Agilent Technologies Classroom Series Practical Temperature Measurements 001 Agilent Technologies Classroom Series Agenda Background, history Mechanical sensors Electrical sensors Optical Pyrometer RTD Thermistor, IC Thermocouple Summary & Examples A1 Agilent Technologies Classroom Series What is Temperature? A scalar quantity that determines the direction of heat flow between two bodies A statistical measurement A difficult measurement A mostly empirical measurement 002 Agilent Technologies Classroom Series How is heat transferred? Conduction Metal coffee cup Convection Radiation 003 Agilent Technologies Classroom Series The Dewar Glass is a poor conductor Gap reduces conduction Metallization reflects radiation Vacuum reduces convection 004 Agilent Technologies Classroom Series Thermal Mass Don't let the measuring Senso r device change the temperature of what you're measuring. Response time = f{Thermal mass} f{Measuring device} Senso r 005 Agilent Technologies Classroom Series Temperature errors What is YOUR normal temperature? Thermometer accuracy, resolution Contact time Thermal mass of thermometer, tongue Human error in reading 97.6 98.6 99.6 36.5 37 37.5 006 Agilent Technologies Classroom Series History of temperature sensors 1600 ad 1700 12 ad 96 Fahrenheit 1 Instrument 0 Galileo: First temp. sensor pressure- sensitive not repeatable Early thermometers Not repeatable No good way to calibrate Maker 12*8=96 points Hg: Repeatable One standard scale 007 Agilent Technologies Classroom Series The 1700's: Standardization 1700 ad 1800 ad 0 100 Thomson effect Absolute zero 100 Celsius: Common, repeatable calibration reference points 0 "Centigrade" scale 008 Agilent Technologies Classroom Series 1821: It was a very good year 1800 ad 1900 ad The Seebeck effect Davy: The RTD Pt 100 d @ O deg.C 009 Agilent Technologies Classroom Series The 1900's: Electronic sensors 1900 ad Thermistor 2000 ad 1 uA/K IC sensor IPTS 1968 IPTS 1990 "Degree Kelvin">> "kelvins" "Centigrade">> " Celsius" 010 Agilent Technologies Classroom Series Temperature scales Freezing point H2O Absolute zero Celsius 0 100 Kelvin 273.15 373.15 32 212 -273.15 0 Fahrenheit -459.67 0 Boiling point H O2 Rankine 427.67 671.67 "Standard" is "better": Reliable reference points Easy to understand 011 Agilent Technologies Classroom Series IPTS '90: More calibration points – – 273.16: TP H2O 234.3156: TP Hg Large gap – – – – – – – – – – – – 83.8058: TP Ar 54.3584: TP O2 24.5561: TP 20.3: BP Ne 17 Liq/vapor H2 13.81 TP H2 3 to 5: Vapor H2 He – – 1357.77: FP Cu 1337.33: FP Au 1234.93: FP Ag 933.473: FP Al 692.677: FP Zn 505.078: FP Sn 429.7485: FP In – 302.9146: MP Ga 012 Agilent Technologies Classroom Series Agenda Background, history Mechanical sensors Electrical sensors Optical Pyrometer RTD Thermistor, IC Thermocouple Summary & Examples A2 Agilent Technologies Classroom Series Bimetal thermometer Two dissimilar Forces due to thermal expansion metals, tightly bonded 0 Result 100 200 300 Bimetallic thermometer 400 Poor accuracy Hysteresis Thermal expansion causes big problems in other designs: IC bonds Mechanical interference 013 Agilent Technologies Classroom Series Liquid thermometer; Paints 100 0 Thermally-sensitive paints Liquid-filled thermometer Irreversible change Low resolution Useful in hard-to-measure area Accurate over a small range Accuracy & resolution= f(length) Range limited by liquid Fragile Large thermal mass Slow 014 Agilent