Instrumentation & Data Acquisition Systems

Transcription

1 Instrumentation & Data Acquisition Systems Section 2 - Temperature Robert W. Harrison, PE Bob@TheHarrisonHouse.com Made in USA

2 Measure Temperature of Material in a Vacuum Oven What sensors can be used? What precautions are necessary? 2

3 Methods Thermocouple (TC) Resistance Temperature Detector (RTD) Thermistor Semiconductor (IC) Non Contact (IR) 3

4 Thermocouple Seebeck Principle (Effect / Voltage) In 1852, Thomas Seebeckdiscovered that when two wires of dissimilar metals are joined at both ends a continuous current flow will be produced when the ends are at different temperatures Current Flow Heat Metal 1 Metal 2 Seebeck Effect When the cold junction is opened, a small voltage is generated Heat Metal 1 + V m Metal 2 Seebeck Voltage - 4

5 Thermocouple Seebeck Principle The voltage signal produced is directly proportional to the difference in junction temperatures For small changes in temperature the Seebeck voltage is linearly proportional For larger changes, the temperature is inherently nonlinear; thus, the thermocouple is considered a nonlinear device Heat T P Metal 1 Metal 2 Seebeck Voltage + T Ref V m - V m = (T P -T Ref ) Where, the Seebeck coefficient, is the constant of proportionally 5

6 Thermocouple Thermoelectric Laws Law of Successive Homogenous Circuits An emf can not be generated in a single homogenous wire with the application of heat alone Any current detected must be attributed to nonhomogeneity Law of Intermediate Conductors The sum of emf s in a circuit made up of any number of dissimilar conductors is zero, if the circuit is at a uniform temperature throughout 6

7 Law of Intermediate Temperatures The sum of the emf s generated in any given circuit containing any number of dissimilar homogenous conductors is a function only of the temperature at the junctions + + T 1 T 2 V 1 T 2 T 3 V T 1 T 3 V 3 = V 1 + V 2-7

8 Thermocouple Construction Criteria Material Considerations Pure or alloyed metals Material Properties Ductility Stability Sensitivity Voltage vs. Temperature Relationship Standards 8

9 TC Reference Junction Compensation The generated voltage is proportional to the temperature difference between the process and a reference point All temperatures greater than absolute zero generate a voltage Standard based on Reference Junction (T Ref 0 C (ice point) TC output defined as 0 0 C + T P V m f (T P -T Ref ) T Ref 9

10 TC Reference Junction Compensation The ice bath (0ºC) acts as the reference compensation No external hardware compensation is required Ice bath generates a voltage as noted in the equivalent electrical circuit 10

11 TC Reference Junction Compensation The situation if the configuration has the reference junction at the input terminals of the device reading the temperature In this situation the reference junction must be read to compensate properly + T P V m f (T P -T Ref ) T Ref 11

12 TC Reference Junction Compensation Maintain reference junction at the ice point No Compensation required Maintain reference junction at a constant temperature Compensation required: Constant - temperature dependent Not now used due to required energy to maintain temperaure Measure temperature of reference junction Thermistor - measure junction temperature Isothermal block - maintain uniform temperature Compensation required: Variable - temperature dependent 12

13 Type J TC Reference Junction Example T Ref = 30 C T P = 200 C V 200 C = mV V 30 C = 1.536mV V m = V P -V Ref V m = mV mV = 9.241mV => 172 C (170 C?) V c = V m + V Ref V c = 9.241mV mV = mV => 200 C 13

14 TC Linear Approximation Over short temperature ranges, the thermocouple response can be approximated by a linear relationship The thermocouple response curve is broken into a series of sub sections over short temperature ranges that are approximated by straight lines Only the end points are constrained to be at the save value If the error in a section is unacceptable, break the interval into smaller subsections The response between the end points uses linear interpolation to obtain the desired value 14

15 Type J TC Interpolation Example T l = 115 C (V l = 6.08 mv), T h = 120 C (V h = 6.36 mv) Calculate T m when V m = 6.22 mv T m = T l + (T/ V)*(V m -V l ) T m = T l + ((T h -T l )/(V h -V l ))*(V m -V l ) T m = (( )/( ))*( ) T m = (5/0.28)*0.14 T m = *0.14 T m = T m = C T h T m T l V l V m V h 15

