METROLOGY AND INSTRUMENTATION

Temperature measurement

BY: VIKAS BODDULA (131FA08178)

Topics:- Various principles to temperature measurements Expansion thermometers Resistance thermometers Thermistors Thermocouples pyrometers

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

A temperature is an objective comparative measure of hot or cold. It is measured by a thermometer, which may work through the bulk behavior of

a thermometric material, detection of thermal radiation, or particle kinetic energy.

Several scales and units exist for measuring temperature, the most common being Celsius (denoted °C; formerly called centigrade), Fahrenheit (denoted °F), and, especially in science, Kelvin (denoted K).

Various principles to temperature measurements:-

Mechanical temperature sensors

a) Expansion Liquid in glass thermometers

Bimetallic thermometers

b) Pressure thermometers Liquid filled thermometers

Vapour pressure thermometers

Electrical temperature sensor

Thermocouples

Resistance thermometers

Thermistors

Optical temperature sensors

Radiation pyrometer

Optical pyrometer

Mechanical temperature sensors:-a) expansion thermometers:

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.

Advantages:1)Simplicity in use & low cost.

2)Portable device.

3)Checking physical damage is easy.

4)Power source not require.

Disadvantages:1) Can not used for automatic recording.

2) Time lag in measurement.

3) Range is limited to about 300 °C .

Bimetallic strips:- In an industry, there is always a need to measure and

monitor temperature of a particular spot, field or locality. The industrial names given to such temperature sensors

are Temperature Indicators (TI) or Temperature Gauges (TG).

All these temperature gauges belong to the class of instruments that are known as bimetallic sensors.

Two 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 co-efficient of temperatures. The rate of volumetric change depends on this co-efficient of temperature.

Bimetallic strips:-

The device consists of a bimetallic strip of two different metals They are bonded together to form a spiral or a twisted helix. 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. 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

Advantages:1) Power source not required

2) Robust, easy to use and cheap.

3) Can be used to 500 °C.

Disadvantages:1) Not very accurate.

2) Limited to applications where manual reading is acceptable.

3) Not suitable for very low temperatures because the expansion of metals tend to be too similar, so the device becomes a rather insensitive thermometer

b)Pressure Thermometer: 1.Liquid Pressure Thermometers: a temperature measurement device whose operation is

 based on the thermal expansion of a liquid. 

Liquid-filled thermometers provide a direct reading of the temperature.

The measurement accuracy depends on the depth of immersion of the thermometer in the medium to be measured.

A liquid-filled thermometer should be immersed up to a reference scale division or a specially applied line on  the scale (partial-immersion thermometer).

If this is not possible, a correction is made to the column of mercury that depends on the temperature being measured and the temperature and height of the column.

The main disadvantages of liquid-filled thermometers are slow response time and sometimes inconvenient dimensions.

Liquid Pressure Thermometers: Meteorological thermometers, Beckmann thermomete

rs, and clinical thermometers are specially designed liquid-filled thermometers. 

Clinicalmercury thermometers have an abbreviated scale (34°–42°C) and scale divisions of 0.1 °C

b) Vapour Pressure Thermometers: Vapour pressure thermometers are very much similar in design

and construction to the liquid filled thermometer. However, in this type the bulb is replaced with an immersion

tube. The immersion tube partially filled with low boiling point liquid

and the rest of the tube filled with its vapour. The change in vapour pressure in the tube over the liquid column

is a function of temperature . The saturated vapor pressure of a volatile liquid is used as a

measure of the temperature. These thermometers are very sensitive but the scale is not linear. The measurement accuracy is 1%. The typical temperature range

is from - 20° C to 280° C depending on the nature of the gas

Vapour Pressure Thermometers:

The major disadvantage of this device is that the indication is non-linear with temperature.

Generally, ethylene, ethyl, ether are some of the liquids that can be used in vapour pressure thermometers.

electrical temperature sensors:-a) thermocouples:

Thermocouples are metal couples which work on the Seebeck

effect.

In this effect, any conductor is subjected to a thermal gradient, it will

generate a voltage. 

A Thermocouple is a sensor used to measure temperature.

Thermocouples consist of two wire legs made from different metals.

The wires legs are welded together at one end, creating a junction.

This junction is where the temperature is measured. When the

junction experiences a change in temperature, a voltage is created.

The voltage can then be interpreted using thermocouple reference

tables to calculate the temperature. 

Thermocouples: There are many types of thermocouples, each with its own

unique characteristics in terms of temperature range, durability, vibration resistance, chemical resistance, and application compatibility.

Type J, K, T, & E are “Base Metal” thermocouples, the most common types of thermocouples.

Type R, S, and B thermocouples are “Noble Metal” thermocouples, which are used in high temperature applications (see thermocouple temperature ranges for details). 

