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Thermal Sensor

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					Thermal Sensor
          Thermal Sensors
• The control of process needs to be able to
  sense controlled or regulated variables
• One physical variable that is important in
  industrial process control is temperature
  Definition of Thermal Energy
• Substances around us are composed of
  atoms
• When substance received energy, it will
  vibrates
  – Vibrates at equilibrium – solid
  – Vibrates and rotating – liquid
  – Vibrates and rotating and moving freely - gas
    Definition of Temperature
• Temperature is the measurement of
  thermal energy containing in each
  molecule of substance (Joules/Molecule)
• Example 10,000 J of energy
  – In a glass of water
  – In a tank of water
 Temperature Scale
and Calibration Points
  Absolute Temperature Scale
• Assigns zero temperature to a material
  that has no thermal energy
  – Kelvin (K) Scale
  – Rankine (°R) Scale
• The transformation between the two
  scales is
                          5
                 T ( K )  T ( R)
                          9
   Relative Temperature Scale
• Shift the zero index up; zero degree does
  not mean zero thermal energy
• Celsius scale relates to Kelvin
               T ( C )  T ( K )  273.15


• Fahrenheit relates to Rankine

               T ( F )  T ( R)  459.6
 Relationship between F and C
• To transform from Celsius to Fahrenheit,
  note that the two scales differ by size of
  degree and scale shift of 32


                        9
               T ( F )  T ( C )  32
                        5
Relationship to Thermal Energy
• Average thermal energy WTH of a
  molecule can be found from the absolute
  temperature in K from
                        3
                 WTH    kT
                        2

• Where k = 1.38x10-23 J/K is Boltzmann’s
  constant
Example
 A material has a temperature of 335 K.
 Find the temperature in °R
Example
 Given temperature of 144.5 °C, express
 this temperature in (a) K and (b) °F
Example
 A sample of oxygen gas has a
 temperature of 90 °F. If its molecular
 mass is 5.3x10-26 kg, find the average
 thermal speed of a molecule.
Metal Resistance and Temperature
• Metal is composed of atoms.
• Add up all atoms vibrations constitutes thermal
  energy stored in the metal
• The energy bands of metal is shown below




• Valance band and conduction band are
  overlapped
• Valance electron can stay in the
  conduction band and moves freely,
  conducting current
• As metal received energy, stationary atom
  vibrates more and more
• Conduction electrons collide more and
  more with vibrating atoms. Thus, resists
  the flow of current (increase in resistance)
Graph of Temperature
   vs. Resistance
Resistance vs. Temp
  Approximation
      Resistance vs. Temperature
            Approximation
• A straight line approximation of resistance
  vs. temperature

                       R(T )  R(T0 ) 1  0T 

 Where
  R(T )  approximation of resistance at temperature T
  R(T0 )  resistance at temperature T0
    T  T  T0
     0  fractional change in resistance per degree of temperature at T0
where

               1  R2  R1 
        0          T T 
                   
             R(T0 )  2 1 
Example
 A sample of metal resistance vs.
 temperature has the following measured
 values:




