ECGR 6185Advanced Embedded Systems

TEMPERATURE SENSORS(Thermocouples, RTDs and

Thermistors)

University Of North Carolina CharlotteKarunakar Reddy Gujja

Temperature sensors

Temperature Sensors are the devices which are used to measure the temperature of an object.

These sensors sense the temperature and generate output signals in one of the two forms: change in voltage or change in resistance.

In order to select a sensor for a particular application - accuracy, range of temperature, response time and environment are considered.

Temperature sensors are categorized into two types:

– Contact type sensors

– Non-Contact type sensors

Contact type sensors:

These measure their own temperature i.e., they are in contact with the metal and will be in thermal equilibrium.

Non-Contact type:

These infer temperature by measuring the thermal radiations emitted by the material.

Temperature sensors

Contact type sensors:

• Thermocouples

• Resistive temperature devices

Non-Contact type sensors:

• IR thermometers

-These measure the temperature by detecting the infrared energy emitted by the material.

-This consists of a lens which senses the IR signal and converts it into electrical signal which is displayed in temperature units.

-These are applied when the object is moving, surrounded by EM field or when a fast response is required.

Temperature sensors

Thermocouple Temperature Measurement Sensors

Principle of operation:

Thermocouples work on the principle of Seebeck effect.

They are available in bead type or probe type construction.

They consist of two junctions: cold junction and hot junction.

The voltage developed between two junctions is called Seebeck voltage.

Voltage is in the order of millivolts.

They generate energy in the order of microwatts-milliwatts.

Different types of thermocouples:

Type Composition Range Good for Not recommended for

Cost Sensitivity

Type K Chromel (Ni-Cr alloy) / Alumel (Ni-Al alloy)

−200 °C to 1200 °C Oxidizing or neutral applications Use under 540ºC Low

(11.65$ to 48.63$)

41 µV/°C

Type E Chromel / Constantan (Cu-Ni alloy) −200 °C to 900 °C Oxidizing or inert applications

Low 68 µV/°C

Type J Iron / Constantan −40 °C to 750 °C Vacuum, reducing, or inert apps

Oxidizing or humid environments

Low 52 µV/°C

Type N Nicrosil (Ni-Cr-Si alloy) / Nisil (Ni-Si alloy)

−270 °C to 1300 °C Oxidizing or neutral applications

Low 39 µV/°C

Type T Copper / Constantan −200 °C to 350 °C Oxidizing, reducing or inert apps

Wet or humid environments

Low 43 µV/°C

Type R Platinum /Platinum with 13% Rhodium 0 °C to 1600 °C High temperatures Shock or vibrating equipment

High 10µV/°C

Type S Platinum /Platinum with 10% Rhodium 0 °C to 1600 °C High temperatures Shock or vibrating equipment

High 10µV/°C

Type B Platinum-Rhodium/Pt-Rh 50 °C to 1800 °C High temperatures Shock or vibrating equipment

High 10µV/°C

Thermocouples

Theory of operation:

– Figure 1 shows the typical Type-J thermocouple.

– The emf shown in the figure is the Seebeck voltage which is developed because of the temperature difference.

– Figure 2 shows the cold junction compensation (CJC).

Thermocouples

Calculations:

The voltage generated by the thermocouple is given by the equation:

V= S* ΔT

Where, V= voltage measured (V)

S= Seebeck coefficient (V/°C)

ΔT= difference in temperature between two junctions

Hence the unknown temperature can be calculated using the equation,

T= Tref + V/S in °C

Thermocouples

• Thermocouples are available in wire bead type or probe type.

• Bead type are used for low temperature applications and probe type for high temperature applications.

• In selecting a thermocouple for particular application type, insulation and probe construction is considered.

• Location of the thermocouple plays a major role for accurate measurement. As a ‘rule of thumb’ it is located at 1/3rd distance from the heat source and 2/3rd distance from workload.

Characteristics of Thermocouples:

Characteristics of Thermocouples:

Precautions and considerations for using thermocouples:

– Connection problems

– Lead Resistance

– Decalibration

– Noise

– Common Mode Voltage

– Thermal Shunting

Thermocouples

Advantages:– Self-powered

– Simple in construction

– Rugged

– Wide temperature range

– Wide variety

– Inexpensive

Disadvantages:– Non-linear

– Low voltage

– Less stable

– Reference required

Resistance Temperature Devices

They work by undergoing change in electrical resistance, with change in temperature.

