Modern Instrumentation PHYS 533/CHEM 620

1. Modern InstrumentationPHYS 533/CHEM 620 Lecture 9 Temperature & Light Sensors Amin Jazaeri Fall 2007

2. What are Sensors? • American National Standards Institute (ANSI) Definition • A device which provides a usable output in response to a specified measurand • A sensor acquires a physical parameter and converts it into a signal suitable for processing (e.g. optical, electrical, mechanical) • A transducer • Microphone, Loud Speaker, Biological Senses (e.g. touch, sight,…ect)

3. Detectable Phenomenon

4. Physical Principles • Amperes’s Law • A current carrying conductor in a magnetic field experiences a force (e.g. galvanometer) • Curie-Weiss Law • There is a transition temperature at which ferromagnetic materials exhibit paramagnetic behavior • Faraday’s Law of Induction • A coil resist a change in magnetic field by generating an opposing voltage/current (e.g. transformer) • Photoconductive Effect • When light strikes certain semiconductor materials, the resistance of the material decreases (e.g. photoresistor)

5. Need for Sensors • Sensors are omnipresent. They are embedded in our bodies, automobiles, airplanes, cellular telephones, radios, chemical plants, industrial plants and countless other applications. • Without the use of sensors, there would be no automation !!

6. Choosing a Sensor

7. Temperature Sensor • Temperature sensors appear in building, chemical process plants, engines, appliances, computers, and many other devices that require temperature monitoring • Many physical phenomena depend on temperature, so we can often measure temperature indirectly by measuring pressure, volume, electrical resistance, and strain

8. Types of temperature sensors • RTD (Resistance Temperature Detector) • Thermistor • Thermocouple

9. RTDs (Resistance Temperature Detectors) • Resistivity of metals is a function of temperature. • Platinum often used since it can be used for a wide temperature range and has excellent stability. Nickel or nickel alloys are used as well, but they aren’t as accurate. • In several common configurations, the platinum wire is exposed directly to air (called a bird-cage element), wound around a bobbin and then sealed in molten glass, or threaded through a ceramic cylinder. • Metal film RTDs are new. To make these, a platinum or metal-glass slurry film is deposited onto a ceramic substrate. The substrate is then etched with a laser. These RTDs are very small but aren’t as stable (and hence accurate). • RTDs are more accurate but also larger and more expensive than thermocouples.

10. RTD • How it works: • Utilizes the fact that resistance of a metal changes with temperature. • Make up: • Traditionally made up of platinum, nickel, iron or copper wound around an insulator. • Temperature range: • From about -196°C to 482°C. Thin Film RTD

11. RTD • Resistance temperature device.

12. RTD

13. RTD

14. Advantages: Stable Very accurate Change in resistance is linear Disadvantages: Expensive Current source required Small change in resistance Self heating Less rugged than thermocouples. RTD Advantages and Disadvantages

15. RTD Advantages • Accuracy • At 212 F, a standard RTD is +- 1.22 F • The high accuracy RTD is +- 0.36 F • At 212 F, a Type J or K T/C is +- 4 F • The high accuracy T/C is +- 2 F • Uses standard copper conductor for lead wires • The resistance signal is not susceptible to electrical noise, less need for a transmitter

16. Resistance Measurement • Several different bridge circuits are used to determine the resistance. Bridge circuits help improve the accuracy of the measurements significantly. Bridge output voltage is a function of the RTD resistance.

17. Resistance/Temperature Conversion • Published equations relating bridge voltage to temperature can be used. • For very accurate results, do your own calibration. • Several electronic calibrators are available. • The most accurate calibration that you can do easily yourself is to use a constant temperature bath and NIST-traceable thermometers. You then can make your own calibration curve correlating temperature and voltage.

18. Accuracy and Response Time • Response time is longer than thermocouples; for a ¼ sheath, response time can easily be 10 s.

19. Potential Problems • RTDs are more fragile than thermocouples. • An external current must be supplied to the RTD. This current can heat the RTD, altering the results. For situations with high heat transfer coefficients, this error is small since the heat is dissipated to air. For small diameter thermocouples and still air this error is the largest. Use the largest RTD possible and smallest external current possible to minimize this error. • Be careful about the way you set up your measurement device. Attaching it can change the voltage. • When the platinum is connected to copper connectors, a voltage difference will occur (as in thermocouples). This voltage must be subtracted off.

20. Thermistor, the basics of • How it works: • Like the RTD a thermistor uses the fact that resistance of a metal changes with temperature. • Make up: • Generally made up of semiconductor materials • Temperature Range: • About -45°C - 150°C Thermistor

21. Thermistors • Thermistors also measure the change in resistance with temperature. • Thermistors are very sensitive (up to 100 times more than RTDs and 1000 times more than thermocouples) and can detect very small changes in temperature. They are also very fast. • Due to their speed, they are used for precision temperature control and any time very small temperature differences must be detected. • They are made of ceramic semiconductor material (metal oxides). • The change in thermistor resistance with temperature is very non-linear.

22. Thermistor

23. Thermistor

24. Advantages: Very sensitive (has the largest output change from input temperature) Quick response More accurate than RTD and Thermocouples Disadvantages: Output is a non-linear function Limited temperature range. Require a current source Self heating Fragile Thermistor Advantages and Disadvantages

25. Thermocouple, some more basics • How it works: • Made up of two different metals joined at one end to produce a small voltage at a given temperature. • Make up: • Made of up two different metals. Ex: A type J is made up of Iron and Constantan. • Temperature Range • Type J: 0°C to 750°C A few Thermocouples

26. Thermocouples • Seebeck effect • If two wires of dissimilar metals are joined at both ends and one end is heated, current will flow. • If the circuit is broken, there will be an open circuit voltage across the wires. • Voltage is a function of temperature and metal types. • For small DT’s, the relationship with temperature is linear • For larger DT’s, non-linearities may occur.