Thermocouples are typically selected because of their low cost, high temperature limits, wide temperature ranges, and durable nature

Thermocouple connected to a multimeter displaying room temperature in °C

1 - hot junction;2 - metal A;3 - metal B;4 - connection head;5 extension wires;6, 7 - positive and negative terminals, respectively, of a measuring instrument;8 - measuring instrument.

Generated EMF,

E = a (Ta – Tb) + b (Ta – Tb)2

Where , Ta= temp. of heater element

Tb= temp. of cold metal

a & b are const. depends upon the type of metal we are using

1 25

3

8

6

7

+

-

+

-

4

Figure :Thermocouple and measuring instrument

Advantages:a) Rugged

b) Inexpensive

c) Simply constructed (welded wires composed of dissimilar metals) 

d) Used to measure temperature ranges spanning thousands of degrees

e)  Fast in their response to changes in temperature. 

Disadvantages:a) Thermocouples can not placed for precision work

b) Thermocouples are placed remote from measuring. For maximum accuracy of measurement the compensating wires should be of the same material as the thermocouple wires.

b)Resistance thermometers (RTD):

The resistance of the metal changes with a change in temperature.

This property is utilized for measurement of temperature in resistance thermometers

For this purpose several metals are used, namely, platinum, copper, nickel.

When temperature increases the resistance of these metals increases

They are slowly replacing the use Of thermocouples in many industrial applications below 600 °C,

due to higher accuracy and repeatability

Resistance thermometers (RTD):

a two-conductor connecting scheme:The simplest resistance thermometer configuration uses two wires. It is only used when high accuracy is not required, as the resistance of the connecting wires is added to that of the sensor, leading to errors of measurement. This configuration allows use of 100 meters of cable. This applies equally to balanced bridge and fixed bridge system

three-conductor connection scheme :One conductor is common to both sides of the bridge, while other two connect the RTD to each side of the bridge. Any change in the cable temperature (as the result of variations in ambient temperature) will be cancelled because the resistance of both sides of the bridge change by the same value (providing three connecting cables are at the same temperature)

A four-conductor connection scheme is used when very accurate measurements of temperature are required, up to 0.01 C of accuracy

Advantages:a. High accuracy

b. Low drift

c. Wide operating range

d. Suitability for precision applications

Disadvantages:a. Low sensitivity

b. Higher cost than thermocouples

c. Affected by shock and vibration

d. Requires three or four-wire operation.

c)Thermistors: A thermistor is a non-metallic resistor having a negative

coefficient of resistance , there resistance is decrease with increase in temperature.

Due to this property of high sensitivity (that is huge resistance change for a small change in temperature),

the thermistor is mainly applicable in precision temperature measurement, especially in a lower temperature range of −90 °C to 130 °C

Thermistors: Advantages:a. Small in size and low cost

b. Fast response

c. Suitable for narrow spans

Disadvantages:a. Self heating

b. Current source is required

c. Less stable than RTD

Pyrometers:a) Radiation pyrometer:

A pyrometer is a type of remote-sensing thermometer used to measure the temperature of a surface.

In the modern usage, it is a device that from a distance determines the temperature of a surface from the spectrum of the thermal radiation it emits, a process known as pyrometry and sometimes radiometry.

Any body which is above the absolute zero in temperature emits .the radiant energy emitted from a body increases with temperature and this principle is used in measuring temperatures.

Radiation pyrometer:

a. The hot body radiates heat at some wave length. this energy passing through a lens is converged to a thermopile,which is sensitive to broad wave length spectra.

b. The output of thermopile is proportional to heat received

c. The e.m.f output of thermopile is fed to millivoltmeter or potentiometer for measurement purposes

d. The thermopile which forms the temperature sensing element consists of a group of very small thermocouples

e. The thermocouples are flattened and blackened so that they absorb all the radiant energy reaching the thermopile

Advantages:a. Ability to measure high temperature

b. No need to bring in contact

c. Fast response

d. Moderate cost

disadvantages:a. Emissivity errors are introduced

b. Errors due to the absorption of radiation by carbon dioxide, water or other apparently transparent gases

Optical pyrometer:

a. Optical pyrometer make use of the changes in colour of hot body and interpret this phenomenon in terms of temperature

b. When the body is heated, it initially becomes dark red, turns to orange and finally attains a white colour.

c. The actual temperature measurement is based upon the determination of the variations in colour of the object and Comparing it with known values generated with a heated filament

Advantages:a. FLEXIBLE

b. PORTABLE

c. CONVIENT TO USE

d. LIGHT WEIGHT

e. CAN MONITOR MOVEABLE OBJECTS

f. NON CONTACT TYPE

g. ACCURATE

Diadvantages :a. EXPENSIVE

b. HUMAN ERRORS

c. NOT USEFUL FOR MEASURING TEMP OF CLEAR GAS

d. AT HIGH TEMP. FILAMENT ERODES FREQUENTLY