The document describes a lab experiment aimed at measuring relative humidity using a sling psychrometer, which consists of two thermometers: one for dry-bulb temperature and another covered with a wet cloth for wet-bulb temperature. The experiment teaches how to determine the relative humidity without expensive equipment by utilizing psychrometric tables and understanding the principles of evaporation. Key discussions include the definition of relative humidity, factors affecting it, and observations from different locations.

1. Petroleum department
Thermo dynamic Lab
Name of experiment:
(Relative humidity)
Prepared by:
Bakhtyar Abdul star

2. Introduction :
In this lab, you will learn an easy way to determine relative
humidity without using any expensive equipment.
Relative humidity is a measurement used to describe water
vapor in the air. In general, it expresses how close the air is
to saturation. In this lab, you will use a psychrometer and a
data table to determine the relative humidity of air Wet-bulb
temperature is measured using a thermometer that has its
bulb wrapped in cloth—called a sock—that is kept wet with
distilled water via wicking action. Such an instrument is
called a wet-bulb thermometer. A widely used device for
measuring wet and dry bulb temperature is a sling
psychrometer, which consists of a pair of mercury bulb
thermometers, one with a wet "sock" to measure the wet-
bulb temperature and the other with the bulb exposed and
dry for the dry-bulb temperature. The thermometers are
attached to a swivelling handle which allows them to be
whirled around so that water evaporates from the sock and
cools the wet bulb until it reaches thermal equilibrium

3. Aim of experiment :
The objective of this exp.is to measure and study a relative
humidity by pschrometer , you will use a psychrometer and
a data table to determine the relative humidity of air .

4. Theory:
Relative humidity is a measure of how much water vapor is in the air
compared with the total amount of water vapor the air is capable of holding
at a given temperature. If the relative humidity is 50% at room temperature
then the air in the room is at half holding capacity. Relative humidity is the
commonmeasurementgiven on weather reports. Today you will investigate
water vapor in the atmosphere (humidity) and will attempt to apply your
findings to circumstances found in everyday life by using a sling
psychrometerin differentlocations to measure humidity .
The sling psychrometer is a simple,yet effective instrument used to
determine relative humidity. It does not, however, give a direct reading of
relative humidity. Psychrometric tables must be used to obtain the relative
humidity from the temperature data provided by the psychrometer.In this
lab, we will find the dew point and relative humidity by using a sling
psychrometer.The sling psychrometeris based on evaporation.
The instrument consists of two thermometers mounted on a platform that can
be ventilated by slinging. One of the thermometerbulbs is covered with a
webbed cloth or wick that has been soaked in water. As the two
thermometers are ventilated, the water on the wick evaporates and the “wet
bulb” is the wick evaporates and the “wet bulb” is cooled by the evaporation.
The temperature differenceof the wet and the dry bulb thermometers (
called wet bulb depression(wbd)) is directly affected by the amount of

5. evaporation. Evaporation at any time is dependentupon the current relative
humidity.
Apparatus :
1-water tank
2-wet cotton cloth for covering the wet bulb thermometer
3-dry bulb thermometer
4-wet bulb thermometer

6. Procedures:
a)first of all , we determine the dry temperature by the
dry bulb thermometer
b)fill a water tank ,and let a cotton cloth wetting ,after
that determine a wet temperature by the wet bulb
thermometer
c) find (DT=Dt –Wt )

7. d)Use the "Relative Humidity Chart" to find the relative
humidity.

8. Calculations:
READING
DRY Tem.=22 C
Wet Tem. =18 C
R.H.=?
DT=dry tem – wet Tem
=22 - 18 =3C
Use the "Relative Humidity Chart" to find the relative
humidity.
R.H= 69%

9. Discussion:
-Define ‘’relativehumidity’’ ?
-Relative humidity is a measurement used to describe water
vapor in the air. In general, it expresses how close the air is to
saturation. In this lab, you will use a psychrometer and a data
table to determine the relative humidity of air
-Why does the temperature decrease on the wet-bulb
thermometer?
-feels cool as liquid water on it evaporates (a process that occurs at
a sub-ambient wet-bulb temperature that is dependent on relative
humidity). Water's
-Apply Was the relative humiditydifferent in the two places?
-Why do you think it changed or stayed the same?
Relative humidity was different at the two places chosen. Trees,
windows, the heating system, and other factors may affect the
humidity recorded at a particular location.

10. Reference :
http://en.wikipedia.org/wiki/Relative_humidity
http://hyperphysics.phy-
astr.gsu.edu/hbase/kinetic/relhum.html
http://www.thefreedictionary.com/relative+humid
ity
http://www.shorstmeyer.com/wxfaqs/humidity/h
umidity.html