Temperature Measurement

Temperature Measurements

Chapter 3

1. Mechanical Methods

2. Electrical Methods

Methods of Temperature Measurement

The temperature measuring instruments are classified according to the nature of the change produced in the testing body by the change of temperature. They may be classified as follows:

(i) Expansion thermometers (ii) Filled-system thermometers (iii) Electrical temperature instruments

Methods of Temperature Measurement

Expansion thermometers are classified according to the nature of substance which expands. They may be described under three headings as follows:

(i) expansion of solids• Bimetallic thermometers

(ii)expansion of liquids• Liquid-in-glass thermometers• Liquid-in-metal thermometers

(iii)expansion of gases• Gas thermometers

Expansion Thermometer

Effect of unequal expansion of a bimetallic strip

Bimetallic thermometer

Expansion Thermometer- Bimetallic Thermometer(expansion of solids)

• It consists of two strips of metal welded together, each strip made from a metal having a different coefficient of thermal expansion. Whenever the welded strip is heated, the two metals change length in accordance with their individual rates of thermal expansion.

• The two metals expand to different lengths as the temperature rises. This forces the bimetallic strip to bend towards the side with low coefficient of thermal Expansion.

• If one end of the bimetallic strip is fixed so that it cannot move, the distance the other end bends is directly proportional to the square of the length of the metal strip as well as to the total change in temperature, and is inversely proportional to the thickness of the metal.

Expansion Thermometer- Bimetallic Thermometer

• If the bimetallic element is wound in the form of a spiral, the spiral coil is tightened with increase in temperature.

• As it coils, the counterpost rotates clockwise, and thus a pointer attached to the post also moves on a calibrated temperature scale.

Expansion thermometer with Spiral Bimetallic Element(expansion of solids)

• It consists of a tightly wound helical bimetallic strip located inside the stem of the thermometer with one end fastened permanently to the outer casing.

• A strip is attached to a centre post that extends from the stem to the centre of an indicating dial. A pointer is attached to the centre post.

• When the temperature surrounding the whole stem changes, the bimetal expands and the helical coil winds and unwinds, which rotates the centre post. This causes the pointer to move on the dial to indicate the measured temperature.

Expansion thermometer with Helical Bimetallic Element (expansion of solids)

Expansion thermometer with Helical Bimetallic

Expansion thermometer with Spiral Bimetallic Element

Expansion thermometer with Spiral Bimetallic Element

Thermal expansion methods: Bimetallic sensors (expansion of solids)

Following are the advantages of bimetallic thermometers:

(i) Their cost is low.(ii) They are tough, and cannot easily be broken.(iii) They are easily installed and maintained(iv) They have good accuracy relative to cost.(v) They have fairly wide temperature range.

Thermal expansion methods: Bimetallic sensors Advantages

Following are the disadvantages of bimetallic thermometers:

(i) They are limited to local mounting.(ii) Only indicating type is available.(iii) There is always a possibility of calibration change due to rough handling.(iv) Their accuracy is not as high as glass stem

thermometers.

Thermal expansion methods: Bimetallic sensors disadvantages

• Its operation is based on the fact that liquid expands as the temperature rises.

• In this type of thermometer, the expansion causes the liquid to rise in the tube, indicating the temperature.

• It consists of a small-bore glass tube with a thin-wall glass bulb at its lower end.

• The liquid that fills the bulb and part of the tube is usually mercury.

• As heat is transferred through the well and metal stem and into the mercury, the mercury expands, pushing the column of mercury higher in the capillary above which indicates the temperature.

Liquid-in-glass Thermometer(expansion of liquids)

• They are fragile and not easily adapted to automatic recording or transmission of temperature data. This limits their use in modern industries.

• They can be difficult to read also.

• In the mercury-in-glass thermometer, a large error may be introduced by changes in the size of the bulb due to ageing

Liquid-in-glass Thermometer-Disadvantages(expansion of liquids)

• The liquid mercury-in-steel thermometer works on exactly the same principle as the liquid-in-glass thermometer.

• As mercury in the system is not visible, a bourdon tube is used to measure the change in its volume. The bourdon tube, the bulb and the capillary tube are completely filled with mercury, usually at a higher pressure.

• When the temperature to be measured rises, the mercury in the bulb expands more than the bulb so that some mercury is driven through the capillary tube into the Bourdon tube.

• As the temperature continues to rise, increasing amounts of mercury will be driven into the Bourdon tube, causing it to bend. One end of the bourdon tube is fixed, while the motion of the other end is communicated to the pointer which moves on a

calibrated temperature scale.

Liquid-in-metal Thermometer-(expansion of liquids)

Liquid-in-metal Thermometer-

• “The volume of a gas increases with temperature, if the pressure is maintained constant;”

• “the pressure increases with temperature, if the volume is maintained constant”.

