1

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

Thermodynamics Lab Humidity Measurement

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

air wetmOH kg

Vm = f 3

2W

′′

(1a)

mw = Mass of water vapor

= Volume of wet gas mixture V ′

(slash means here that it is the wet gas mixture)

is also called the density of the water vapor. f ′

b) Related to the unit volume of the dry gas mixture

2

airdry mOH kg

Vm = f 3

2w

(1b)

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.

airdry kgOH kg

mm = x 2

L

w

(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) (p1, t1, x1), is

the relative humidity ϕ. It is defined as the ratio of the available absolute humidity in state (1)

humidity , to the maximum possible humidity at the same temperature t1, i.e. the

absolute humidity in the saturation state:

´f ´sf

)t(’ f)/t(f = 1s1′ϕ (3)

1

S

x

h 1+x

t1 t =const.

1

ϕ =con

st.

ϕ=1

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 . 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.

)x - x( m = m 21Lw

1

2

x

h 1+x

tTpkt

ϕ=1

t2

x2 x1

Taupunkt

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

1

x

h 1+x

t1ϕ=1

tF

x1

Kühlgrenze

For the representation of the concentration of water vapor in the gas mixture the mass fraction

mmw , the mole fraction

nnw and the partial pressure pD = p

nnW are commonly used. In general

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

, VT Rm = p WWDand

VT Rm = p GGG (4)

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 . It can be shown that for total

pressure p <10 bar the saturated partial pressure (t) can be approximated (better than 1%)

with the vapor pressure of pure water at temperature “t” which is given by the vapor

pressure curve as a temperature function (Table 1). The water vapor pressure in

unsaturated air can be calculated from pD = pWS(tTpkt), as

pDS

pDS

pWS

Dp

(t) f p n

n = pDW ≠ remains constant

at dew point temperature and at dew point it is saturated.

With equation (4) it follows for equation (3)

(t)p)t(p

= (t)p

p = W

TpktW

D

D

S

S

S

ϕ

The difference of both pressure pD and is called saturation deficit Δp. pDS

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 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.

30 40 50 60 70 80 90 100

trocken normal feucht

20

Feuchtigkeit in %Ablesung im Grenzfeld rosa-blau

Abb. 1 Zwei Typen von Feuchte-Indikatoren mittels Farbumschlag

Ablesung der relativen Feuchte in %am Übergang rosa-blau, oder wo Strei-fen der Untergrundfarbe entspricht

Feuchte-Indikator

20

30

40

50

60

70

8075

65

55

45

35

25

normal

trocken

feucht

10

0

10

20

30

4060

40

20

10

5

2

SkalaTemperatur

in °C

SkalaSättigungsdruck

in Torr

Haarbündel

3015 10

5

0

10080

503010

0

°C

%rel. Feuchte

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 3.4 10-5 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

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 tU and the dew point temperature tTpkt 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 tTpkt = tU - Δt

- the partial pressure of water vapor pD = φ pDs [equation (5)]

- the absolute humidity ~ partial pressure of water vapor pD 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).

1000

400

200

1006040

20

1064

2

1-20 0 20 40 60 80 100

Dam

pfdr

uck

mbar

Temperatur

°C

Abb. 3 Dampfdruckkurve des Wassers

üb. unterkühltem Wasser

50

40

30

20

10

00-10 10 20 30 40°C

Gas-Temperatur

abso

lute

Feu

chte

f'g

/

m

Luft

H O 2

3

100

90

80

70

60

50

40

30

20

Abb. 4a Abhängigkeit von relativer, absoluter Feuchte un Temperatur im Bereich von -10 °C bis +40 °C

φ

9

500

°CGas-Temperatur

abso

lute

Feu

chte

f'g

/ m

Luft

H O 2

3

450

400

350

300

250

200

150

100

50

030 40 50 60 70 80 90 100

80

70

90100

60

50

40

30

20

10

φ

Abb. 4b Abhängigkeit von relativer, absolute Feuc und Temperatur im Bereich von 30 - 100 °

10

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.