16 TC T P Junction Location The thermocouple temperature is traditionally measured at the end point (T P ) as shown in Fig. 1 The temperature may also be measured at an arbitrary point along the thermocouple, where the metals change as shown in Fig. 2 Metal 1 + Metal 2 T P Metal 1 + T P Fig. 1 Metal 2 T Ref Fig. 2 T Ref 16

17 TC Reading with Lead Reversal The thermocouple is installed in reverse swapping leads (+ to -, and to +) at the junction box This creates additional thermocouples at the points of reversal in each pair of lead wires The original thermocouple will give a decreasing value as the temperature increases and an increasing value as the temperature decreases T 1 + T p Normal T 1 T Ref +T 1 + Reversed -T p +T 1 T Ref 17

18 Sources of TC Errors Reference (Cold) Junction Compensation Extension Wires Noise and Grounding Due to the inherently small voltages, ground loops can be a significant problem Environment Wire Contamination 18

19 ISA Designated Thermocouples Class Type Composition Positive Negative J Iron Constantan Base K Chromel Alumel T Copper Constantan E Chromel Constantan B Platnum - 30% Rhodium Platnum - 6% Rhodium Noble R Platnum - 13% Rhodium Platnum S Platnum - 10% Rhodium Platnum Refractory Tungsten Rhenium 19

20 Thermocouple Characteristics Type Range ( F) Description J -350 to 1,400 Least Expensive, Most Widely Used K -450 to 2,500 Most Linear T -450 to 750 High Resistance to Corrosion from Moisture E -450 to 1,800 Highest emf/dt, Suited for Moise & Sub Zero B 32 to 3,210 Low emf Output R -60 to 3,210 Low emf Output, Fast Response, Industrial Standard S -60 to 3,210 Low emf Output, International & Lab Standard 20

21 Thermocouple Selection Criteria Temperature Range Environment Level of emf Output Linearity Junction Configurations Cost 21

22 TC EMF vs. Temperature 22

23 Thermocouple Junction Types 23

24 Thermocouple Junction Type Comparisons 24

25 2.50 TC Response Time Time (sec) Probe Diameter (in) Exposed Grounded Ungrounded 25

26 Thermowells Inserted into and remain in process vessel Sensor inserted into thermowell Repair or replace sensor leaving process online Introduces additional delay in temperature response time Degrades accuracy 26

27 Thermowell Response units Time (sec) Response 27

28 Color Coding for Thermocouples (The Three Entries are for Overall Insulator Color [First Color], Positive W ire Insulation Color [Second Color], and Negative W ire Insulation Color [Third Color]). Type USA a British b German b Japanese b French b IEC Brown Black Blue Yellow Black Black J W hite Yellow Red Red Yellow Black Red Blue Blue W hite Black W hite Brown Red Green Blue Yellow Green K Yellow Brown Red Red Yellow Green Red Blue Green W hite Purple W hite Brown Brown Black Purple Purple Violet E Violet Brown Red Red Yellow Violet Red Blue Black W hite Black W hite Brown Blue Brown Brown Blue Brown T Blue W hite Red Red Yellow Brown Red Blue Brown W hite Blue W hite Brown Pink N Orange None None None None Pink Red White Green W hite Black Green Orange R or S None W hite Red Red Yellow Orange Blue W hite W hite Green W hite Gray Gray B None None Red Red None None White White Notes: a. Still in use. b. Superseded by IEC color coding. 28

29 Thermocouple Advantages Simple to make and versatile in their application and forms Very durable in most forms and sizes Can be made in very small sizes and have faster response time than a RTD of similar size Relatively accurate when used correctly Wide operating range from about -450 to 3,200 F Self powered and can be read with a simple analog meter circuit Relatively inexpensive for most types: $2 to 50 typical Very suitable for applications requiring a tip sensitive probe Highest resistance to vibration 29

