Midterm Exam. on your way out. Sign - in. Pick up the TAKEHOME Midterm. Complete by Wednesday 8:30am. Turn in beginning of class Wednesday

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1 Midterm Exam Sign - in Pick up the TAKEHOME Midterm on your way out Complete by Wednesday 8:30am Turn in beginning of class Wednesday

2 Journals Due next Monday NEXT Week At start of class!

3 Process Variables

4 Process Variables Temperature Strain Level Flow Pressure

5 Process Variables Weight Viscosity Vibration ph Conductivity

6 Process Variables Density Distance Position Altitude Acceleration

7 Process Variables Colour Radiation Force Humidity Specific Gases

8 Process Variables The number one variable that is measured and controlled in the manufacturing environment

9 Temperature Processes, as well as humans, are sensitive to changes in temperature.

10 Temperature Before we start, SAFETY FIRST

11 Temperature SAFETY

12 Temperature SAFETY

13 Temperature SAFETY

14 Temperature SAFETY Due to the possibility of explosions use the safety barrier circuitry.

15 Temperature The four most common devices to measure temperature are: 1. Thermocouple 2. Resistance Thermal Device 3. Thermistor 4. Infrared Pyrometer / IC Sensors

16 What is a thermocouple? It consists of two dissimilar metals, joined together at one end, which produce a small unique voltage at a given temperature.

17 Thermocouples are available in different combinations of metals or calibrations.

18 Each calibration has a different temperature range and environment, although the maximum temperature varies with the diameter of the wire used in the thermocouple.

19 Choosing a thermocouple type Because thermocouples measure in wide temperature ranges and can be relatively rugged, they are very often used in industry. The following criteria are used in selecting a thermocouple:

20 Choosing a thermocouple type Temperature range Chemical resistance of the thermocouple or sheath material Abrasion and vibration resistance

21 Choosing a thermocouple type Installation requirements (may need to be compatible with existing equipment; existing holes may determine probe diameter).

22 Temperature Thermocouples Basic Principle of the Thermocouple Seebeck Effect When two wires composed of dissimilar metals are joined at both ends and one of the ends are heated, there is a continuous current which flows in the thermoelectric circuit.

23 Temperature Thermocouples Metal A Metal C Metal B

24 Temperature Thermocouples If the circuit is broken at the center, the net open circuit voltage (Seebeck voltage) is a function of the junction temperature and the composition of the two metals. All dissimilar metals exhibit this effect.

25 Temperature Thermocouples Metal A E AB = Seebeck voltage Metal B eab=sc * T

26 Temperature Thermocouples Cu Fe Cu C Voltmeter Reference Block

27 Temperature Thermocouples Why use them? (benefits) CHEAP!

28 Temperature Thermocouples Very Wide Temperature Use -450ºF +4200ºF

29 Temperature Thermocouples Interchangeable to some degree Medium to fast response time Fair linearity Tip sensitive Small to large packaging

30 Temperature Thermocouples Why not to use them? (problems)

31 Temperature Thermocouples Long term stability poor to fair Accuracy medium Repeatability poor to fair Sensitivity - low

32 Temperature Thermocouples The most common combinations are: E, J, K R, S, T

33 Temperature Thermocouples Type E Nickel Chromium vs. Constantan -100 deg. C to 1000 deg. C +/- 0.5 deg. C

34 Temperature Thermocouples Type J Iron vs. Constantan 0 deg. C to 760 deg. C +/- 0.1 deg. C

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36 Temperature Thermocouples Type K Nickel Chromium vs. Nickel Aluminum-Silicon 0 deg. C to 1370 deg. C +/- 0.7 deg. C

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38 Temperature Thermocouples Type R Platinum Rhodium vs. Platinum 0 deg. C to 1000 deg. C +/- 0.5 deg. C

39 Temperature Thermocouples Type S Platinum Rhodium vs. Platinum 0 deg. C to 1750 deg. C +/- 1 deg. C

40 Temperature Thermocouples Type T Copper vs. Constantan -160 deg. C to 400 deg. C +/- 0.5 deg. C

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42 Temperature Curves

43 Junction Types Grounded The thermocouple wires are physically attached to the inside of the probe wall. This results in good heat transfer from the outside, through the probe wall to the thermocouple junction.

44 Junction Types Ungrounded The thermocouple junction is detached from the probe wall. Response time is slowed down from the grounded style, but the ungrounded offers electrical isolation of 1.5 M1/2 at 500 Vdc in all diameters.

45 Junction Types Exposed This type offers the best response time, but is limited in use to noncorrosive and nonpressurized applications

46 Material Maximum Temperature Application Atmosphere 304 SS 900E C (1650E F) Inconel E C (2100E F) Oxidizing Hydrogen Vacuum Very Good Very Good Good Good Very Good Very Good Inert Very Good Very Good

47 Polarity In the thermocouple industry, standard practice is to color the negative lead red. The negative lead of bare wire thermocouple is approximately 1/4" shorter than the positive lead, and the large pin on a thermocouple connector is always the negative conductor.