• If a certain volume of inert gas is enclosed in a bulb, capillary and Bourdon tube then the pressure indicated by the Bourdon tube may be calibrated in terms of the temperature of the bulb.

Gas Thermometer-(expansion of gases)

An advantage of the gas-filled thermometer is that the gas in the bulb has a low thermal capacity than a similar quantity of liquid, so that the response of the thermometer to temperature changes will be more rapid than that for a liquid-filled system with a bulb of the same size and shape.

Gas Thermometer-(expansion of gases)Advantages

Filled system thermometers

It consists of: Bulb Capillary tube Pressure element Scale

• When in use, the thermometer bulb is installed inside the substance to be measured. This causes the filling liquid inside the bulb to heat or cool until its temperature matches the temperature of the measured substance. This change in temperature causes the filling liquid to expand or contract and thus the bourdon tube moves. With increase in temperature (heating) the liquid expands and this expansion forces the bourdon tube to uncoil.

• With decrease in temperature (cooling) the liquid contracts and it forces the bourdon tube to coil more tightly. The movement of the bourdon tube may be used to drive a pointer for indicating temperature or to drive the pen on a strip-chart.

Filled system thermometers

• They have a rugged construction.• They require low maintenance.• There is no need for electric power since they are self-

sufficient.• They possess satisfactory time response sensitivity

and accuracy for most industrial applications • Their cost is low.• They deliver enough power to drive not only a pointer

or recording pen but also a controller mechanism.• Three (or more) separate systems can be put in a

single instrument case.

Filled system thermometers - Advantages

• They need a large bulb for the sake of accuracy.

• The entire system usually has to be replaced in case of failure.

• Their accuracy, sensitivity, and temperature span is much lower compared to electrical temperature instruments.

• Maximum spans are not as narrow as in the bimetallic thermometer or electrical systems.

• They have limited maximum temperature compared to some electrical measuring systems.

Filled system thermometers - Disadvantages

There are three types of electrical temperature instruments (temperature sensors) which are generally used in industries.

• Resistance thermometer(RTD)

• Thermocouple

• Thermistor

Electrical temperature instruments

• How it works:– Utilizes the fact that

resistance of a metal changes with temperature.

• Make up:– Traditionally made up of

platinum, nickel, iron or copper wound around an insulator.

• Temperature range:– From about -196°C to

482°C.

Thin Film RTD

Resistance Temperature Detector- RTD


• It is a positive temperature coefficient

device, which means that the

resistance increases with temperature.

The industry standard is the platinum wire RTD (Pt100) whose base resistance is exactly 100.00 ohms at 0.0 °C.

Platinum Wire RTDs (PRTs)

PRTs have established themselves as the de-facto industry standard for temperature measurement, and for many reasons: linear temperature sensors

Resistance vs temperature characteristics are stable and reproducible

linear positive temperature coefficient (-200 to 800 °C)

very accurate and suitable for use as a secondary standard


Platinum Scale ( 0 to 100 °C )


International Practical scale for Temperature (0 to 650. 30 °C)


International Practical scale for Temperature (Below 0 °C)


Only practical if the RTD lead wires are short.

In many applications the RTD is located far from the conditioning circuit adding extra resistance because the length of the copper lead wire.

Cu = 0.0302 Ω per ft. Most RTD’s have an extra wire to compensate for the length of lead wire.

RTDs with a bridge circuit

A temperature sensitive resistance element is fabricated in a suitable form to insert in the medium whose temperature is to be measured, and is connected by leads to a wheatstone bridge

C

LRLRXBA 21

• The bridge consists of a sensing element resistance X having high temperature coefficient and resistances A, B and C whose ohmic values do not alter with change of temperature. LR1 and LR2 are the lead wire resistances of the sensing element.

• The principle of wheat-stone bridge states that in balanced condition (when no current flows through galvanometer).

The ratio of resistances is given by:

• Now, when resistance X changes, the wheatstone bridge becomes unbalanced and thus galvanometer will give deflection which can be calibrated to give suitable temperature scale.


The requirement for the resistance materials used in RTDs are

• High temperature coefficient of resistance in order to give

substantial change in resistance for relatively small change

in temperature or larger sensitivity

• High resistivity

• Linearity of relation between resistance and temperature

for convenience in measurement.

• Stability of electrical characteristics of the material and resistance

to contamination for good repeatability.

• Sufficient mechanical strength


(i) They possess high accuracy of measurement.

(ii) They have a wide temperature range from 200 to 650°C.

(iii) They are small in size.

(iv) They are fast in response.

(v) They have good reproducibility.

(vi) They have shown stable and accurate performance over many years.

(vii) Temperature compensation is not required.

