This document describes an experiment to measure humidity using a psychrometer. It includes sections on the aim, introduction, theory, instructions, calculations, discussion, and references. The introduction explains that humidity is the amount of water vapor in air and defines relative humidity. The theory section describes how vapor pressure relates to temperature and humidity. The instructions explain how to use a psychrometer to measure the dry and wet bulb temperatures and determine relative humidity from a table.

1. 1
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

2. 2
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

3. 3
Aim of this experiment:
To determine the humidity of air (to measure the amount of
water in air).

4. 4
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:

5. 5
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}

6. 6
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.

7. 7
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}

8. 8
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}

9. 9
Calculation:
19=0
cdT
= 160
cwT
3=0
cT∆
Relative Humidity = %

10. 11
Discussion:
We use distilled water because...1.
. we have to use NEUTRAL(according to PH factor) water in someA
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 resultsC
the water.changes slightly according to impurity which is in
.though if there is no impurity in regular filtered water(rare case) weD
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.

11. 11
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 dry6.
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

12. 12
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