27. Measuring the Thermocouple Voltage • If you attach the thermocouple directly to a voltmeter, you will have problems. • You have just created another junction! Your displayed voltage will be proportional to the difference between J1 and J2 (and hence T1 and T2). Note that this is “Type T” thermocouple.

28. External Reference Junction • A solution is to put J2 in an ice-bath; then you know T2, and your output voltage will be proportional to T1-T2.

29. Other types of thermocouples • Many thermocouples don’t have one copper wire. Shown below is a “Type J” thermocouple. • If the two terminals aren’t at the same temperature, this also creates an error.

30. Isothermal Block • The block is an electrical insulator but good heat conductor. This way the voltages for J3 and J4 cancel out. Thermocouple data acquisition set-ups include these isothermal blocks. • If we eliminate the ice-bath, then the isothermal block temperature is our reference temperature

31. Time Constant vs. Wire Diameter

32. Time Constant vs. Wire Diameter, cont.

33. Thermocouple Types If you do your own calibration, you can usually improve on the listed uncertainties.

34. Thermocouple Types, cont. • Type B – very poor below 50ºC; reference junction temperature not important since voltage output is about the same from 0 to 42 ºC • Type E – good for low temperatures since dV/dT (a) is high for low temperatures • Type J – cheap because one wire is iron; high sensitivity but also high uncertainty (iron impurities cause inaccuracy) • Type T – good accuracy but low max temperature (400 ºC); one lead is copper, making connections easier; watch for heat being conducted along the copper wire, changing your surface temp • Type K – popular type since it has decent accuracy and a wide temperature range; some instability (drift) over time • Type N – most stable over time when exposed to elevated temperatures for long periods

35. Potential Problems, cont. • Shunt impedence • As temperature goes up, the resistance of many insulation types goes down. At high enough temperatures, this creates a “virtual junction”. This is especially problematic for small diameter wires. • Galvanic Action • The dyes in some insulations form an electrolyte in the water. This creates a galvanic action with a resulting emf potentially many times that of the thermocouple. Use an appropriate shield for a wet environment. “T Type” thermocouples have less of a problem with this.

36. Potential Problems, cont. • Thermal shunting • It takes energy to heat the thermocouple, which results in a small decrease in the surroundings’ temperature. For tiny spaces, this may be a problem. • Use small wire (with a small thermal mass) to help alleviate this problem. Small-diameter wire is more susceptible to decalibration and shunt impedence problems. Extension wire helps alleviate this problem. Have short leads on the thermocouple, and connect them to the same type of extension wire which is larger. Extension wire has a smaller temperature range than normal wire. • Noise • Several types of circuit set-ups help reduce line-related noise. You can set your data acquisition system up with a filter, too. • Small-diameter wires have more of a problem with noise.

37. Thermocouples

38. Thermocouples

39. Thermocouples

40. Thermistor Non-Linearity

41. Temperature-Sensetive Diodes • Forward Bias Current • Reversed Bias Current

42. Temperature-Sensetive Diodes

43. Infrared Thermometry • Infrared thermometers measure the amount of radiation emitted by an object. • Peak magnitude is often in the infrared region. • Surface emissivity must be known. This can add a lot of error. • Reflection from other objects can introduce error as well. • Surface whose temp you’re measuring must fill the field of view of your camera.

44. Benefits of Infrared Thermometry • Can be used for • Moving objects • Non-contact applications where sensors would affect results or be difficult to insert or conditions are hazardous • Large distances • Very high temperatures

45. Field of View • On some infrared thermometers, FOV is adjustable.

46. Emissivity • To back out temperature, surface emissivity must be known. • You can look up emissivities, but it’s not easy to get an accurate number, esp. if surface condition is uncertain (for example, degree of oxidation). • Highly reflective surfaces introduce a lot of error. • Narrow-band spectral filtering results in a more accurate emissivity value.

47. Spectral Effects • Use a filter to eliminate longer-wavelength atmospheric radiation (since your surface will often have a much higher temperature than the atmosphere). • If you know the range of temperatures that you’ll be measuring, you can filter out both smaller and larger wavelength radiation. Filtering out small wavelengths eliminates the effects of flames or other hot spots. • If you’re measuring through glass-type surfaces, make sure that the glass is transparent for the wavelengths you care about. Otherwise the temperature you read will be a sort of average of your desired surface and glass temperatures.

48. Price and Accuracy • Prices range from $500 (for a cheap handheld) to $6000 (for a highly accurate computer-controlled model). • Accuracy is often in the 0.5-1% of full range. Uncertainties of 10°F are common, but at temperatures of several hundred degrees, this is small.

49. Non-Electronic Temperature Gages • Crayons – You can buy crayons with specified melting temperatures. Mark the surface, and when the mark melts, you know the temperature at that time. • Lacquers – Special lacquers are available that change from dull to glossy and transparent at a specified temperature. This is a type of phase change. • Pellets – These change phase like crayons and lacquers but are larger. If the heating time is long, oxidation may obscure crayon marks. Pellets are also used as thermal fuses; they can be placed so that when they melt, they release a circuit breaker. • Temperature sensitive labels – These are nice because you can peel them off when finished and place them in a log book.

50. Thin-Film Heat Flux Gauge • Temperature difference across a narrow gap of known material is measured using a thermopile. • A thermopile is a group of thermocouples combined in series to reduce uncertainty and measure a temperature difference. From Nicholas & White, Traceable Temperatures.