Universität Duisburg-Essen Fak. 5, IVG Fachgebiet Thermodynamik Laborleiter: Dr. Siddiqi. 4. Semester. Thermodynamics Lab. Humidity Measurement

Transcription

1 Universität Duisburg-Essen Fak. 5, IVG Fachgebiet Thermodynamik Laborleiter: Dr. Siddiqi 4. Semester Thermodynamics Lab Humidity Measurement 1

2 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 m 3 ) 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 mw = V f 3 kg H 2O m wet air (1a) m w = 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

3 m = V w f 3 kg H 2O m 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. (1b) m x= m w L kg H 2O kg dry air (2) 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) (p 1, t 1, x 1 ), 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 at the same temperature t 1, i.e. the absolute humidity in the saturation state: f s ϕ = f ( t1 )/ f ( t1 ) (3) s h1+x 1 t 1 ϕ =const. S t =const. 1 ϕ =1 x 3

4 Another possibility to describe the degree of the saturation of moist air exists in the comparison of the actual air temperature t 1 and the so-called dew point temperature (Taupunkt) t Tpkt. 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 x 1. If the air is cooled below the dew point temperature to t 2, the air loses water vapor. The mass of the water lost (due to condensation) is = ml ( x1 - x ). The information (expression) of the dew point mw 2 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. h 1+x t Tpkt t ϕ =1 Taupunkt x 2 x 1 x The third possibility to describe the state of moist air at a certain temperature t 1, makes use of the cooling limit temperature (Kühlgrenztemperatur) t F. 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

5 h 1+x t 1 1 ϕ =1 t F Kühlgrenze x 1 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 p D = p are commonly used. In general m n n p = p G + p D 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 pd = mwr W, (4) G V V and it means that every component behaves in such a way as if it were alone in the volume V (Dalton s law). R G and R L are the specific gas constants for dry gas mixture and water vapor. The partial pressure of the water vapor at saturation is p DS. It can be shown that for total pressure p <10 bar the saturated partial pressure (t) can be approximated (better than 1%) p DS with the vapor pressure p WS 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 unsaturated air can be calculated from p D = p WS (t Tpkt ), as at dew point temperature and at dew point it is saturated. = p D n n W p f (t) remains constant With equation (4) it follows for equation (3) p p ( t ) D W Tpkt S ϕ = = p (t) p (t) D S W S The difference of both pressure p D and is called saturation deficit Δp. p DS 5

6 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 p H ) 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

7 Feuchtigkeit in % Ablesung im Grenzfeld rosa-blau trocken normal feucht Feuchte-Indikator Ablesung der relativen Feuchte in % am Übergang rosa-blau, oder wo Streifen der Untergrundfarbe entspricht trocken normal feucht Skala Temperatur in C C % rel. Feuchte 80 Skala Sättigungsdruck in Torr Haarbündel 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 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

8 small changes in length. Hair hygrometers are not as accurate as their counterparts and register significant errors at very high and very low relative humidity. The disadvantages of the method are: - The accuracy of the measurement is approx. 3-5% in the relative humidity value. It shows an additional hysteresis effect of 8%. - It can be used in the relative humidity range of 30 90% and below 70 C (at this temperature the hair are affected by the heat). Also the use below - 30 C is not recommended as the reading is not shown promptly (it is sluggish). - To avoid false measurements, the hair must be "regenerated" in 2-weeks cycle. It is done by keeping the instrument in saturated atmosphere for ½ hour. The hair hygrometer used by us is in the form of a Polymeter. So besides hair hygrometer it is also provided with a thermometer which shows the surrounding temperature as well as the saturation pressure in Torr at this temperature. Other directly readable quantities are: - the relative humidity (Feuchte) on the lower scale of the round indicator - the difference Δt between ambient temperature t U and the dew point temperature t Tpkt on the upper scale of the round indicator. In this case the left point of the triangle gives the value at +20 C surroundings temperature, the middle point the value at +10 C surroundings and the right point at 0 C surroundings (at other surrounding temperatures the values must be estimated accordingly). - the saturation water vapor pressure (in Torr) is shown on the right side of the thermometer besides the ambient temperature. From it one can find immediately. - the dew point temperature t Tpkt = t U - Δt - the partial pressure of water vapor p D = φ p Ds [equation (5)] - the absolute humidity ~ partial pressure of water vapor p D in Torr. This is a coarse approximation in the range of -30 C to + 45 C (see Fig./Abb. 3). The exact values must be taken from tables (Table 1) or diagrammes (Fig. 4a and Fig./Abb. 4b). 8

