Humidity-Measuremente

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Koya University Faulty of Engineering

School of Chemical & Petroleum Engineering

Chemical Engineering department

EXPERIMENT NUMBER FIVE

Thermodynamics

Humidity-Measurement

Instructor: Mr.Rebwar & Mr.Omer

Author Name: Aree Salah Tahir

Experiment Contacted on: 4/2/2013

Report Submitted on: 11/2 /2013

Group:A

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List of content:

Aim of this experiment: ………………………3

Introduction: ………………………………….4 , 5

Theory: …………………………………………..6 , 7

Instructions for use: ……………………………8

Calculation: …………………………………………9

Discussion: ………………………………..10 ,11

Reference: ………………………………………..12

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Aim of this experiment:

To determine the humidity of air (to measure the amount of

water in air).

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Introduction

Air, in our normal environment, always holds humidity. The number of

water molecules in the air can vary substantially, e.g. it can be as dry as in

a desert or as humid as in the tropics. There is an upper limit for the

amount of humidity which air can hold at a given temperature. Beyond

this limit saturation occurs. If for some reason the humidity level is

pushed up to this limit, condensation occurs and fog or water droplets

form. Relative humidity tells you what percentage of this maximum

amount of humidity is present in the air. In contrast to relative humidity,

absolute humidity denotes the absolute amount of humidity in the air

regardless of the saturation level expressed as the total mass of water

molecules per air volume.

The maximum possible amount of humidity as well as the actual present

amount of humidity in the air are defined by so called water vapor

pressures. According to Dalton’s law, total air pressure is the sum of the

partial vapor pressures of its components and water vapor pressure is one

of them:

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The maximum amount of humidity, which air can hold, is defined by the

so-called saturation water vapor pressure. This is a function of

temperature.

See: Figure 1 Saturation water vapor pressure.

If the partial water vapor pressure is equal to the saturation water vapor

pressure, condensation occurs. Mathematically, relative humidity is

expressed as the ratio of the partial water vapor pressure divided by the

saturation water vapor pressure as a percentage.

If temperature rises or falls in a closed system, the saturation vapor

pressure will increase or decrease. As a consequence, the relative

humidity will drop or rise.

Saturation water vapor pressure is not a function of total air pressure, but

partial water vapor pressure is. If for example the total air pressure in a

closed system is increased, relative humidity will increase as well,

because the partial water vapor pressure increases proportionally to the

overall pressure increase according to Dalton’s law and saturation vapor

pressure stays the same.{1}

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Theory:

Air is a mix of different gases. Under regular ambient conditions these

gases behave quasi "ideal". That means that each gas molecule can act

totally independent. This leads to the Dalton's law:

p = p1 + p2 + p3 + ...

The total pressure of an ideal mixture of gases is the sum of the single

pressures (partial pressure) of its components.

Vapour pressure mostly behaves like an ideal gas too and appears as an

additional element in the Dalton's law:

p = p1 + p2 + p3 + ...+pW = pda + pW

pD: Partial water vapour pressure, pda: Partial pressure of dry air

Vapour pressure over water

In a self-contained box which is partly filled with water an equilibrium

will be reached between the process of evaporation and condensation. A

low concentration of water molecules in the vapour leads to an increased

evaporation that will increase the vapour pressure. A higher concentration

on the other side lowers the vapour pressure by condensation. This

molecule movement depends on the kinetic energy of the molecules in

water. Hereby the temperature of the medium plays an important role.

The molecules in the water vapour phase behave similar. The

equilibrium between the evaporation and condensation leads to a vapour

pressure which is dependant on the temperature in the box.

At the thermal equilibrium the partial water vapour pressure pW is the

maximum value at the temperature t which is reached in the box at this

temperature and is called:

Saturation vapour pressure pS over water at the temperature t.

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Relative humidity rh (%)

The relative humidity rh is defined as the ratio between the actual water

vapour pressure pW and the saturation vapour pressure pS over water

rh = pW / pS * 100 [%]

Since the partial water vapour pressure pW can never exceed the

saturation perssure pS the maximum posible value of rh = 100%

{2}

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Instructions for use:

This wet bulb psychrometer is used to measure

The humidity of air with using the table

Above. The percentage values are valid for still Temperatures are

measured in 'C.

• The tubular glass tank in the middle is to be filled with -dean water. The

wick attached to the thermometer should dip into the water approx. 2 cm.

• To determine the humidity read of the temperatures at the dry and the

wet thermometer shortly after-each other and determine the difference.

• Search the values in the table and you can determine the relative

humidity of the air,

{3}

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Calculation:

19 = 0c dT

= 16 0c wT

3 = 0cT ∆

Relative Humidity = %

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Discussion:

We use distilled water because...1.

. we have to use NEUTRAL(according to PH factor) water in some A

reactions.other wise reaction goes wrong.

water always contain some type of mineral impurity like some .regularB

salts and in some reactions we have to avoid the effect of those

impurities

.if we are doing experiment with regular water ,some time results C

the water. changes slightly according to impurity which is in

.though if there is no impurity in regular filtered water(rare case) we D

can't say that it is NEUTRAL and don't contain any impurity by only

seeing it with naked eye so it is best way to use distilled water.

2.What does humidity measures?

humidity measures the amount of water vapor in the air. Water

concentration in air or water vapour.The amount of water vapor

in the air. water vapor moisture

3.What is relative humidity measured in?

Relative humidity is measured in %.

4.Can relative humidity be measured with an instrument?

Yes, you can check the whether with an instrument. this

instrument is called a thermometer. this is how it works... when

it's freezing cold it should be below 0. when it's hot it should be

at least above 10 degrees. These thermometers should be kept

outside so the thermometer is absolutely right or it's close to the

right answer. However, a hygrometer can be used to use relative

humidity. Or it is how much water vapor is in the air.

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5.Relative humidity can be measured with a?

Hygrometer

We used wet cloth on a humid day there is moisture in the air. On a dry 6.

day the moisture in the clothes more easily evaporates.

7.There are many reasons to change the temperature and makes the

temperature be unstable like condition in the room and being close to the

thermometer

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Reference:

1/ http://www.elektronika.rs.ba/files_bin/projekti/TermoHigrometar/Introduction_to_Relative

_Humidity_E.pdf

2/

-air-dp-http://www.novasina.com/wEnglisch/Knowledge_Center/Theorie/Theorie

Luftfeuchte_und_Temperatur.php-aw/Theorie

3/ d_561.html-measurement-http://www.engineeringtoolbox.com/humidity

4/ http://wiki.answers.com/Q/What_is_the_Bernoulli_theory#slide=2