Technologies Classroom Series Agenda Background, history Mechanical sensors Electrical sensors Optical Pyrometer RTD Thermistor, IC Thermocouple Summary & Examples A3 Agilent Technologies Classroom Series Optical Pyrometer Infrared Radiation-sensitive Photodiode or photoresistor Accuracy= f{emissivity} Useful @ very high temperatures Non-contacting Very expensive Not very accurate 015 Agilent Technologies Classroom Series Agenda Background, history Mechanical sensors Electrical sensors Optical Pyrometer RTD Thermistor, IC Thermocouple Summary & Examples A4 Agilent Technologies Classroom Series Resistance Temperature Detector Most accurate & stable Good to 800 degrees Celsius Resistance= f{Absolute T} Self-heating a problem Low resistance Nonlinear 016 Agilent Technologies Classroom Series RTD Equation R= 100 Ohms @ O C Callendar-Van Deusen Equation: For T>OC: R=Ro(1+aT) - Ro(ad(.01T)(.01T-1)) R Ro=100 @ O C a= 0.00385 / - C for Pt d= 1.49 300 200 100 0 Nonlinearity 200 400 600 800 T 017 Agilent Technologies Classroom Series Measuring an RTD: 2-wire method Rx 100d Pt Rlead + Rlead - V I ref= 5 mA R= Iref*(Rx + 2* Rlead) Error= 2d /.385= more than 5 degrees C for 1 ohm Rlead! Self-heating: For 0.5 V signal, I= 5mA; P=.5*.005=2.5 mwatts @ 1 mW/deg C, Error = 2.5 deg C! Moral: Minimize Iref; Use 4-wire method If you must use 2-wire, NULL out the lead resistance 018 Agilent Technologies Classroom Series The 4-Wire technique Rx 100d + Rlead=1d - V I ref= 5 mA R= Iref * Rx Error not a function of R in source or sense leads No error due to changes in lead R Twice as much wire Twice as many scanner channels Usually slower than 2-wire 019 Agilent Technologies Classroom Series Offset compensation Voffset + 100d - V I ref (switched) Eliminates thermal voltages Measure V without I applied Measure V With I applied R= V I 020 Agilent Technologies Classroom Series Bridge method 100 d d 1000 V d 1000 100d High resolution (DMM stays on most sensitive range) Nonlinear output Bridge resistors too close to heat source 021 Agilent Technologies Classroom Series 3-Wire bridge 100d 1000d Rlead 1 V 1000d Sense wire 3-Wire PRTD Rlead 2 Keeps bridge away from heat source Break DMM lead (dashed line); connect to 100d RTD through 3rd "sense" wire If Rlead 1= Rlead 2, sense wire makes error small Series resistance of sense wire causes no error 022 Agilent Technologies Classroom Series Agenda Background, history Mechanical sensors Electrical sensors Optical Pyrometer RTD Thermistor, IC Thermocouple Summary & Examples A5 Agilent Technologies Classroom Series Electrical sensors: Thermistor Rlead=1 5k d d Rlead=1d + - Hi-Z; Sensitive: 5 k V d I= 0.1 mA @ 25C; R = 4%/deg C Limited range 2-Wire method: R= I * (Rthmr + 2*Rlead) Lead R Error= 2 d /400= 0.005 degrees C Low thermal mass: High self-heating Very nonlinear 023 Agilent Technologies Classroom Series I.C. Sensor I= 1 uA/K + 5V - 100 d V = 1mV/K 960d High output Very linear Accurate @ room ambient Limited range Cheap 024 Agilent Technologies Classroom Series Summary: Absolute T devices RTD Thermistor I.C. Most accurate Most stable Fairly linear High output Fast 2-wire meas. High output Most linear Inexpensive Expensive Slow Needs I source Self-heating 4-wire meas. Very nonlinear Limited range Needs I source Self-heating Fragile Limited variety Limited range Needs V source Self-heating 025 Agilent Technologies Classroom Series Agenda Background, history Mechanical sensors Electrical