9 absolute Feuchte f' g / m Luft H O Dampfdruck 1000 mbar üb. unterkühltem Wasser C 100 Temperatur Abb. 3 Dampfdruckkurve des Wassers φ C 40 Gas-Temperatur Abb. 4a Abhängigkeit von relativer, absoluter Feuchte un Temperatur im Bereich von -10 C bis +40 C 9

10 absolute Feuchte f' g / m Luft H O φ C 100 Gas-Temperatur Abb. 4b Abhängigkeit von relativer, absolute Feuc und Temperatur im Bereich von

11 2.3 Aspiration-Psychrometer (Assmann) The psychrometric measuring procedure, also called Dry and wet thermometer procedure, uses the cooling effect which is observed when an unsaturated gas stream flows over a moistened thermometer thereby evaporation. The process which takes place is a combined heat and mass transfer process. Figure 5 (Abb. 5) shows the basic construction of an Aspiration-Psychrometer. Uhrwerk mit Ventilator feuchtes Thermometer trockenes Thermometer Luft Abb. 5 Prinzip-Aufbauskizze eines Aspirations- Psychrometers nach Assmann 11

12 It consist of two thermometers. The bulb of one thermometer is kept wet (by means of a thin, wet cloth wick) so that the cooling that results from evaporation makes it register a lower temperature than the other (called dry bulb) thermometer. The flowing air (at the inlet of the measuring device) has the water vapor partial pressure p D and the temperature t u, which is shown by the dry thermometer. If p D is smaller than the saturation pressure p S at this temperature, the water evaporates from the wet bulb wick and saturates the passing gas stream. The needed energy for the vaporization of water is supplied by the internal energy of the gas(unsaturated air), the thermometer bulb and the wet wick. Through this, the temperature of the wet thermometer drops below that of the gas to a typical value, the cooling limit temperature t F. This depends on the saturation deficit of the gas. After the equilibrium temperature is reached the necessary heat energy for the continuous vaporization of water is taken only from the gas stream. We equate the heat amount which is taken away from the gas by cooling to the heat amount necessary for the evaporation and get the expression for the water vapor partial pressure p D : p D = p F p c p Δt - * R hv where p = Barometer pressure Δt = (t u - t F ) psychrometric temperature difference t u = Temperature of the measuring air (6) p F = Water vapor saturation pressure at t F c p = spec. heat capacity of the gas at constant pressure R * = R L /R W, R L is the specific gas constant for dry air and R W for water vapor h v = specific enthalpy of vaporization for water For technical measurements we mostly use (using the empirical approximation formula p D = p F - p A Δt, with A = Psychrometer constant) the formula from SPRUNG (1888) for the measurements in air, which simplifies equation (6) to: p D = p F C p 755 Δt, (7) 12

13 where the constant 0.5 refers to Assmann aspirations psychrometer with an air speed > 2.0 m/s. For practical evaluation mostly the tables, graphical representations or special slide rules or discs are used. Thus the absolute humidity can be determined by means of Table 1 with the help of the water vapor partial pressure, or one can get the relative humidity directly from the psychrometric temperature difference (Δt) using Fig./Abb. 6 ("Graphical Psychrometeric Table") or with the help of the humidity slide rule. Very accurate results are read from the Psychrometer Table provided by the manufacturer, because it uses the Psychrometer constant very precisely which depends on the respective construction details, the wind speed and the gas kind. The Aspiration psychrometer comes from the meteorology where it is also used intensively even today because of its mobility. The accuracy in the humidity measurement is limited by the accuracy of the temperature readings on the thermometers. With the calibrated accuracy of the thermometers available in our experiment, we reach an accuracy better than 0.2 K in the psychrometric temperature difference; this corresponds at an ambient temperature of 25 C the exactness in humidity measurements of 1% of relative humidity. For accurate measurement the side effects that have an influence on the psychrometric effect should be taken into account. Primarily, we have to keep the wind speed to a defined value because the saturation of air strongly depends on its value. As many experiments show the speeds over 2 m/s also give acceptable values. Also, for lower speeds calibrations are possible, nevertheless then the variations in wind speed are possible which enhance the measuring errors. It is favorable anyway to provide artificial ventilation (e.g. by fan) for the necessary gas speed. However it is to be assured that the ventilation does not raise the temperature of the gas before the measurement. Beside the necessary least gas speed of 2 m/s, the application of the Psychrometers is limited furthermore by the necessary moistening of the wet thermometer bulb which should remain the same during the measurement and the extreme sensitivity against impurities (e.g. dust particles) in the air which can lower the vapor pressure of the water and thus affect the psychrometric effect. For these reasons continuous measurements are only very hardly possible with this device. There are of course some devices which can continuously humidify the wet bulb thermometer but due to the impurities in air the wet wick has to be changed regularly in short periods. 13

