By: Engr. Irfan Ahmed HalepotoAssistant Professor

LECTURE#04TEMPERATURE SENSORS

AUTOMATION & ROBOTICS

TRANSDUCER • Transducer is a device which transforms energy from

one type to another, even if both energy types are in the same domain.– microphone (converts sound into an electrical

signal).• Typical energy domains are mechanical, electrical,

chemical, magnetic, optical and thermal.

• Transducer can be further divided into Sensors, which monitors a system and Actuators, which impose an action on the system.

TEMPERATURE SENSORSTemperature is a scalar quantity that determines the direction of

heat flow between two bodies.Sensing methods: contact and non-contact

– Contact

• Sensor is in direct physical contact with the object to be sensed

• To monitor solids, liquids, gases over wide range

– Non-contact

• Interprets the radiant energy of a heat source to energy in electromagnetic spectrum

• Monitor non-reflective solids and liquidsTemperature sensors generate output signals in one of two ways:

1. through a change in output voltage

2. through a change in resistance of the sensor‘s electrical circuit

TEMPERATURE SENSORS CLASSIFCATION: Contact Sensing

• Thermocouple (Thermoelectric)• Thermistor (Thermal Resistors)

– Negative temperature coefficient device (NTC)– Positive temperature coefficient device (PTC)

• Resistive Temperature Detector (RTD)• Semiconductor Temperature sensors • Liquid-in-Glass Thermometers• Bimetallic Thermometers

TEMPERATURE SENSORS CLASSIFCATION: Non-Contact Sensing

• Radiation Thermometers– Infrared thermal Imaging– Scanners– Spot Radiometers

• Thermal Imagers• Ratio Thermometers

THERMOCOUPLES• Most common temperature sensing device. • Accurate temperature measurements can be made

with thermocouples sensors at low cost with shop-built probes and ordinary low-level voltmeters.

• Thermocouples can measure temperature at a point in a range of -250C to +3500C.

Typical Industrial Thermocouple General Thermocouple

Thermocouple Concept • Principle of operation is based on the Seebeck effect,

discovered by Thomas Seebeck in 1822,– Electrons flow from one wire to other, due to different

energy potentials of alloys– As temperature changes, current flows– Voltage is measured between the two alloys (Small

voltage, less than 10 mV)

• Seebeck effect: when any conductor subjected to thermal gradient, generates a voltage.

• A temperature gradient along a conductor creates an EMF.

Thermocouple Concept…… • If two conductors of different materials are joined at one

point, an EMF is created between the open ends which is dependent upon the temperature of the junction.

• As T1 increases, so does Voltage (V). • EMF also depends on the temperature of the open ends T2.• The junction is placed in the process, the other end is in

iced water at 0C. This is called the reference junction.

Thermocouple Working Principle• To measure this thermal

gradient– connect another conductor.-

• voltage depends on type of metal used.

• Difference –1 – 70 micro volts per degree C

• Thermocouple gives difference in temperature not the absolute value

• Current continues to flow as long as T2>T1

• Emf e= a1 + a2 () 2+…….+ an () n

• Finally reduced to e= a1 • Thermocouples behaves

according to thermo electric laws

Thermo electric laws• Three laws govern operation of thermocouples:• Law 1. A thermoelectric current cannot be established in a

homogeneous circuit by heat alone. – This law establishes the need for junctions of dissimilar

materials since a single conductor is not sufficient.Law 2. The algebraic sum of the thermoelectric forces in a

circuit composed of any number and combination of dissimilar materials is zero if all junctions are at uniform temperatures. – Additional materials may be connected in the

thermoelectric circuit without affecting the output of the circuit as long as any junctions added to the circuit are kept at the same temperature.

– voltages are additive so that multiple junctions may be connected in series to increase the output.

Thermo electric laws

• Law 3.If two junctions at temperatures T1 and T2 produce Seebeck voltageV2 and temperatures T2 and T3 produce voltage V1, then temperatures T1 and T3 produce V3=V1+V2.

– This law establishes methods of calibration of thermocouples.