sensors Optical Pyrometer RTD Thermistor, IC Thermocouple Summary & Examples A6 Agilent Technologies Classroom Series Thermocouples The Gradient Theory Ta Tx The WIRE is the V= V sensor, not the junction Tx The Seebeck e(T) dT Ta coefficient (e) is a function of temperature 026 Agilent Technologies Classroom Series Making a thermocouple Ta Tx B V Two wires make a A Ta thermocouple V= e Ta A Voltage output is Ta Tx dT + e B dT nonzero if metals are not the same Tx 027 Agilent Technologies Classroom Series Gradient theory also says... Ta Tx A V If wires are the A Ta Ta Tx e dT + A V= Ta e A dT = 0 same type, or if there is one wire, and both ends are at the same temperature, output= Zero. Tx 028 Agilent Technologies Classroom Series Now try to measure it: a Fe b Tx Theoretically, Vab= f{Tx-Tab} Con But, try to measure it with a DMM: Cu Tx V Cu Cu Fe Con = Fe V Tx Con Cu Result: 3 unequal junctions, all at unknown temperatures 029 Agilent Technologies Classroom Series Solution: Reference Thermocouple Problems: a) 3 different thermocouples, b) 3 unknown temperatures Solutions: a) Add an opposing thermocouple b) Use a known reference temp. Isothermal block Fe Cu Cu Fe Tx Add Tx Con V V Con Tref Tref = 0 oC Cu Fe Cu Fe 030 Agilent Technologies Classroom Series The Classical Method Cu Fe Tx V Con Tref o =0C Cu Fe If both Cu junctions are at same T, the two "batteries" cancel Tref is an ice bath (sometimes an electronic ice bath) All T/C tables are referenced to an ice bath V= f{Tx-Tref} Question: How can we eliminate the ice bath? 031 Agilent Technologies Classroom Series Eliminating the ice bath Don't force Tref to icepoint, just Cu V Fe Con Cu Fe measure it Compensate for Tref Tx mathematically: V=f{ Tx - Tref } Tice Tref Tice If we know Tref , we can Tice compute Tx. 032 Agilent Technologies Classroom Series Eliminating the second T/C Cu Fe Tx V Extend the isothermal block If isothermal, V1-V2=0 2 Con Cu Fe Tref Cu Fe 1 Tx V Tref 2 Con Cu 1 033 Agilent Technologies Classroom Series The Algorithm for one T/C Cu V Tref Cu Fe IC or thermistor Tx Measure Tref: RTD, o Tref ==> Vref @ O C for Type J(FeCon C) Know V,Tx Know Vref: Compute Vx Solve for using Vx Compute Vx=V+Vref Vx Vref 0 o Tref V Tx 034 Agilent Technologies Classroom Series V Linearization Small sectors 0 o Tref Tx T 2 3 V +.... a9 V Polynomial: T=a +a V +a V +a 0 1 2 3 9 Nested (faster): T=a 0 +V(a 1 +V(a 2 +V(a 3 2 +.......))))))))) 0 Small sectors (faster): T=T +bV+cV Lookup table: Fastest, most memory 035 Agilent Technologies Classroom Series Common Thermocouples mV E 60 Platinum T/Cs Base Metal T/Cs K J N 40 20 T 0 500 RS 1000 2000 deg C All have Seebeck coefficients in MICROvolts/deg.C 036 Agilent Technologies Classroom Series Common Thermocouples Type J K T S E N Metals Seebeck Coeff: uV/C Fe-Con Ni-Cr Cu-Con Pt/Rh-Pt Ni/Cr-Con Ni/Cr/Si-Ni/Si 50 40 38 10 59 39 Microvolt output is a tough measurement Type "N" is fairly new.. more rugged and higher temp. than type K, but still cheap 037 Agilent Technologies Classroom Series Extension Wires Possible problem here Large extension wires Small diameter measurement wires Extension wires are cheaper, more rugged, but not exactly the same characteristic curve as the T/C. Keep extension/TC junction near room temperature Where is most of the signal generated in 038 this Agilent Technologies Classroom Series Noise: DMM Glossary Normal Mode ac NOISE DMM Input Resistance HI LO Normal Mode dc SIGNAL