14 Psychrometrische Differenz 35 C % % 20 % 30 % 40 % 50 % 5 60 % 70 % 80 % 90 % C 50 0 Temperatur des trockenen Thermometers Abb. 6 Graphische Psychrometertafel für stark bewegte Luft Luftgeschwindigkeit 2 m/s > = 14

15 A consideration of the Psychrometer formula (6) or (7) shows that at low temperatures and water contents even small errors in temperature readings can result in very high errors in the humidity values because of the small value of the psychrometric temperature difference (Δt). This fixes the lower temperature limit for the accuracy in psychrometeric measurements. The upper limit is where the cooling limit temperature reaches the boiling temperature of water, because then water does not exist anymore in liquid aggregate state. The relative humidity ϕ can be read in h-x-diagram [Fig.(Abb.) 7] comfortably in the intersection of the cooling border fog isotherm with the ambient temperature. 15

16 h [kj/kg] 1+x Δh 1+x /Δx [kj/kg] t=60 C 60 ϕ=0,1 ϕ=0,2 t=50 C 50 t=30 C t=30 C t=35 C h =50 kj/kg 1+x t=40 C t=0 C (Wasser) t=45 C ϕ=0,4 t=40 C 40 h =200 kj/kg 1+x ϕ=0,6 h =175 kj/kg 1+x ϕ=0,5 ϕ=0,8 30 t=25 C p* =0,04 bar d p* =0,03 bar d p* =0,05 bar d p* =0,06 bar d p* =0,07 bar d p* =0,08 bar d t=0 C (Eis) p* =0,02 bar d t=-10 C Abb. 7 Mollier-h,x-Diagramm für feuchte Luft p* =0,09 bar d t=20 C t=10 C Δh 1+x /Δx [kj/kg] p=1,01325 bar p* =0,1 bar d h =150 kj/kg ϕ=1,0 1+x h =125 kj/kg 1+x t=20 C 20 h =100 kj/kg 1+x h =75 kj/kg t=15 C t=10 C 10 1+x 0 h =25 kj/kg 1+x h =0 kj/kg 1+x -10 x [g/kg] 60

17 ~ 2.4 LiCl-dew point measuring instrument The measurement principle is based on the hygroscopic nature (ability of a substance to attract water molecules) of lithium chloride. The hygroscopic property of LiCl to draw moisture from the environment and the strong dependency of the electrical conductivity of LiCl solution on its water content is used to measure the dew-point of a gas mixture (air). Figure 8 shows the basic construction of a LiCl- dew point cell. The sensor (e.g. a resistance thermometer/widerstandsthermometer) consists of a reel covered with an absorbent fabric (glass fibre/ Glasgewebe) and a bifilar winding (two insulated wires, with current traveling through them in opposite directions) of inert electrodes. The reel is coated with lithium chloride. An alternating current is passed through the winding and the lithium chloride solution, causing resistive heating. Widerstandsthermometer Elektroden Glasgewebe mit LiCl Abb. 8 Ausführungsprinzip einer LiCl-Taupunktzelle If an alternating current is supplied to the precious metal wires of the hot probe (low water content in LiCl solution) then first of all only a low current flows which is proportional (via electrical resistance) to the water content in the LiCl solution. As the time passes the water content rises slowly due to the hygroscopic effect of the LiCl, which leads to a reduction of 17

