ t1 t 1 = c 1 o n s t. x 3 Another possibility to describe the degree of the saturation of moist air exists in the comparison of the actual air temperature t1 and the so-called dew point temperature (Taupunkt) tTpkt. The dew point temperature is the temperature at which the first drop of water condenses when we cool the air in state (1) at constant water content x1. If the air is cooled below the dew point temperature to t2, the air loses water vapor. The mass of the water lost (due to condensation) is m = m ( x - x21Lw ) . The information (expression) of the dew point temperature is well suited in the climate technology because it gives the temperature up to which a test atmosphere (room) can be cooled without any condensation takes place. 1 x ϕ 1 + = 1 h t Tpkt Taupunkt 2 t2 x2 x1 x The third possibility to describe the state of moist air at a certain temperature t1, makes use of the cooling limit temperature (Kühlgrenztemperatur) tF. The cooling limit (Kühlgrenze) lies in h, x- diagram on the intersection of the extended nebulous isotherms of the state point (1) of the air to and the line ϕ = 1. The temperature at this point is the temperature which the water, initially at any temperature, attains if it is brought in contact with unsaturated air of state (1). 4 x + 1 1 ϕ=1 h t1 t F Kühlgrenze x1 x For the representation of the concentration of water vapor in the gas mixture the mass fraction mw nw nW , the mole fraction and the partial pressure pD = p are commonly used. In general m n n p = pG + pD is valid for every vapor gas mixture. In the special case of vapor gas mixture if we apply the ideal gas law, then T T p = mG RG and p = Rm WW , (4) G V D V and it means that every component behaves in such a way as if it were alone in the volume V (Dalton´s law). RG and RL are the specific gas constants for dry gas mixture and water vapor. The partial pressure of the water vapor at saturation is p . It can be shown that for total DS pressure p <10 bar the saturated partial pressure p (t) can be approximated (better than 1%) DS with the vapor pressure pWS of pure water at temperature “t” which is given by the vapor pressure curve as a temperature function (Table 1). The water vapor pressure p D in nW unsaturated air can be calculated from pD = pWS(tTpkt), as p = p ≠ f (t) remains constant D n at dew point temperature and at dew point it is saturated. With equation (4) it follows for equation (3) p p ( tTpkt ) ϕ = D = W S p (t) p (t) DS W S The difference of both pressure pD and p is called saturation deficit Δp. DS 5 Lab Experiment Comparative measurements of the humidity in a moist chamber should be made with different measuring methods and the values will be represented in different humidity units. Two different climates are to be examined. 1.Object of the Experiment The humidity of air in chamber should be measured with different measuring instruments. The following instruments will be used to measure the humidity. The working principle, and the advantages and disadvantages will be described. - Humidity indicator - Hair hygrometer as a polymeter - Assmann Aspirations-Psychrometer - LiCl dew point measuring instrument - Mirror dew point measuring instrument. The different humidity values are determined by suitable conversions, tables and diagrams. 2. Working principles 2.1 Humidity indicator The humidity indicator is a chemically preserved paper strip which reacts like litmus paper (used for pH) by changing its color with a change of the relative humidity. With increasing humidity the color segments will become pink (rosa), with increasing dryness they becomes blue (blau). This process can be repeated many times and the humidity value can be read in % of relative humidity (relative Feuchtigkeit) on a printed scale (kept beside). The both strips used for the experiment are shown in Fig. 1(Abb. 1). The advantage of these measuring stripes is primarily, the low price and lower place requirement. For a continuous view control with low demand for accuracy (5 - 10%) the indicators can be used even in very small places (self sticking strips, basic implementation: 50 x 19 mm). Their use is not possible in air containing ammonia and with the appearance of liquid water on it’s surface. 6 Skala 60 Skala Feuchtigkeit in % Temperatur 40 Sättigungsdruck 40 Ablesung im Grenzfeld rosa-blau in °C 30 in Torr 20 30 40 50 60 70 80 90 100 20 20 10 10 trocken normal feucht 0 5 Haarbündel 10 2 Feuchte-Indikator Ablesung der relativen Feuchte in % am Übergang rosa-blau, oder wo Strei- fen der Untergrundfarbe entspricht 20 25 °C 30 15 10 5 trocken 35 30 40 30 50 0 0 80 45 1 % 1 0 rel. Feuchte 0 normal 50 0 55 60 feucht 65 70 75 80 Abb. 1 Zwei Typen von Feuchte-Indikatoren mittels Farbumschlag Abb. 2 Haarhygrometer in Polymeterform 2.2 Hair polymeter The hair-polymeter represents the easiest and most inexpensive instrument for hygrometric measurements. In hygrometric procedures the physical properties of substances which are dependent on humidity are used for the measurement. These can be mechanical (length + twist) and electrical properties (surface resistance, conductivity) as well as heat evolution, diffusion, infrared absorption and the electrolysis. For the mechanical properties the substances which change their length with the humidity seem to be a simple and good choice. It is also required that the change in length should be reproducible, i.e. it should come to the initial value when the humidity changes back to its initial value. Hair fulfills this requirement and is also inexpensive. It has a thermal coefficient of expansion of about 3.4x10-5 per degree which is almost temperature independent. It is, therefore, used for the purpose of making a device to measure the humidity. The working principle of hair hygrometer is that the length of hair changes with the increase or decrease in humidity. The hair hygrometer (Fig. /Abb. 2) uses hair under tension to measure humidity. When relative humidity increases, hair becomes longer, and when it drops, hair becomes shorter. This instrument uses strands of human or horse hair attached to levers that magnify 7 small changes in length.

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