Lecture 3. In The Name of Allah. Instrumentation. Dr. Ali Karimpour Associate Professor Ferdowsi University of Mashhad

Transcription

1 In The Name of Allah Instrumentation Dr. Ali Karimpour Associate Professor Ferdowsi University of Mashhad

2 Temperature measurement Topics to be covered include: v v v v v v v v Resistive Thermometers Thermistor Thermometers Thermocouple Thermometers Semiconductor Junction Thermometers Bimaterials Thermometers (Bimetal) Pyroelectric(Infrared) Thermometers Fiber-Optic Thermometers. 2

3 Resistive Thermometers v PTC Positive Temperature Coefficient. v NTC Negative Temperature Coefficient. 3

4 Resistive Thermometers Sensitivity is defined as the amount of resistance change of the sensor per degree of temperature change. Nickel is very good for sensitivity. However, have a limited temperature range, Copper has a very linear resistance-totemperature relationship. However, copper oxidizes at moderate temperatures and cannot be used above 150 C. 4

5 Resistive Thermometers Platinum is the best metal for RTD elements. It follows a very linear resistanceto-temperature relationship. It follows its resistance-totemperature relationship in a highly repeatable manner over its temperature range. It has the widest temperature range among the metals used to make RTDs. Platinum is not the most sensitive metal; however, it is the metal that offers the best long-term stability. 5

6 Resistive Thermometers Wire-wound RTD. Thin film RTD. 6

7 Resistive Thermometers Industrial Platinum Resistance Thermometers (IPRTs) are designed to withstand industrial environments and are almost as durable as thermocouples. 7

8 8 Linearity (least square method) b au y i i T i i i b a u y 1 N t T i i N N y N Z V ), ( N i T i y i N N i i i N i T i i N y ˆ b a u i i, 1 Calibration b a u u u y y y N N A Y Y A A A T T N ˆ 1 Resistive Thermometers

9 Resistive Thermometers Calibration R 2 3 R 1 ( T T ) ( T T ) ( T T )

10 Resistive Thermometers Calibration Exercise 3-1: Resistance of an RTD at 60 C is 106Ω. The resistance Variation with time is: a) Derive a first order model for system and find FSO%(Suppose is full scale). b) Derive a second order model for system and find FSO %(Suppose is full scale). 10

11 Resistive Thermometers PTC100. It is often from platinum. Its resistance at 0 C is 100 Ω. Its sensitivity is Example 3-1: What is the resistive of PTC100 at 300 C? Ω. Platinum RTD usually used in -200 C800 C Nickel RTD usually used in -100 C260 C 11

12 Thermistor Thermometers A thermistor is a thermally sensitive resistor whose primary function is to exhibit a change in electric resistance with a change in body temperature. Unlike a wire-wound or metal film resistance temperature detector (RTD), a thermistor is a ceramic semiconductor. Thermistors have a large temperature coefficient compare to RTD but it is nonlinear. Thermistors usually used in -90 C130 C 12

13 Thermistor Thermometers Calibration R R RTD 1 ( T T ) dr dt R 0 1 dr R dt 0 1 Its sensitivity is Calibration R dr RdT R e 1 ( T 1 T 0 Thermistor ) dr dt T T T: Degrees Kelvin 0 Its sensitivity is R 13

14 Thermistor Thermometers Calibration Calibration curve of Thermistor by Steinhart-Hart equation: 1 T A Bln R C(ln R) 3 T: Degrees Kelvin 14

15 Thermistor Thermometers Calibration Exercise 3-2: TR91 Series C Thermistor calibration table is: a) Derive β for TR91 and find maximum error in ohms. Hint :derive ln of R R e ( ) T T 0 and use LSM. b) Derive A, B and C for Steinhart-Hart equation of TR91 and find maximum error in ohms. Hint :use LSM. 15

16 RTD and Thermistor Thermometers Suitable circuits to convert R V Voltage divider. Voltage divider with follower. V OUT R SEN R1 R 1 V DD Advantages: Simplicity Detection of open sensor (failure) Disadvantages: Poor noise rejection Voltage is non-linear function of resistance 16

17 RTD and Thermistor Thermometers Suitable circuits to convert R V Voltage divider by current source. Two wire Four wire V R OUT SEN I SEN Advantages: Linearity of resistance to voltage conversion. Robust in noise. Disadvantages: 17 Cost.

