2. Thermocouples (cont’d) and Variable Resistors (Thermistors) 23 Feb 2000 Introduction to Temperature Sensors

3. Kirchoff’s Current Law Convention +_+_ +_+_ Using –  + = +, we have +V b - IR = 0 or V b = IR Using +  – = +, we have - V b + IR = 0 or - V b = - IR or V b = IR Same Result! Just be consistent. clockwise current flow VbVb IR

4. Language of Thermocouples zTemperature Range y-270 o C to 2320 o C depends on the type of the thermocouple zPassive yDoes not require external power source zNon-intrusive yDoes not affect the system much zLinearity yIn general, it is a nonlinear function of temperature zSensitivity yFrom 0.011 mV/ o C for type R to 0.076 mV/ o C for type E

5. Millivolt Output of Common Thermocouples (Reference Junction at 0 o C) K T J E R S

6. Characteristics of Standard Thermocouples

7. Common Sheathed Thermocouple Probe Place the wires and junction and ceramic insulation inside a stainless steel or inconel sheath to protect the thermocouple wires from damage or chemical contamination.

8. Thermopile When n thermocouples are connected in series in a device called a thermopile Which increases the sensitivity of the system, however, it provides a method to average several thermocouples which are distributed in a spatial region. T1T2

9. Example: A type R thermocouple with an ice reference gives a voltage reading of 2.415 mV. What is its temperature? Thermoelectric voltage in mV 2.415

10. Example: A type R thermocouple with an ice reference gives a voltage reading of 2.415 mV. What would it read with a 30 °C reference? Thermoelectric voltage in mV V = 2.415 mV - 0.171 mV = 2.244 mV

11. Example: A type R thermocouple with 30 °C reference gives a voltage reading of 2.244 mV. What is its temperature? Thermoelectric voltage in mV 0.010 mV 2.244 0.008 mV NO!

12. Thermoelectric voltage in mV 2.244 + 0.171 = 2.415 Example: A type R thermocouple with 30 °C reference gives a voltage reading of 2.244 mV. What is its temperature? Correct the voltage FIRST! (30 o voltage)

13. Thermistors - work by changing resistivity,  change L and A (strain gauge) change  (thermistor)

14. Thermistors zCharacteristics yResistive Device, active, non-linear yLess accurate than TCs and RTDs y-100 to 300 o C zApplications yAutomobile engine water temperature yVCR overheating controller yElectronic circuits temperature dependency compensator

15. Thermistor Types zRods zChips zFlakes zBeads ya) a bare bead yb) a glass-encapsulated bead yc) an assembly with extension wires

16. Thermistors zNon-linear zSmall Range:-100 to +300 o C zSensitivity High coefficient of resistivity: 4 to 6% per o C or greater!

17. Thermistor Example: Our Lab#6 zResistance = 34.17 k  at 22 o C zChange Temperature by 1 to 23 o C zResistance is now 32.71K a difference of y1.46 k  or 1460  yChange of 4.2% from the original reading zDoes this sound readable?

18. Thermistors zConversion Equations: yT o is typically 25 o C  R o ranges from 50  to 2 M  The Steinhart-Hart equation:

19. Measuring the Resistance Voltage Divider Circuit Simple to build Find the output voltage given the temperature: xSolve for the voltage divider for R T xLook up the temperature for a given resistance VsVs R1R1 RTRT V out

20. Basic Thermistors zVoltage Divider Circuit zThe range: VsVs R1R1 RTRT V out

21. Thermistor’s Linearized zLinearizing with more hardware: T2T2 R2R2 V out + - VsVs R1R1 T1T1 zYou will do this this week in lab!