Law 4. Law of intermediate temp. Eac= Eab + Ebc

Law of intermediate metals: • States that net emf in a circuit remains unaltered if a third

metal is introduced provided that the two junctions of third metals are at same temperature.

Thermo electric laws

Practical Thermocouple Construction• A thermocouple construction consist of two conductors,

welded together at the measuring point and insulated from each other long the length.

• It will usually have an outer protection sheath.

Thermocouple Materials Classificationthermocouple materials are classified as• BASE METAL : Types E, J, K, N, and T

• NOBLE METAL: Types R, S, and B

Comparison of Thermocouples & Applications

Type J:

• Type J is useable up to 720°C.

• It is not very susceptible to aging up to about 540°C.

• It is very cost effective and is the thermocouple of choice in the plastics processing industry where temperatures rarely exceed 400 °C.

• The iron conductor is subject to oxidation (due to the iron wire) at higher temperatures and when unprotected.

• generates about 50 µV/°C (28 µV/°F)

Type K:

• Type K is useable up to 1150°C in an oxidizing atmosphere.

• Metallurgical changes can cause a calibration drift of 1 to 2°C in a few hours, increasing to 5 °C over time.

• generates about 40 µV/°C (22 µV/°F)

Comparison of Thermocouples & Applications

Type E:• Type E is useable up to 820 °C.• It has the highest mV output of all the thermocouples and has similar

calibration drift to that of Type K so the same precautions are recommended.

Type T:• Type T oxidization of the copper limits the useable temperature to

about 370 °C. • Type T is thermocouple of choice for applications down to –200 °C.• generates about 40 µV/°C (22 µV/°F).Type N :• Type N is useable up to1260°C.• It was developed to overcome several problems inherent in Type K

TCs.• Aging in the 300 to 600 °C range is considerably less. • Also Type N has also been found to be more stable than Type K in

nuclear environments, where Type K has been the sensor of choice.

Comparison of Thermocouple & Applications

Types R and S: • Types R and S are usable up to 1480°C. • They are extremely stable but reducing atmospheres are

particularly damaging. • This type should be protected with a gas-tight ceramic tube

and a secondary tube of porcelain, silicon carbide or metal outer tube, as conditions require.

• Type R delivers some 15% more mV than type S.

Type B• Type B is usable up to 1700 °C. Also easily contaminated,

and damaged by reducing atmospheres. • The same protective measures for R and S shown above

apply to type B thermocouples.

Thermocouples used in industrial process

• Type K thermocouples are the most linear of the three

Types J and E have better relative output than type K

Thermocouples Capabilities

• Wide Range

• Fast Response

• Passive

• Inexpensive

Thermocouples Limitations

• Non-Linear• Accuracy

– Often between 0.5 and 2.2ºC, depending on TC type• Noise

– Long leads can attract electrical signals– Already low signal from thermocouple

• Thermal shunting– Heating of wire mass can affect measurements by

absorbing energy• Corrosion

– High alkali or water environments can modify calibration

Thermistors: THERMal resISTORS• Thermistor is a combination of the words thermal and resistor. • Thermistor was invented by Samuel Ruben in 1930• A thermistor is a type of resistor used to measure temperature

changes, relying on the change in its resistance with changing temperature. – Typically have a negative temperature coefficient (NTC), – Resistance decreases with increasing temperature

• Thermistor can measures across the range of -40~150 ±0.35 °C

Leads, coated Glass encased Surface mount

Relationship b/w resistance & temperature

• Assume a simple linear relationship between resistance and temperature for the following discussion:

ΔR = k ΔT

Where

• ΔR = change in resistance

• ΔT = change in temperature

• k = first-order temperature coefficient of resistance

Thermistors Classification• Thermistors can be classified into two types depending on

the sign of k. • If k is positive, the resistance increases with increasing

temperature, and device is called a positive temperature coefficient (PTC) thermistor, some time also known as Resistance Temperature Detector (RTD) – If Platinum and nickel are used as metal in RTD.

• If k is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor.

• Resistors that are not thermistors are designed to have the smallest possible k, so that their resistance remains almost constant over a wide temperature range.