18 RTD and Thermistor Thermometers Suitable circuits to convert R V Wheatstone bridge Two wire U O R 1 R 2 R 2 R 3 R3 R RTD U s Two wire Three wire 18

19 RTD and Thermistor Thermometers RTDs and Thermistors self heating The ability of the sensor to dissipate heat is defined by its dissipation factor, δ, which has units of mw/ C. The definition for δ is the amount of power that it takes to the raise the body temperature of the sensing element 1 C. Example 3-2: An RTD with R=500Ω at 20 C and sensitivity coefficient / C with dissipation factor δ=30 mw / C let R 1 =R 2 =500 Ω. Derive R 3 for balance of bridge in 0 C. U s =10V R 3 =450 Ω. In the case of no dissipation. R 3 =454 Ω. By considering of dissipation. 19

20 Thermocouple Thermometers The Seebeck effect is the conversion of temperature differences directly into electricity (Thomas Johann Seebeck 1821) For example, the Seebeck coefficients of Cu is 1.5 μv/k, and of Al 1.5 μv/k. 20

21 Thermocouple Thermometers The Peltier effect is the presence of heating or cooling at an electrified junction of two different conductors(jean Charles Athanase Peltier 1834.) 21

22 Thermocouple Thermometers The Seebeck effect: How to use? 22

23 Thermocouple Thermometers 23

24 Thermocouple Thermometers Seebeck Coefficients for Some Metals and Alloys, Compared to Platinum. 24

25 Thermocouple Thermometers Properties of several different thermocouple types 25

26 Thermocouple Thermometers Two additional thermocouple: External ice-point reference junction: Thermocouple refrigeration method: 26

27 Thermocouple Thermometers Using two ovens to simulate ice-point reference: Using a thermometer to sense the temperature: 27

28 Thermocouple Thermometers Scanning many thermocouples. 28

29 Thermocouple Thermometers Electronic ice-point compensation. 29

30 Thermocouple Thermometers Resistive, thermistor and thermocouple thermometer The sensitivity of RTD is very small. The thermistor s output is quite nonlinear. The RTD and thermistor s output is... The thermocouple output is a very small signal that is nonlinear with temperature. 30

31 Semiconductor Junction Thermometers A silicon temperature sensor 730 mv at 40 C and 300 mv at 150 C The narrowly specified V be ranges between 580 mv -620 mv at 25 C. 31

32 Semiconductor Junction Thermometers Linearity error (in mv) vs. temperature for a silicon temperature sensor 32

33 Semiconductor Junction Thermometers LM335 provides 10 mv/ K (A zener diode) Or LM335 provides mv/ C. Output: +10 mv/ C. LM34 provides 10 mv/ F AD592 provides ma / K 33

34 Semiconductor Junction Thermometers LM34 provides 10 mv/ F 5 F 50 mv 300 F 3.00 V -5 F?? -5 F -50mV 300 F 3.00 V Sensors are not near to source?? 34

35 Semiconductor Junction Thermometers Sensors are not near to source?? We use two wires not three! We transmit current instead of voltage! I Q is 70 μa 35

36 Semiconductor Junction Thermometers 36

37 Most available thermometers 37

38 Bimaterials Thermometers (Bimetal) Bimetallic thermometers make use of change of volume with temperature. 38

39 Pyroelectric(Infrared) Thermometers An infrared (IR) thermometer is a noncontact radiant energy detector. IR thermometers capture the invisible infrared energy that is naturally emitted from all objects warmer than absolute zero (0 K). IR thermometers are ideal for moving targets such as glass, plastic, and steel. They do not interfere with the process. The TC or RTD will not respond to these temperature changes quickly enough to permit close control. The IR thermometer will instantly measure the actual product temperature, not the environment surrounding the product. 39

40 Pyroelectric(Infrared) Thermometers 40

41 Pyroelectric(Infrared) Thermometers Distance does not affect the measurement. Models are available that can measure from 0.3 to 91 m. However, the farther away from the target the sensor is, the larger the spot. 41

42 Pyroelectric(Infrared) Thermometers The infrared thermometer must be carefully aimed to avoid focusing on reflected solar radiation. 42

43 Fiber-Optic Thermometers 43

44 Photon Thermometers 44

45 Some more thermometers Temperature sensing by expansion Temperature sensing by displacement Temperature sensing by pressure. 45

46 System Design Two-point control: (also called on off control) is the simplest type of closed-loop control strategy. Inverting Amplifier V error R3 R3 VPV ( VSP ) V R R 2 2 SP V PV 46

47 System Design Hysteresis circuit 47

48 System Design Exercise 3-3: Consider following TR91 Series C Thermistor calibration table. Design a circuit that turn on the fan of a car at 95 C and turn it off at 85 C. 48

49 References v Industrial Control Electronics J.M. Jacob, Prentice-Hall, 1989 مبانی اندازه گیری در سیستمهای ابزار دقیق حمید رضا تقی راد و سید v علی سالمتی انتشارات دانشگاه صنعتی خواجه نصیرالدین طوسی 1392 v Principles and Practice of Automatic Process Control, Third edition, C. A. Smith, A. Corripio, Wiley 2006 v Instrument Control Engineers' Handbook Fourth Edition, Béla G. Lipták, editor-in-chief. CRC Press

50 Some Useful websites for the course v v v 50