18 the electric resistance and an increase of current flow. The more and more Joule s energy released thereby causes, through the evaporation of water and a growing counter hygroscopic effect, the flow of water from the LiCl solution to the surroundings. When the sensor has been heated up so much that the mass flow of the evaporated water becomes larger than the diffusion flow which results from the hygroscopic effect of the LiCl, then the water content in the solution decreases again, the resistance increases and the electric current decreases. The sensor cools to the extent that the proportion of the mass flow changes, i.e. the hygroscopic mass flow is again just higher than the flow of evaporated water. The apparatus comes to an equilibrium state where the temperature and the water content of the LiCl solution reach a state where the mass flow of the evaporated water is equal to the diffusion flow as a result of the hygroscopic effect of the LiCl solution. In this equilibrium state the water vapor pressure over the LiCl solution is equal to the partial pressure of the water in the surrounding air. Starting the measuring process with a saturated probe (very high water content) would lead, due to the low electric resistance of the solution, to the damage of the sensor by overheating. The temperature of the LiCl layer can be determined by the thermometer arranged inside and with this the water vapor partial pressure from the water vapor partial pressure curve of saturated LiCl solution. This will be equal to the water vapor partial pressure of surrounding air at room temperature (surrounding temperature) due to the material equilibrium. Also this is equal to the saturation pressure at the dew point and hence with the help of the vapor pressure curve the relation between the sensor temperature and the dew point is established. The display can be calibrated directly in such a way that it indicates the dew point temperature, which as mentioned already in the introduction is suitable for humidity characterization. Moreover, one can determine the relative humidity from the vapor pressure curve (Fig. 3). The main advantage of this method of humidity measurement lies in its wide range of application (extremely low and high temperatures, -30 C to +100 C) and in its insensitivity against dirt or electrolytically reactive impurities (by the strong hygroscopic properties of LiCl are already dry at the equilibrium temperature and do not contribute to electrical conductivity). For this reason this measuring method is well suited for continuous measurements and for automatic check arrangements (the electrical signal can be used for 18

19 control), also in industries where the humidity is to be controlled. The measuring inaccuracy is in general ± 1 C in dew point temperature, so that for the relative humidity an error of approx. 3% can arise. The disadvantage is the long time required for the setting time of the device which is about 1 min for every 1 C dew point temperature. 2.5 Mirror dew point measuring instrument This utilizes the occurrence of dew at dew point temperature when air containing water vapor is cooled. A mirror is cooled until it reaches the dew point of the gas in question. As dew condensation forms, it changes the light reflected from the mirror. When the mirror surface reaches an equilibrium state whereby evaporation and condensation are occurring at the same rate, the temperature of the mirror is equal to the dew point temperature of the tested gas. With the determined dew point temperature the relative humidity can be determined with the help of h- x diagram. 3. Experimental procedure The aim of the experiment is the measurement of the humidity in a humidity chamber with the help of the methods introduced in chapter 2. The experiment results are to be noted in the enclosed test protocols. A comparison of the results using different methods reveals the advantages and disadvantages as already given in chapter 2. The experiment devices are accommodated in a chamber. The following eleven points of the experimental procedure are to be done under two different conditions (2 test protocols). The first measurement is done under the condition that the window of the chamber is open, i.e. the measurement is done under ambient condition. These are recorded in Protocol 1. The second measurement is done after changing the humidity of the chamber and the values are recorded in Protocol 2. Before the second measurement is carried out the following things are to be done: a) Carefully close the window. b) Turn on the humidifier: For this the three switches on the right side of the humidifier are pressed. (do not change the thermostat setting!) 19

20 c) Wait, until the humidifier switches off itself automatically (approx. 10 min.). Then Switch off the humidifier through the three switches on the right side of the humidifier. (During the waiting period, the calculations of the first test protocol can be carried out.) d) Wait further 30 minutes before you start the second measurement. The experiment should be carried out in the following steps: 1. Check whether the switch on the "Dew-Point" measuring device (LiCl-dew point measuring device) on position "Standby" or "Dew-Point" stays. If not, immediately inform the instructor, he will switch on the dew point apparatus! (Then an additional half-hour will be required for the heating of the sensor) 2. When the switch is on the position "Dew-Point" note the current time as start measuring time" τ e under point 4 in the protocol. 3. Read the relative humidity according to the printed instruction from both color indicators hanging on the working place and note it under point 1 on the protocol. 4. Read (or evaluate) the following properties from the Hair Polymeter according to the methods described in chapter 2.2 and note under point 2 in the protocol. -The relative humidity in % -The room temperature in C - The dew point temperature in C - The saturation pressure in Torr on the right side of the thermometer beside the room temperature value - The water vapor partial pressure in Torr - The absolute humidity in g/m 3 5. Preparation for the measurement with Psychrometer: The thermometer provided with the "wick" (green tape) is to be made wet with the help of water. For this small amount of water filled in the syringe (or a dropper) is sprayed over the wick. 6. Psychrometer measurements: After moistening the "wick" the ventilating fan is pulled completely with the help of the key provided. During the running time of the ventilating fan ( 3 to 4 minutes) both thermometers (especially the wet thermometer) 20