6 0

05

04

03

02

01

0

01

6 0

05

04

03

02

01

0

01

Uhrwerkmit Ventilator

trockenesThermometer

feuchtesThermometer

Luft

Abb. 5 Prinzip-Aufbauskizze eines Aspirations- Psychrometers nach Assmann

11

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 pD and the temperature tu , which is

shown by the dry thermometer. If pD is smaller than the saturation pressure pS 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 tF. 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 pD:

h Rt c p

- p = pV

*p

FD ⋅Δ⋅⋅

(6)

where p = Barometer pressure

Δt = (tu - tF) psychrometric temperature difference

tu = Temperature of the measuring air

pF = Water vapor saturation pressure at tF

cp = spec. heat capacity of the gas at constant pressure

R* = RL/RW , RL is the specific gas constant for dry air and RW for water vapor

hv = specific enthalpy of vaporization for water

For technical measurements we mostly use (using the empirical approximation formula pD =

pF - p ⋅ A ⋅ Δt, with A = Psychrometer constant) the formula from SPRUNG (1888) for the

measurements in air, which simplifies equation (6) to:

,t 755

p C150 - p = p FD Δ⋅°. (7)

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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.

Abb. 6 Graphische Psychrometertafel für stark bewegte Luft Luftgeschwindigkeit 2 m/s>=

°CTemperatur des trockenen Thermometers

020 30 40 50

5

10

15

20

25

30

35

10

Psyc

hrom

etris

che

Diff

eren

z

°C

0 %

10 %

20 %

30 %

40 %

50 %

60 %70 %

80 %90 %

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.

16

9000

8000

1000

0

7000

6000

5000

4500

4250

4000 3900

38003700

36003500

3400

3300

3200

3100

3000

2900

2800

2700

2600

2500

2400

2300

22002100200019001750

16501500

12501000

5000

h =50 kJ/kg

1+x

Δh /Δx [kJ/kg]1+x

t=60°C

t=30°C

t=50°C

t=0 °C(Eis)

t=0 °C(W

asser)t=-10°C

p* =0,09 bard

p* =0,08 bard

p* =0,07 bard

p* =0,06 bard

p* =0,05 bard

p* =0,04 bardp* =0,03 bard

p* =0,02 bard

0 10 20 30 40 50 70

h

[kJ/

kg]

1+x

Δh /Δx [kJ/kg]1+x

t=10°C

t=20°C

t=30°C

t=35°C

t=25°C

t=45°C

p* =0,1 bard

p=1,01325 bar

ϕ=0,5

-100

1020

3040

5060

Abb. 7 Mollier-h,x-Diagramm für feuchte Luft

h =200 kJ/kg

1+x

h =175 kJ/kg

1+x

t=40°C

h =150 kJ/kg

1+x

h =125 kJ/kg

1+x

h =100 kJ/kg

1+x

ϕ =0,4

ϕ=0,6

ϕ=0,8

ϕ =0,2

ϕ =0,1

t=15°C

h =75 kJ/kg

1+x

h =25 kJ/kg

1+x

t=10°C

t=20°C

h =0 kJ/kg

1+x

t=40°C

ϕ=1,0

x [g/kg] 60

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

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!)

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" τe 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/m3

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)

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) (pF from Table 1) and write down the values. With

pD and you can determine the relative humidity from equation (5) and then from

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.

pDS

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 is

noted.

T Tpkt

10. With the help of humidity slide rule (disc) the relative humidity and the absolute

humidity (in g/m3) 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.

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 TTpkt 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

Table 1 Saturated pressure of the water vapor (ps), water content of the saturated air (xs) and the absolute humidity of the saturated air, with reference to moist (humidified) state as a function of temperature between 0 to 30 °C. )’sf(