48 Extension Wire Thermocouple alloy wire must always be used to connect a thermocouple sensor to the instrumentation to assure accurate measurements.

49 RTD s Resistive temperature devices capitalize on the fact that the electrical resistance of a material changes as its temperature changes

50 RTD s A basic physical property of a metal is that its electrical resistivity changes with temperature. All RTD's are based on this principle.

51 RTDs rely on resistance change in a metal, with the resistance rising more or less linearly with temperature

52 The heart of the RTD is the resistance element. Two varieties are: semi-supported wire-wound fully supported bifilar wound glass thin film

53 A typical RTD consists of a fine platinum wire wrapped around a mandrel and covered with a protective coating. Usually, the mandrel and coating are glass or ceramic.

54 The mean slope of the resistance vs temperature plot for the RTD is often referred to as the alpha value

55 Alpha standing for the temperature coefficient. The slope of the curve for a given sensor depends somewhat on purity of the platinum in it.

56 The most commonly used standard slope, pertaining to platinum of a particular purity and composition, has a value of (assuming that the resistance is measured in ohms and the temperature in degrees Celsius).

57 A resistance vs temperature curve drawn with this slope is a so-called European curve, because RTDs of this composition were first used extensively on that continent.

58 Complicating the picture, there is also another standard slope, pertaining to a slightly different platinum composition. Having a slightly higher alpha value of , it follows what is known as the American curve.

59 If the alpha value for a given RTD is not specified, it is usually However, it is prudent to make sure of this, especially if the temperatures to be measured are high.

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61 Advantages 1. RTDs operate from about -250 to 850 C 2. Most stable over time 3. Most Accurate 4. Most repeatable teperature measurement 5. Very resistant to contamination and corrosion

62 Disadvantages 1. High cost 2. Slowest response time 3. Low sensitivity 4. Sensitive to vibration 5. De-calibrates if used outside of range 6. Somewhat fragile

63 THERMISTORS Thermistors are based on resistance change in a ceramic semiconductor; the resistance drops nonlinearly with temperature rise.

64 The resistance-temperature relationship of a thermistor is negative and highly nonlinear

65 The difficulty can be eased by using thermistors in matched pairs, in such a way that the nonlinearities offset each other

66 There are vendors that offer panel meters and controllers that compensate internally for thermistors' lack of linearity

67 Thermistors are usually designated in accordance with their resistance at 25 C.

68 The most common of these ratings is 2252 ohms; among the others are 5,000 and 10,000 ohms

69 If not specified to the contrary, most instruments will accept the 2252 type of thermistor.

70 1. Thermistors have a more restrictive span, being commonly used between -40 and 150 C 2. Fragile 3. Some internal accuracy drift 4. Decalibration if used outside range 5. Lack of standards for replacement

71 RTD and Thermistor Problem Thermistors and RTDs share a very important limitation. They are resistive devices, and accordingly they function by passing a current through a sensor. Even though only a very small current is generally employed, it creates a certain amount of heat and thus can throw off the temperature reading.

72 Temperature Sensor Attributes Criteria Thermocouple RTD Thermistor Cost-OEM Quality Low High Low Temperature Range Selection, selection, selection Very wide -450ººF +4200ººF Wide -400ººF +1200ººF Shot to medium -100ººF +500ººF Interchangeability Good Excellent Poor to fair Long-term Stability Poor to fair Good Poor Accuracy Medium High Medium Repeatability Poor to fair Excellent Fair to good Sensitivity (output) Low Medium Very high Response Medium to fast Medium Medium to fast Linearity Fair Good Poor Self Heating No Very low to low High Point (end) Sensitive Excellent Fair Good Size/Packaging Small to large Medium to small Small to medium

73 INFRARED An infrared thermometer measures temperature by detecting the infrared energy emitted by all materials which are at temperatures above absolute zero, (0 Kelvin).

74 In many process situations, the energy is in the infrared region. As the temperature goes up, the amount of infrared radiation and its average frequency go up.

75 Different materials radiate at different levels of efficiency. This efficiency is quantified as emissivity, a decimal number or percentage ranging between 0 and 1 or 0% and 100%.

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77 To function properly, an infrared measurement device must take into account the emissivity of the surface being measured. This can often be looked up in a reference table.

78 An infrared device is like a camera, and thus covers a certain field of view. It might, for instance, be able to see a 1-deg visual cone or a 100-deg cone. When measuring a surface, be sure that the surface completely fills the field of view

79 If the target surface does not at first fill the field of view, move closer, or use an instrument with a more narrow field of view.

80 Infrared sensors, though relatively expensive, are appropriate when the temperatures are extremely high. They are available for up to 3,000 C (5,400 F), far exceeding the range of thermocouples or other contact devices.

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82 Benefits 1. No contact with the product 2. Response time as fast if not faster than thermocouples 3. No corrosion or oxidation 4. Good stability 5. High repeatability

83 Disadvantages 1. HIGH initial cost 2. Complex 3. Variations in emissivity affects readings 4. Field of view and spot size important 5. Measurements affected by dust, smoke, background radiation ect.

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