21 are observed whether they reach a constant final value. These values are noted under point 3 on the protocol. If no constant value is reached during the run time, the ventilating fan must be drawn up again. During the temperature reading it is to assured that the temperature values are not influenced by the breathing. 7. Read the pressure on the mercury barometer in the Lab. 8. Determine the relative humidity from Fig. 6 (Abb. 6), calculate the water vapor partial pressure from equation (7) (p F from Table 1) and write down the values. With p D and you can determine the relative humidity from equation (5) and then from p DS Fig. 4 the absolute humidity and the dew point temperature can be read. The saturation deficit is to be calculated according to the definition given in the introduction of the experiment. 9. Before reading the dew point temperature on the LiCl-dew point measuring device the time is to be noted under number 3 on the protocol. It is to be assured that the instrument is on for more than 30 minutes. The value of the temperature noted. 10. With the help of humidity slide rule (disc) the relative humidity and the absolute humidity (in g/m 3 ) can be calculated from the ambient temperature and the dew point temperature. With the saturation pressure from table 1 and the read water vapor partial pressure the saturation deficit can be calculated. T Tpkt is The above mentioned humidity variables can also be evaluated with the figures and equations given above instead of using the humidity slide rule. 11. For the "Reflecting mirror dew point instrument" the air pump is started. Then the depth of the reflecting stick into the ice water is so adjusted with the help of the spindle that the mirror is just fogged. The reflecting mirror temperature T Tpkt is read on the thermometer. The relative humidity is to be read from Fig 7. As a check the relative humidity can also be calculated with the help of equation (5). 21

22 Table 1 Saturated pressure of the water vapor (p s ), water content of the saturated air (x s ) and the absolute humidity of the saturated air, with reference to moist (humidified) state ( f ) s as a function of temperature between 0 to 30 C. t p s x s f s C mbar Torr g/kg dry g/m 3 moist

23 Table 1 Saturated pressure of the water vapor (p s ), water content of the saturated air (x s ) and the absolute humidity of the saturated air, with reference to moist (humidified) state ( f ) s as a function of temperature between 31 to 70 C. t p s x s f s C mbar Torr g/kg dry g/m 3 moist

24 Table 1 Saturated pressure of the water vapor (p s ), water content of the saturated air (x s ) and the absolute humidity of the saturated air, with reference to moist (humidified) state ( f ) s as a function of temperature between 71 to 100 C. t p s x s f s C mbar Torr g/kg dry g/m 3 wet

25 Protocol 1 t u = p = p DS = 1. Color indicator Color indicator 1 Color indicator 2 ϕ 2. Polymeter ϕ = Δt = t Tpkt = f = p D = Δp = 3. Aspirations-Psychrometer t F = Δt = ϕ = [Fig. (Abb.) 6] p F = p D = [Eq.(7)] ϕ = [Eq.( 5)] f = Δp = 4. LiCl-Dew point measuring device τ e = τ m = > τ e + 30 min t Tpkt = p D = Δ p = ϕ = f = 5. Reflecting mirror dew point measuring device t Tpkt = p D = Δp = ϕ = f = 25

26 Protocol 2 t u = p = p DS = 1. Color indicator Color indicator 1 Color indicator 2 ϕ 2. Polymeter ϕ = Δt = t Tpkt = f = p D = Δp = 3. Aspirations-Psychrometer t F = Δt = ϕ = [Fig.(Abb.) 6] p F = p D = [Eq.(7)] ϕ = [Eq.( 5)] f = Δp = 4. LiCl-Dew point measuring device τ e = τ m = > τ e + 30 min t Tpkt = p D = Δ p = ϕ = f = 5. Reflecting mirror dew point measuring device t Tpkt = p D = Δp = ϕ = f = 26