Thermistor constructionNTC Thermistors• NTC Thermistors are made from the oxides of metals

such as manganese, cobalt, nickel and copper. • The metals are oxidized through a chemical reaction,

ground to a fine powder, then compressed and subject to very high heat.

• Some NTC thermistors are crystallized from semiconducting material such as silicon and germanium.

PTC Thermistors• PTC Thermistors are made by introducing small

quantities of semiconducting material into a polycrystalline ceramic.

• When temperature reaches a critical point, the semiconducting material forms a barrier to the flow of electricity and resistance climbs very quickly.

PTC working principle

• Resistance of these types of thermistors increases with the rise in temperature.

• Due to the special Resistance- Temperature-characteristic, there is no additional temperature regulation or safety device necessary while reaching high heat-power level when using the low resistance area .

NTC working principle • Resistance of NTC thermistors decreases proportionally

with increases in temperature.• Thermistor resistance-temperature relationship can be

approximated by,

where:  • T is temperature (in Kelvin), • TRef is the reference temperature, usually at room temp (25°C;77°F; 298.5 K),• R is the resistance of the thermistor (W), • RRef is the resistance at TRef, • b is a calibration constant depending on the thermistor material, usually between 3,000 and 5,000 K. 

PTC Thermistor :Application configuration• There are very few commercial

applications involving PTC thermistors that are based upon the resistance -temperature characteristic.

• Most PTC thermistor applications are based upon either the steady state self-heated condition (voltage-current characteristic) or upon the dynamic self-heated condition (current -time characteristic) or a combination of both.

• Dramatic rise in resistance of a PTC at and above the transition temperature makes it ideal for over current protection.

PTC Thermistor: Industrial Applications

Time Delay, Motor Starting, Degaussing• These three applications are somewhat similar in that they

all rely on the dynamic operation (Current-Time Characteristic) of a self-heated PTC thermistor.

• In each case, current is allowed to pass through a series circuit for a prescribed amount of time before the thermistor self-heats into a high resistance condition.

Time Delay Circuit:In the time delay circuit, as the thermistor self-heats and increases its resistance, the voltage drop on the fixed resistor decreases to a minimum.

PTC Thermistor: Industrial ApplicationsMotor StartingIn the motor start circuit, PTC has a low resistance at turn-on so that a significant amount of current is permitted to flow in the starting winding of the motor. After the thermistor has self-heated into a high resistance state, the current through the starting winding becomes negligible.

degaussing circuitIn the degaussing circuit, current through the demagnetizing coil is initially large and decreases to a negligible value as the thermistor self-heats. In all of these cases, the length of time required for the switching action to occur depends upon the amount of power applied to the PTC and its thermal characteristics.

NTC Thermistor :Application configuration

• NTC thermistor is a versatile component that can be used in a wide variety of applications where the measured is temperature dependent.

• Thermistor applications are grouped according to one of the three fundamental electrical characteristics;

The current-time characteristics The voltage-current characteristic The resistance-temperature characteristic

NTC Thermistor :Application configuration

• Application based on Current-Time characteristic Time delay, surge suppression, inrush current limiting and

sequential switching represent some of the earliest, high volume uses of thermistors.

• Application based on Resistance -temperature characteristic Temperature measurement, control, and compensation.

NTC Thermistor :Application configuration

Linear Voltage Divider• Simplest thermistor network used in

many applications is the voltage divider circuit

The output voltage is taken across the fixed resistor.

This has the advantages of providing an increasing output voltage for increasing temperatures and allows the loading effect of any external measurement circuitry to be included into the computations for the resistor, R .

The loading will not affect the output voltage as temperature varies

Thermistors Specifications

• Thermistor-choice is based on the nominal resistance you want at the operating temperature range, on the size, and on the time constant.

• Time constants are about 5-10 seconds. (Check this out with your thermistor).

Thermistor Linearizing Circuit

Application of PTC Thermistors

• They are used as resettable fuses.

• They are used in time delay circuits.

• PTC Thermistors are used in motor starting circuits.

• They are also used in Degaussing circuitry.