RTD Advantages

(i) Their cost is high.

(ii) They need a bridge circuit, power supply.

(iii) They show inaccuracy resulting from the current flowing through the bridge circuit and thereby heating the resistance element.

(iv) They have larger bulb size than thermocouples.

RTD disadvantages

Thermocouples

• Principle of operation: Thermo-electric effect.

• When 2 dissimilar metals are joined together to form a junction, an emf is produced which is proportional to the temperature being sensed.

• The amount of the current produced depends on the difference in temperature between the two junctions and on the characteristics of the two metals. This was first observed by Seebeck in 1821 and is known as Seebeck effect.

• Instrument which record the variations in current flow are calibrated in terms of temperature and are known as Thermocouple Pyrometers

Typical Industrial Thermocouple Assembly

The relation between the emf and the difference of temperature of hot and cold junctions over a limited temperature range is given by the expression:

)(21)( 22

oo TTTTe

Where e is the thermo-electric emf in volts,

T is the absolute temperature of hot junction,

To is the absolute temperature of cold junction

and are constants whose value depends on the thermo electric power of the two metals

Thermocouples

A copper constantan thermocouple has = 37.5 V/C and = 0.0045 V/C. Determine the emf developed by the thermocouple when its hot junction is at 200 C and cold junction is kept in ice.

Given: = 37.5 V/C, = 0.0045 V/C

Temperature at hot junction = 200 + 273 = 473K

Temperature at cold junction = 0 + 273 =273 K

Emf developed by the thermocouple,

)(21)( 22

oo TTTTe

mV84.7

)273473(100045.021)273473(105.37 2266

Thermocouples - Example

• They have rugged construction.• They are inexpensive.• They are simpler to use than resistance thermometers.• There is no need of a bridge circuit.• They have extremely wide temperature ranges from -270°C to 2800°C.• They have wide variety of designs to both standard and special

applications.• Their electrical output is adaptable to a variety of readout and/or

control devices.• They have high response speed compared to filled systems

thermometers.• They possess good accuracy.• Calibration checking is easily made.• They possess long transmission distances.• They have good reproducibility.

Thermocouples - Advantage

• They have limited use in temperature spans of less than about 33°C because of the relatively small change in junction voltages with temperature.

• Extension leads must be housed in metal conduit, as low junction voltage can cause the device to pickup stray electrical signals.

• They need to hold reference junction temperatures constant or compensation for any deviations.

• Their temperature-voltage relationship is non-linear.• They hold chances of stray voltage pickups.• Temperature spans are not as narrow as filled system or resistance

thermometers.• They require much of an amplifier for many applications.• They need expensive accessories for control applications.

Thermocouples - Disadvantage

Thermistor

Thermistor

The thermistors can be in the shape of a rod, bead or disc. Manufactured from

oxides of nickel, manganese, iron, cobalt, magnesium, titanium and other metals.

Thermistor

• Advantages:– Very sensitive (has the

largest output change from input temperature)

– Quick response– More accurate than

RTD and Thermocouples

• Disadvantages:– Output is a non-linear

function– Limited temperature

range.– Require a current

source– Self heating– Fragile

Thermistor - Advantages and Disadvantages

The word that best describes the thermistors is “sensitive”

Resistance Thermometers Comparison

Lets Experiment!• In lab a RTD, thermistor, and

thermocouple were placed in a beaker of 750mL of water and readings were taken from 19°C to 80°C.

• The next two slides show the results.


TemperatureThermocoupl

e RTD Thermistor(degrees Celsius) (mille-Volts) (ohms) (kilo-ohms)

19 -0.10 108.00 105.6020 -0.10 108.40 99.8021 0.00 108.70 94.2022 0.00 109.00 88.2023 0.00 109.50 83.8024 0.10 110.00 79.7025 0.10 110.40 75.9026 0.10 110.90 73.3027 0.20 111.30 70.0028 0.20 111.50 68.4029 0.30 112.00 63.4030 0.40 112.90 60.5032 0.50 113.20 54.8034 0.70 114.10 49.2036 0.70 114.80 45.50

Thermocouple

-0.50

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

0 10 20 30 40 50 60 70 80 90

Temperature (∘C)

Vol

tage

(mV

)

Thermistor

0.00

20.00

40.00

60.00

80.00

100.00

120.00

0 10 20 30 40 50 60 70 80 90

Temperature (∘C)

Res

ista

nce

(KΩ

)

RTD

100.00

105.00

110.00

115.00

120.00

125.00

130.00

135.00

0 10 20 30 40 50 60 70 80 90

Temperature (∘C)

Res

ista

nce

(Ω)


Summary of Temperature Sensor Characteristics

Summary of Temperature Sensor Characteristics

Documents

Temperature Measurement