ECGR 6185Advanced Embedded Systems : ECGR 6185Advanced Embedded Systems TEMPERATURE SENSORS(Thermocouples, RTDs and Thermistors)
University Of North Carolina Charlotte
Karunakar Reddy Gujja Temperature sensors : 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 : 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.
These infer temperature by measuring the thermal radiations emitted by the material. Temperature sensors Slide 4: Contact type sensors:
Resistive temperature devices
Non-Contact type sensors:
-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 : 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: : Different types of thermocouples: Thermocouples : 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 : 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 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: Characteristics of Thermocouples: : Characteristics of Thermocouples: Precautions and considerations for using thermocouples: : Precautions and considerations for using thermocouples: Connection problems
Common Mode Voltage
Thermal Shunting Thermocouples : Thermocouples Advantages:
Simple in construction
Wide temperature range
Reference required Resistance Temperature Devices : 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:
Thermistors Resistance Temperature Detectors (RTDs) : 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)
Tungsten (rare) RTDs : RTDs Calculations:
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)) RTDs : Advantages:
Stable output for a long period of time
Ease of recalibration
Accurate readings over narrow temperature range
Smaller temperature range when compared to thermocouples
High initial cost and less rugged to environmental vibrations
Self heating RTDs 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 : 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). Thermistors : Advantages:
Two wire ohms measurement Disadvantages:
Limited temperature range
Self heating Thermistors Thermistor symbol Thermistors : 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) : 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 Controllers : Temperature Controllers What 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:
PID controller Temperature controllers : 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 : 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 : Temperature controllers PID controller:
It is a closed loop control system. Conclusion : 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. Slide 28: END