ECGR 6185 Advanced Embedded SystemsTemperature sensorsTemperature sensorsSlide 4Thermocouple Temperature Measurement SensorsDifferent types of thermocouples:ThermocouplesSlide 8Slide 9Characteristics of Thermocouples:Slide 11Precautions and considerations for using thermocouples:Slide 13Resistance Temperature DevicesResistance Temperature Detectors (RTDs)RTDsSlide 17Slide 18ThermistorsSlide 20Slide 21Software aspect: (Thermistor and RTD application)Temperature ControllersTemperature controllersSlide 25Slide 26ConclusionSlide 28ECGR 6185Advanced Embedded SystemsTEMPERATURE SENSORS(Thermocouples, RTDs and Thermistors)University Of North Carolina CharlotteKarunakar Reddy GujjaTemperature 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 sensorsContact 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 sensorsContact type sensors:•Thermocouples•Resistive temperature devicesNon-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 sensorsThermocouple Temperature Measurement SensorsPrinciple 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 forCost SensitivityType 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/°CType E Chromel / Constantan (Cu-Ni alloy) −200 °C to 900 °C Oxidizing or inert applications Low 68 µV/°CType J Iron / Constantan −40 °C to 750 °C Vacuum, reducing, or inert apps Oxidizing or humid environmentsLow 52 µV/°CType N Nicrosil (Ni-Cr-Si alloy) / Nisil (Ni-Si alloy)−270 °C to 1300 °C Oxidizing or neutral applicationsLow 39 µV/°CType T Copper / Constantan −200 °C to 350 °C Oxidizing, reducing or inert apps Wet or humid environmentsLow 43 µV/°CType R Platinum /Platinum with 13% Rhodium 0 °C to 1600 °C High temperatures Shock or vibrating equipmentHigh 10µV/°CType S Platinum /Platinum with 10% Rhodium 0 °C to 1600 °C High temperatures Shock or vibrating equipmentHigh 10µV/°CType B Platinum-Rhodium/Pt-Rh 50 °C to 1800 °C High temperatures Shock or vibrating equipmentHigh 10µV/°CThermocouples 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* ΔTWhere, V= voltage measured (V) S= Seebeck coefficient (V/°C) ΔT= difference in temperature between two junctionsHence the unknown temperature can be calculated using the equation, T= Tref + V/S in °CThermocouples •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 ShuntingThermocouples Advantages:–Self-powered –Simple in construction –Rugged–Wide temperature range–Wide variety–Inexpensive Disadvantages:–Non-linear–Low voltage–Less stable –Reference requiredResistance 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•ThermistorsResistance 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)RTDsCalculations: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 RTDsApplications:•They are used for precision process temperature control.•Widely used in industrial applications.•Directly used in recorders, temperature controllers, transmitters and digital ohmmeters RTDsThermistorsThese are similar to RTDs.These work on negative temperature coefficient.These are made up of semiconductor devices.Variation is
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