t

ps

xs ’sf

°C

mbar

Torr g/kg dry g/m3 moist

0 1 2 3 4 5 6 7 8 9 10

11 12 13 14 15

16 17 18 19 20

21 22 23 24 25

26 27 28 29 30

6.11 6.5 7.1 7.6 8.1 8.7

9.3

10.0 10.7 11.5 12.3

13.1 14.0 14.9 16.0 17.1

18.1 19.3 20.6 22.0 23.4

24.9 26.4 28.1 29.9 31.7

33.6 35.6 37.7 40.0 42.4

4.58 4.9 5.3 5.7 6.1 6.5

7.0 7.5 8.0 8.6 9.2

9.8

10.5 11.2 12.0 12.8

13.6 14.5 15.5 16.5 17.5

18.7 19.8 21.1 22.4 23.8

25.2 26.7 28.3 30.0 31.8

3.78 4.07 4.37 4.70 5.03 5.40

5.79 6.21 6.65 7.13 7.63

8.15 8.75 9.35 9.97 10.6

11.4 12.1 12.9 13.8 14.7

15.6 16.6 17.7 18.8 20.0

21.4 22.6 24.0 25.6 27.2

4.84 5.2 5.6 6.0 6.4 6.8

7.3 7.8 8.3 8.8 9.4

10.0 10.7 11.4 12.1 12.8

13.6 14.5 15.4 16.3 17.3

18.3 19.4 20.6 21.8 23.0

24.4 25.8 27.2 28.7 30.3

22

Table 1 Saturated pressure of the water vapor (ps), water content of the saturated air

(xs) and the absolute humidity of the saturated air, with reference to moist (humidified) state as a function of temperature between 31 to 70 °C. )’sf(

t

ps

xs ’sf

°C

mbar

Torr g/kg dry g/m3 moist

31 32 33 34 35

36 37 38 39 40

41 42 43 44 45

46 47 48 49 50

52 54 56 58 60

62 64 66 68 70

44.9 47.6 50.3 53.2 56.3

59.5 62.8 66.3 69.9 73.7

77.7 82.0 86.4 91.0 95.0

100.9 106.1 111.6 117.3 123.3

136.1

150 165

181.5 199.2

218.4 239.4 261.4 285.6 311.6

33.7 35.7 37.7 39.9 42.2

44.6 47.1 49.7 52.4 55.3

58.3 61.5 64.8 68.3 71.9

75.7 79.6 83.7 88.0 92.5

102.1 112.5 123.8 136.1 149.4

163.8 179.3 196.1 214.2 233.7

28.8 30.6 32.5 34.4 36.6

38.8 41.1 43.5 46.0 48.8

51.7 54.8 58.0 61.3 65.0

68.9 72.8 77.0 81.5 86.2

96.6 108 121 136 152

171 192 216 244 276

32 34 35 37 39

41 44 46 48 51

53 56 59 62 65

68 72 75 79 83

90 99 108 119 130

142 154 168 182 198

23

Table 1 Saturated pressure of the water vapor (ps), water content of the saturated air

(xs) and the absolute humidity of the saturated air, with reference to moist (humidified) state as a function of temperature between 71 to 100 °C. )’sf(

t

ps

xs ’sf

°C

mbar

Torr g/kg dry g/m3 wet

72 74 76 78 80

82 84 86 88 90

92 94 96 98

100

339.4 369.6 401.8 436.4 473.4

513.2 555.7 601.2 649.4 701.1

755.9 814.5 876.7 943.0

1013.3

254.6 277.2 301.4 327.4 355.1

384.9 416.8 450.9 487.1 525.8

567.0 610.9 657.6 707.3 760.0

313 357 409 471 546

638 755 907

1110 1396

1827 2549 3994 8348

-

214 232 251 271 293

316 340 367 394 423

454 487 522 559 597

24

Protocol 1 tu =

p = pDS

=

1. Color indicator

Color indicator 1 Color indicator 2

ϕ

2. Polymeter ϕ =

Δt = tTpkt =

f ′ =

pD = Δp =

3. Aspirations-Psychrometer tF =

Δt = ϕ = [Fig. (Abb.) 6]

pF =

pD = [Eq.(7)] ϕ = [Eq.( 5)]

f ′ = Δp =

4. LiCl-Dew point measuring device τe =

τm = > τe + 30 min

tTpkt =

pD = Δ p =

ϕ =

f ′ =

5. Reflecting mirror dew point measuring device tTpkt =

pD = Δp =

ϕ =

f ′ =

25

Protocol 2 tu =

p = pDS

=

1. Color indicator

Color indicator 1 Color indicator 2

ϕ

2. Polymeter ϕ =

Δt = tTpkt =

f ′ =

pD = Δp =

3. Aspirations-Psychrometer tF =

Δt = ϕ = [Fig.(Abb.) 6]

pF =

pD = [Eq.(7)] ϕ = [Eq.( 5)]

f ′ = Δp =

4. LiCl-Dew point measuring device τe =

τm = > τe + 30 min

tTpkt =

pD = Δ p =

ϕ =

f ′ =

5. Reflecting mirror dew point measuring device tTpkt =

pD = Δp =

ϕ =

f ′ =

26