These are low cost and low temperature range sensors.

These are of two types:

• RTDs

• Thermistors

Resistance Temperature Detectors (RTDs)

They work on the principle of positive temperature coefficient.

RTDs are used to measure the temperatures ranging from -196 to 482 deg C

or (-320 to 900 deg Fahrenheit)

Common Resistance Materials for RTDs:

• Platinum (most popular and accurate)

• Nickel

• Copper

• Balco (rare)

• Tungsten (rare)

RTDs

Calculations: R(T)=R0*(1+a*T+ b*T^2) – R (T) = Resistance at temperature T

– R0 = Resistance at Nominal Temperature

– a and b are calibration constants, where

a= 3.9692 * 10^-3 /°C

b= -5.8495 * 10^-7 /°C

The relationship between voltage and RTD’s resistance is given by:

V= (Vref*R(T))/(R(0)+R(T))

Advantages:• Stable output for a long period of time

• Ease of recalibration

• Accurate readings over narrow temperature range

• Linear output

Disadvantages:• Smaller temperature range when compared to thermocouples

• High initial cost and less rugged to environmental vibrations

• Not self-powered

• Self heating

RTDs

Applications:

• They are used for precision process temperature control.

• Widely used in industrial applications.

• Directly used in recorders, temperature controllers, transmitters and digital

ohmmeters

RTDs

Thermistors

These are similar to RTDs.

These work on negative temperature coefficient.

These are made up of semiconductor devices.

Variation is non-linear.

Thermistors are used to measure the temperatures ranging from -45 to 260 deg C or (-50 to 500 deg Fahrenheit).

Advantages:•High output

•Fast response

•Two wire ohms measurement

Disadvantages:•Non-linear

•Limited temperature range

•Not self-powered

•Self heating

Thermistors

Thermistor symbol

Applications:

• Can be used as a liquid level indicator or as a liquid level controller

• To measure temperature in Medical Applications

• Temperature Control

Thermistors

Software aspect: (Thermistor and RTD application)

• Application of RTD for detecting the environment temperature.

• This uses the microcontroller board which has an inbuilt Thermistor which is used to compare the readings of both sensors.

• The environmental temperature is measured and displayed on the LCD screen of the microcontroller and updated every 1 second.

• RTD is connected to one of the ADCs of the microcontroller and this value is also displayed on the LCD and updated for every 1 second.

Temperature ControllersWhat are temperature controllers?

How to select a controller?

The following items should be considered when selecting a controller:

–Type of input sensor (thermocouple, RTD) and temperature range

–Type of output required (electromechanical relay or analog output)

–Control algorithm needed (on/off, proportional, PID)

–Number and type of outputs (heat, cool, alarm, limit)

Different types of controllers:•On/Off controller

•Proportional controller

•PID controller

Temperature controllers

On-Off controller:

• This is a simple mechanism for temperature control device, whenever temperature crosses the set point, controller switches the output.

• It is a cyclic process.

• In order to prevent the continual operation, a differential or hysteresis is used.

• It is used in slow temperature change applications.

Eg: Temperature alarm system.

Temperature controllers

Proportional controller:• It eliminates the cyclic problem of on-off controller.

• This slows down the time at which heater approaches the set point by decreasing the average power supplied.

• This time proportioning phenomenon controls the ON time and OFF time of the controller.

• Proportioning action occurs within a proportional band.

• Output is ON within the band (below set point) and OFF outside the band (above the set point).

Temperature controllers

PID controller:• Proportional-Integral-Derivative controller.

• It is a closed loop control system.

Conclusion

Thermocouples,– Produce a difference voltage in response to a temperature gradient

developed along its length.

– Must be referenced to a known temperature reference, a ‘cold junction’ for accurate measurement.

– Requires linearization for best over-temperature linearity response.

Resistance temperature devices, RTD produce fast response than thermocouples at low temperatures and

is accurate and stable when compared to other sensors. Thermistors are sensitive and less expensive compared to RTDs.

END

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