30 Thermocouple Disadvantages Very low output signal easily disturbed by other emf s in the circuit Requires elaborate electronics for signal conditioning Signals in millivolts (mv) with changes in microvolts (V) Requires reference junction or other compensating devices for accurate readings Prone to inaccuracies due to drift, aging, and hysteresis in TC materials Prone to misuse because people underestimate the care required to setup an accurate TC measurement system 30

31 Thermocouple (One Wire) Transmitter A thermocouple transmitter provides signal conditioning and typically converts the voltage signal to a current loop signal (4-20 ma) An alternative method is to transmit the signal in digital format to a field bus interface Signal conditioning normally includes reference junction compensation, signal linearization, and calibration Additional processing may include alarm checking and status reporting These capabilities are commonly referred to as a smart sensor 31

32 Thermocouple Demonstration Embedded device server TC-01.vi Interactive LabVIEW Alarm Example TC-01_custom_alarms.vi 32

33 RTD Definition Resistance Temperature Detectors (RTDs) are based on the principle that the resistance of a material to the passage of electric current is temperature dependent Resistivity is also dependant on purity and deformation Total Temperature + Impurities + Deformation RTDs are manufactured from metals who resistance increaseswith temperature 33

34 RTD Materials & Coefficient of Resistance Material Temperature Coefficient of Resistance (Ohms / Ohm C) Copper Nickel Platinum Tungsten Alpha () = The slope of the resistance vs. temperature curve 34

35 RTD Standards Comparison Standard Alpha (α) Nominal 0 C (T 0 ) IEC DIN (A& B) American ITS

36 Alpha Industrial Standard The 100 platinum RTD is the industrial standard = /C 0C: R 0 = C: R 100 = = (R 100 -R 0 )/(R 0 * T 100 ) = ( )/(100 *100 C) = 38.5 /10,000 C = /(C) Resistance Ratio R/R 0 = 1 + α* T 100 R/R 0 = =

37 RTD Construction Ceramic and glass wire wound 3 wire connection 37

38 RTD Configuration 38

39 Platinum RTD Most Predictable and Repeatable Resistance (R) vs. Temperature (T) Relationship Used as International Standard Pure, Easily Drawable into Fine Wire Can be Contaminated by Gases in a Reducing Atmosphere Can act as a Catalyst when Certain Hydrocarbons are Present 39

40 Nickel RTD Useful Measurement Range is about -100 F to 500 F Highest Resistance Change between 32 F to 212 F Sensitivity Drops Sharply Above 500 F T vs. R Relationship is Quite Nonlinear Very Susceptible to Contamination Sulfur and phosphorous 40

41 Copper RTD Easily Refined and Drawn Very Susceptible to Oxidation Poor Stability and Reproducibility Widely Used due to Cost 41

42 Tungsten RTD High Melting Point allows use above 1,800 F High Tensile Strength 10x platinum Excellent for small wire sizes Corresponds to higher resistance T vs. R Relationship is Not as well Understood as Others 42

43 RTD R vs. T Curves 43

44 RTD Measurement Criteria The most important criteria are Stability excellent in RTDs (about 0.05 C over a 5 year period) Repeatability Secondary importance to the above is absolute accuracy If an RTD in a C environment reads C, the electronics can easily compensate for the error 44

45 RTD Sources of Error Strain Impurities Conduction Induced Errors Thermal EMF Effect Self Heating Leadwire Resistance Primary source of error Compensated for by various leadwire configurations 2, 3 and 4 wire connections to Wheatstone Bridge 45

46 50 Leadwire Resistance Errors Error in F Resistance () Between Leadwires 46

47 RTD Wire Configuration Types 2-Wire RTD Few applications are suitable for 2-wire RTDs Leadwire resistance is part of the measurement and can lead to serious errors Connect directly to receiving device, do not use extension wires Fair temperature measurement 3-Wire RTD Difference in leadwire resistance is part of the measurement Can use extension leadwires Third wire compensates for lead length Wires must have same resistance values Care must be taken to not stress or bend the extension wires Use with caution 47