• The PTC Thermistor can provide a combination of heater and thermostat in one device

• They are used as ‘liquid level’ and ‘flow sensors’.

• General industrial applications Industrial process controls Plastic laminating equipment Fiber processing & manufacturing Hot mold equipment (thermoplastics) Solar energy equipment• Automotive & Transportation

Application Emission controls Engine temperatures Aircraft Temperatures.

Application of NTC Thermistors

Medical ApplicationsFever ThermometersFluid temperatureDialysis Equipment

Consumer/Household ApplicationsBurglar alarmRefrigeration & air conditioningFire detectionOven temperature control

Thermistor Advantages• High resistance (1 kΩ to 100 kΩ)

– Eliminates problems with resistance in lead wires• Highly non-linear RT vs. T relationships

– Mostly negative temperature coefficients (NTC) from metal oxides, but positive temperature coefficient (PTC) models are available from barium and strontium titanate mixtures

– Can be linearized• Small physical size• Fast response time• Not as small as thermocouples• Lower cost than RTDs• Easy to manufacture in bulk• Wide temperature range• Very high sensitivity and resolution• Up to 1000 times more sensitive than RTDs• Can withstand shock and vibration• Accurate

Thermistor Disadvantages• Limited temperature range, typically -100 ~ 150 °C (-148 ~

302 °F).• Nonlinear resistance-temperature relationship, unlike RTDs

which have a very linear relationship. • Errors can result from self excitation currents being

dissipated by the thermistors.• They get de-calibrated on exposure to higher temperatures• The nonlinear response requires extra circuitry• Limited range of linear response with this additional circuitry• Narrow “linear” operating range for a single sensor• The glass can break if mishandled• Requires an excitation current• High resistance leads to self-heating errors (more so than

RTDs)• Less stable than RTDs

Resistance Temperature Detector- RTD

• RTD is a temperature sensitive resistor, It is a

positive temperature coefficient device, which

means that the resistance increases with

temperature.• This type of sensors is based on the observation

that different materials can have different resistive profiles at different temperatures. – Properties are mainly electrical in nature.

• Industrial RTDs are very accurate: the accuracy can be as high as ±0.1°C.

• The ultra high accurate version of RTD is known as Standard Platinum Resistance Thermometers (SPRTs) having accuracy at ±0.0001°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 linear positive temperature coefficient (-200 to 800 °C) Resistance vs. temperature characteristics are stable and

reproducible very accurate and suitable for use as a secondary standard

Platinum Scale ( 0 to 100 °C )

Platinum Wire RTDs (PRTs)

Thin-Film RTDs• Thin-film RTD design is a newer technology and is gaining favor due to lower cost.

• It is designed to minimize strain on the platinum due to thermal expansion since strain also cause changes in resistance, R =(L/A).

RTD: small resistance change measurement

• Bridge circuit:– Can detect small resistance changes– If R1=R4, RRTD= R2(Vs-2Vo)/(Vs+2Vo)

R1 R2

RRTDR4

VoVs

“Supply”Voltage

Circuits Used to Determine the Resistance of an RTD

• Two-wire: Non-linear relationship between the measured voltage and the RTD resistance.

• Three-wire: Better results.

• Four-wire: Resistance is a linear function of the measured voltage.

Four Wire Design

RTDs: Characteristics and Applications

• Characteristics:– Resistive device, active, linear– Large range: -200 to +850oC for Platinum– High accuracy: 0.001oC– Low sensitivity: 0.39 % per oC– Don’t need reference temperature

Applications: Industries and laboratories where high accuracy of

temperature measurements are required.

Selecting A Temperature SensorThree things that we need to keep in mind when selecting temperature sensor:

1) What is the desired temperature range, the tolerable limit to the error in measurement and the conditions under which the measurement is to be performed?

2) Is it possible to touch the object and if so would the sensor or the temperature of the object be likely to be seriously affected by the contact?

If the answer is yes, then a non-contact temperature sensor is needed. If no, then the answer probably lies with one of the other sensor types.

3) If a contact sensor appears satisfactory, then questions revolve more around temperature measuring range, satisfying the conditions of use and meeting the acceptable error allowance.