2. Thermocouples (continued) Introduction to Temperature Sensors ISAT 300 Instrumentation and Measurement 02/21/2000

3. Language of Thermocouples zTemperature Range y-270 o C to 2320 o C depends on the type of the thermocouple zPassive yDose 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

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

5. Characteristics of Standard Thermocouples

6. 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.

7. 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.

8. Example: A type K thermocouple with an ice reference gives a voltage reading of 0.900 mV. What is its temperature? Thermoelectric voltage in mV 0.040 mV 0.900 0.021 mV

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. The Importance of 0 o C Reference Junction

11. 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

12. 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!

13. Thermoelectric voltage in mV 2.415 = 2.244 + 0.171 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!

14. First Order System zA change at the input is NOT seen immediately at the output. zExample: Move a temperature probe from cold water to hot water. Cool Bath Hot Bath

15. A Differential Equation zAssumed that the water temperature of the hot bath (T w ) does not cool down much (non-intrusive), and can be treated as a constant. zThen this equation simply just a first-order differential equation, a time dependent temperature equation of T p (t), with an initial condition of T p (0)=T po.

16. Solution for This Differential Equation Here, C is the heat capacity, R is the thermal resistance of the temperature probe, they are the probe material properties should be given or known to us.

17. The Time Constant, 