48 RTD Wire Configuration Types 4-Wire RTD Highest degree of accuracy and reliability The 4 wire circuit cancels out all errors due to Lead length Resistance imbalance between leadwires Can use lighter wire than 2 and 3-wire RTDs Suitable for all applications High accuracy Corrosive atmospheres Remote sensor locations 48

49 Wheatstone Bridge 49

50 Wheatstone Bridge 50

51 Wheatstone Bridge 51

52 Wheatstone Bridge Vout = ((R-1)/2*(R+1))*Vin R2 = R3 = R4 R1 (*R) Vout Vout Wheatstone Bridge R1 (*R) 52

53 Wheatstone Bridge Simplify Bridge Circuit Vm Vn Vm Vn Vm Circuit A Circuit B Since the impedance measuring Vout is very large, the legs of the bridge are independent, so the center measuring portion of the bridge can be removed (Circuit A) This forms two independent circuits, so the circuit can be further simplified (Circuit B) The voltage across R2 is Vm The voltage across R3 is thus Vn -Vm 53

54 RTD Interchangeable vs. Calibrated Accuracy Calibration Procedure Each RTD is calibrated at several points (typically 10) Calibration constants (R 0 and ) are generated Constants are used to generate a resistance vs. temperature table Information on table can be used to increase system accuracy 54

55 Improving Accuracy of Standard RTDs Specify that RTDs be made to a tighter interchangeability to the standard curve Select units with a closer R 0 tolerance DIN (Deutsches Institut für Normung) standard German Institute for Standardization German ISO Member Body IEC-751 matches DIN standard Specify the RTD be Calibrated at Specified Points Generate an R vs. T curve Adjust equipment to match the RTD Specify the RTD be Calibrated at a Specific Point of Interest Tight tolerance (<0.25 C) 55

56 RTD Temperature Measurement RTD Failure Points Wire is small and fragile Many points where wire could break 56

57 Thin Film RTDs Manufactured by depositing a very thin layer of platinum on a ceramic substrate Platinum layer is then etched to provide a resistance circuit Protective coating is deposited Batches of wafers are then sliced into individual sensors The resultant sensor must be protected from mechanical or chemical damage Type of packaging is application dependent 57

58 Thin Film RTD Advantages Higher nominal resistance compared to wire wound Increases sensitivity Reduces lead wire effects Smaller package provides more packaging versatility R vs. T characteristics are highly consistent Conforms to internationally recognized standards Excellent long term resistance stability Lower cost 58

59 Thin Film RTD Limitations Self heating errors can be a problem because of the reduced size of the sensors Contamination can be a problem if not adequately addressed during manufacturer Strain gauge effects can result if substrate and coating materials are not carefully matched to the thermal expansion characteristics of the platinum thin film 59

60 Advantages of RTDs Increased System Accuracy Requires no Reference Junction Increased Sensitivity to Small Temperature Changes RTD Signal Less Susceptible to Noise and Thermoelectric Effects RTDs do Not Require Special Leadwire Platinum Inherently has Less Drift Easy, Accurate Calibration using Decade Box Reproducibility is Not Affected by Temperature Change RTDs Generally do Not Fail a Little Bit 60

61 Disadvantages of RTDs More Expensive than Thermocouples Limited to Below 800 C Less Rugged than Thermocouples, more susceptible to vibration Thermocouples have Faster Response Time Self Heating may be a Problem Supply Excitation Voltage and Bridge 61

62 Thermistors(Thermal Resistors) Temperature sensitive semiconductor (resistor) Functions similar to a thin film RTD Construction Sintered metal oxide in a ceramic matrix Prone to damage by moisture Passivated by glass or epoxy encapsulation Often mounted in a stainless steel sheath Common form is a bead with two wires attached 62

63 Thermistors Properties Much higher resistance than a RTD High resistance yields high sensitivity Read with a bridge circuit as a RTD Does not require 3 or 4 lead connections Standards Only standard is resistance normalized to 25 C (77 F) KΩ 5 KΩ 10 KΩ 63

