Temperature Measurement

• Introduced by : Mohammad Nashat Abu SabbahAli Mousa alowaidat

Ahmad Saleh MansourAnas Yacoub

• Supervised by : Dr. Sameh Al-saqoor• Date: Mon (8-12-2014)

Introduction

The accurate measurement of temperature is vital across abroad spectrum of human activities,

Including industrial processes (e.g. making steel) Manufacturing;

Monitoring (in food transport and storage),

Health and safety. In fact, in almost every sector, temperature is

one of the key parameters to be measured

The means of accurately measuring temperatures has long fascinated people.

Different people will have different perceptions of what is hot and what is cold.

To make objective measurements, we must use thermometer in which some physical property of a substance changes with temperature in a reliable and reproducible way.

Scale Temperature is a measure of the thermal energy

in the body. Normally measured in degrees [°]using one of the following scales.

1.Fahrenheit.[°F]2.Celsius or centigrade. [°C]3.Kelvin .[°K]

1.Bulb: The reservoir for containing most of the thermometric liquid (mercury).

2.Stem: The glass tube having a capillary bore along which the liquid moves with changes in temperature.

3. Scale: A narrow-temperature-range scale for reading a reference temperature .

Construction

1.Liquid – in – Glass Thermometer

• The volume of mercury changes slightly with temperature; the small change in volume drives the narrow mercury column a relatively long way up the tube.

• The space above the mercury may be filled with nitrogen or it may be at less than atmospheric pressure, a partial vacuum.

Advantages1)Simplicity in use & low cost.2)Portable device.3)Checking physical damage is easy.4)Power source not require.

Disadvantages1) Can not used for automatic recording.2) Time lag in measurement.3) Range is limited to about 300 °C .

2-Bimetallic ThermometerTwo basic principles of operation is to be

followed in the case of a bimetallic sensor.

1) A metal tends to undergo a volumetric dimensional change (expansion/contraction), according to the change in temperature.

2) Different metals have different coefficient of temperatures. The rate of volumetric change depends on this coefficient of temperature.

Construction

• The device consists of a bimetallic strip of two different metals .

• They are bonded together to form a spiral or a helical. • Both these metals are joined together at one end by

either welding or riveting.• It is bonded so strong that there will not be any

relative motion between the two. • The image of a bimetallic strip is shown below.

A change in temperature causes the free end of the strip to expand or contract due to the different coefficients of expansion of the two metals.

This movement is linear to the change in temperature and the deflection of the free end can be read out by attaching a pointer to it.

This reading will indicate the value of temperature. Bimetallic strips are available in different forms like helix type, cantilever, spiral, and also flat type.

Construction

Advantages1)Power source not required2)Easy to use .3)Can be used to 500 °C.4)Inexpensive.

Disadvantages1) Low accuracy.2) Limited to applications.3) Not suitable for very low temperatures

because the expansion of metals tend to be too similar.

The vapor pressure thermometer is a filled system thermometer type that used to measure temperature by pressure of vapor.

3-Vapor Pressure Thermometers.Main components1-bulb2-bourdon tube3-capillary4-pointer

Range temp. available( C) ̊ Boiling point( C) ̊ liquid120-220 100 water0-50 -23.6 Methyle chloride30-120 -10 Sulphur dioxide30-100 12.5 Ethyle chloride

Table : Temperature ranges of liquid used in vapour-pressure system

PrincipleIn this system the bulb is filled with liquid while the capillary and bourdon tube is filled with vapor. The substance that we want to be measured heats the liquid in the bulb and the liquid is boils and vaporizes operation which creates a vapor inside the capillary and bourdon tube and the pressure of this vapor expansion the bourdon tube and move the pointer ,the liquid continuous to boil until the pressure in the system equal the vapor pressure of boiling liquid at this point the liquid stops boiling. When the substance is cold the vapor is condense and the vapor pressure is decrease and the bourdon tube is contract and move the pointer.

Vapor pressure thermometer

Pyrometer

• A pyrometer is a device that is used for the temperature measurement of an object.

• The device actually tracks and measures the amount of heat that is radiated from an object.

• The thermal heat radiates from the object to the optical system present inside the pyrometer.

• The optical system makes the thermal radiation into a better focus and passes it to the detector.

• In an optical pyrometer, a brightness comparison is made to measure the temperature.

• The device compares the brightness produced by the radiation of the object whose temperature is to be measured,

• For an object, its light intensity always depends on the temperature of the object.

• After adjusting the temperature, the current passing through it is measured using a multimeter, as its value will be proportional to the temperature of the source when calibrated.

• The working of an optical pyrometer is shown in the figure below.

As shown in the figure above, an optical pyrometer has the following components.

1. An eye piece at the left side and an optical lens on the right.

2. A reference lamp, which is powered with the help of a battery.

3. A rheostat to change the current and hence the brightness intensity.

4. So as to increase the temperature range which is to be measured, an absorption screen is fitted between the optical lens and the reference bulb.

Working1.The radiation from the source is emitted and the

optical objective lens captures it. 2.The lens helps in focusing the thermal radiation

on to the reference bulb. 3.The observer watches the process through the eye piece

and corrects it in such a manner that the reference lamp

filament has a sharp focus and the filament is super-imposed

on the temperature source image. 4.The observer starts changing the rheostat values and the

current in the reference lamp changes. 5.This in turn, changes its intensity.

This change in current can be observed in three different ways.

The filament is dark. That is, cooler than the temperature source.

Filament is bright. That is, hotter than the temperature source.

Filament disappears. Thus, there is equal brightness between the filament and temperature source

At this time, the current that flows in the reference lamp is measured, as its value is a measure of the temperature of the radiated light in the temperature source, when calibrated.

• Advantages1. Provides a very high accuracy with +/-5º Celsius.2. The biggest advantage of this device is that,

there is no direct contact between the pyrometer and the object whose temperature is to be found out.

• Disadvantages1. As the measurement is based on the light

intensity, the device can be used only in applications with a minimum temperature of 700º Celsius.

2. The device is not useful for obtaining continuous values of temperatures at small intervals.

• Applications1.Used to measure temperatures of liquid

metals or highly heated materials.2.Can be used to measure furnace

temperatures.