 Find the linear approximation of resistance
 vs temperature between 60° and 90° F
Resistance Temperature Detectors
             (RTD)
• A temperature sensor based on the
  changed of resistance of metal due to the
  change in temperature
• Metals used are
  – Platinum (Repeatable, expensive)
  – Nickel (No quite repeatable, inexpensive)
       Characteristics of RTD
• Sensitivity
  – Value of α0 is sensitivity
  – on the order of 0.004/°C for platinum and
    0.005/°C for nickel
• Response time
  – Due to the slowness of thermal conductivity,
    the response time is 0.5 to 5 s or more
• Construction
  – Metal wire is wound into coil and is protected
    in sheath or productive tube
• Signal conditioning
  – The RTD is generally used in a bridge circuit
    with compensate line
• Dissipation constant
  – Because RTD is resistance, there is an I2R
    power dissipation in the device (Self-heating)
  – Can cause erroneous reading
  – Dissipation constant (PD) is provided in RTD
    spec.
     • A power required to raise the RTD temp by 1 C
     • The increase in temperature is found as
                               P
                        T 
                               PD
    T  temperature rise becuase of self-heating in C
      P  power dissipated in the RTD from the circuit in W
     PD  dissipation constant of the RTD in W/ C
Example
 An RTD has α0 = 0.005/°C , R = 500 , and
 a dissipation constant of PD = 30 mW/°C at
 20 °C. The RTD is used in a bridge circuit
 with R1 = R2 = 500  and R3 a variable
 resistor used to null the bridge. If the
 supply is 10 V and the RTD is placed in a
 bath at 0 °C, find the value of R3 to null the
 bridge.
             Thermistors
• Thermistor is made of semiconductor
  instead of metal
• The gap energy ΔWg exists between the
  band
• When the temperature of material
  increases, valance electrons gain
  additional energy until exceeds the band
  gap
• Valence electrons are free to move and
  conduct current as temperature increase
• As current conducts more, resistance
  decreases
Resistance vs. Temperature
    Thermistor Characteristics
• Sensitivity
  – Typically, 10% resistance change per 1 °C
• Construction
  – Can be fabricated in discs, bead, and rods
• Response time
  – Typically, 0.5 s
  – For poor thermal contact, response time can
    be 10 s
• Signal conditioning
  – Bridge circuit
• Dissipation constants
  – In milliwatts/°C
Example
 A thermistor is to monitor room
 temperature. It has a resistance
 of 3.5 k at 20 °C with a slope of
 -10%/C. The dissipation
 constant is PD = 5 mV/C. It is
 proposed to use the thermistor in
 the divider circuit as shown to
 provide a voltage of 5.0 V at 20 °
 C. Evaluate the effect of self-
 heating
           Thermocouples
• When any conductor (such as a metal) is
  subjected to a thermal gradient, it will
  generate a voltage
• Two different metals A and B are used to
  close the loop with connecting junction at
  T1 and T2
Seebeck Effect
• Seebeck effect
                       T2

                       QA  QB dT
                       T1




• where

            emf produced in voltage
      T1 , T2  junction temperatures in K
    QA , QB  thermal transport constants of the two metals
• In practice, an approximation linear
  relationship exists as

                     T2  T1 

• where

                constant in V/K
           T1 , T2  junction temperatures in K
Example
 Find the Seebeck emf for a material with α
 = 50 V/°C if the junction temperatures are
 20 °C and 100 °C.
 Thermocouple Characteristics
• The setup of thermocouple is arranged as
  shown




• Junction TM is exposed to temperature to be
  measured
• The reference junction TR is a known
  temperature
• The voltage produce is a function of (TM-
  TR)
       Thermocouple Types
• Certain standard configuration of
  thermocouples have been adopted
Each type has its particular features such as range,
linearity, inertness to hostile environments sensitivity
       Thermocouple Polarity
• If the reference temperature is less than
  the measured temperature, the output
  would be positive
        Thermocouple Tables
• Table gives the voltage output related to
  reference and measured temperature with
  TR = 0 °C                          increment




   TM            VM
• To find a corresponding temperature to a
  certain output voltage, need interpolation

                       TH  TL 
            TM  TL            VM  VL 
                      VH  VL 

• To find a measured voltage for a particular
  temperature
                      VH  VL 
            VM  VL            TM  TL 
                       TH  TL 
Example
 A voltage of 23.72 mV is measured with a
 type K thermocouple at a 0 °C reference.
 Find the temperature of the measurement
 junction.
Example
 Find the voltage of a type J thermocouple
 with a 0 °C reference if the junction
 temperature is -172 °C.
      Thermocouple Sensors
• Sensitivity
  – Type J: 0.05 mV/°C
  – Type K: 0.006 mV/°C
• Construction
  – Welded or twisted
    junction between 2
    metals
  – Protective cover
• Time response
  – Related to size of the wire and any protective
    material
  – Small TC, 10 to 20 ms
  – Large TC, 10 to 20 s
• Signal conditioning
  – Output is very small, need high gain
    differential amplifier

				
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posted:9/10/2012
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