64 Temp C R/R25 R25=10K K Thermistor Resistance/Temperature K K K K K K K K K K K K K K K K K R/R Deg C 64

65 Thermistors Response Nonlinear resistance vs. temperature curve Accurate approximation according to Steinhart-Hart equation 1/T = A + B*ln(R) + C*(ln(R)) 3 Linear over small ranges First-order approximation R=k*T R = Change in resistance (Ω) T = Change in temperature (K) k = Proportionality coefficient Types defined by k k<0: Negative Temperature Coefficient (NTC), most used for temperature measurement k>0: Positive Temperature Coefficient (PTC), at high temperature reverts back to NTC. Used for circuit protection, motor control k=0: Standard resistors 65

66 Thermistors Advantages Significantly larger resistance vs. temperature change than RTDs High sensitivity measurement More resolution over a given range Higher lead wire resistance than RTDs No need for lead wire compensation use 2-wire bridge configuration Longer and smaller lead wires Small size and mass Wide range of package assemblies React faster to temperature changes than RTDs Generally less expensive than RTDs 66

67 Thermistors Disadvantages Resistance vs. temperature characteristics are highly nonlinear Linear over small ranges Relatively small temperature measuring range Lack of industry standards Problem when replacing sensors Problem complicated if acquiring replacement from source other than original manufacturer 67

68 Response Comparisons 68

69 Sensor Comparisons Parameter Thermocouple RTD Thermistor Accuracy 1-10ºF (L) ºF (H) 0.1-1ºF (M) Stability 1ºF / year (L) <0.1% in 5 years (H) 2ºF / year (M) Sensitivity 5-50V/ºF (L) Ω/ºF (M) 50-5,000Ω/ºF (H) Linearity Moderate High Low Range ,308ºF (H) ,621ºC ,562ºF (M) ºC ºF (L) ºC Ruggedness High Low Low Overall High Temperatures Best Ruggedness Highest Accuracy Best Stability Best Sensitivity 69

70 Integrated Circuit (IC) Sensors Uses variable resistance properties of semiconductor materials Provides a linear voltage or current output, especially at low temperatures Provides a digital temperature signal Eliminates the need for an A/D converter Since IC sensors can have memory, they can be very accurately calibrated May operate in multisensor environments Applications such as communications networks Most IC sensors provide an output value that is linear and proportional with temperature These devices are usually supplied with a standard accuracy assigned Include the ability to be calibrated at a specific temperature 70

71 IC Sensor Characteristics Sensors are based on a silicon semiconductor The electrical resistance of silicon also changes with temperature Useful temperature limits are typically on the order of 55 C to 150 C (-65 F to 300 F) Can be assembled into immersion packages similar to those used in thermistors or RTDs Can also be PC board or surface mounted May operate in multisensor environments Applications such as communications networks 71

72 IC Sensor Advantages Within their effective temperature range, IC sensors provide good linearity at low cost Output values that are proportional with temperature Do not require the additional signal conditioning necessary for RTDs, thermocouples, and thermistors Many IC sensors also offer communication protocols for use with bustype data acquisition systems Some have addressability and data storage and retrieval capabilities 72

73 IC Sensor Disadvantages The major drawback of IC sensors is their limited temperature range Sensors are typically larger than RTDs and thermistors Require larger package sizes for immersion-type measurements 73

74 Temperature Transmitters Handles thermocouples and RTDs Reference junction compensation in transmitter Scaleable 4-20 ma is stable and accurate Input may be linearized 2-wire loop powered Stops ground loops with signal isolation up to 1,500Vrms Typical sample cycle 8/sec Sensor diagnostics detects sensor problems and provides error message 74

75 InfraRed Noncontact Measurements Wide Temperature Range and Applications Can Emulate Thermocouple or Other Temperature Sensors Infrared Temperature Measurement Theory Application 75

76 InfraRed Field of View 8:1 Meter 16 inches from target, spot size at least 2 inches Measurements normally made less that 2 feet from target From further distances Measurement may be affected by external light sources Spot size may be so large that it encompasses surfaces outside the area to be measured Spot Diameter (in) Spot Diameter Distance to Target (in) 76

77 Measure Temperature of Material in a Vacuum Oven Thermocouple RTD Thermistor IC IR 77