Normal Mode: In series with input Common Mode: Both HI and LO HI DMM Input Resistance terminals driven equally LO Common Mode ac NOISE 039 Agilent Technologies Classroom Series Generating noise Electrostatic Noise DMM Input Resistance Magnetic Noise HI Normal Mode LO dc SIGNAL Large surface area, high Rlead: Max. static DMM Input Resistance R leak coupling HILarge loop area: Max. magnetic coupling Large R lead, small R leak: R lead Max. LO common mode noise Common Mode Current Common Mode ac source 040 Agilent Technologies Classroom Series Eliminating noise Electrostati c Noise DM M Inpu tR Magneti c Noise HI Normal Mode dc SIGNAL L O Filter, shielding, small loop area (Caution: filter slows down the measurement) HI DM M Input R R leak Make R leak close to L O - + Common Mode Current Common Mode ac source 041 Agilent Technologies Classroom Series Magnetic Noise Magnetic coupling DMM Input Resistance Induced I Minimize area Twist leads Move away from strong fields 042 Agilent Technologies Classroom Series Reducing Magnetic Noise Equal and opposite induced currents DMM Input Resistance Even with twisted pair: Minimize area Move away from strong fields 043 Agilent Technologies Classroom Series Electrostatic noise AC Noise source Stray capacitances DMM Input Resistance Inoise Stray resistances Stray capacitance causes I noise DMM resistance to ground is important 044 Agilent Technologies Classroom Series Reducing Electrostatic AC Noise source Coupling DMM Input Resistance HI LO Rleak Shield shunts stray current For noise coupled to the tip, Rleak is still important 045 Agilent Technologies Classroom Series A scanning system for T/Cs One thermistor, multiple T/C channels Noise reduction CPU linearizes T/C DMM must be very high quality OHMs Conv. Isolators HI uP LO Integrating A/D Floating Circuitry ROM Lookup uP I/O (HP-IB, RS-232) To Computer Grounded Circuitry 046 Agilent Technologies Classroom Series Errors in the system Ref. Block Thermal gradient T/C Calibration & Wire errors Thermal emf Ref. Thermistor cal, linearity OHMs Conv. Reference Thermistor Ohms measurement Linearization algorithm Isolators HI uP LO Integrating A/D Floating Circuitry ROM Lookup DMM offset, linearity, thermal emf, noise uP Extension wire junction error I/O (HP-IB, RS-232) Grounded Circuitry 047 Agilent Technologies Classroom Series Physical errors Shorts, shunt impedance Galvanic action Decalibration Sensor accuracy Thermal contact Thermal shunting 048 Agilent Technologies Classroom Series Physical Errors Water droplets cause galvanic action; huge offsets Hot spot causes shunt Z, mete shows the WRONG temperatur Exceeding the T/C's range can cause permanent offset Real T/C's have absolute accuracy of 1 deg C @ 25C: Calibrate often and take care 049 Agilent Technologies Classroom Series Physical error: Thermal contact Surface probe Make sure thermal mass is much smaller than that of object being measured 050 Agilent Technologies Classroom Series Physical errors: Decalibration 350 C 300 C 200 C 100 C 975 C 1000 C This section Don't exceed Tmax of T/C produces the Temp. cycling causes work-hardening, ENTIRE signal decalibration Replace the GRADIENT section 051 Agilent Technologies Classroom Series Agenda Background, history Mechanical sensors Electrical sensors Optical Pyrometer RTD Thermistor, IC Thermocouple Summary & Examples A7 Agilent Technologies Classroom Series The basic 4 temperature sensors RTD Thermistor Most accurate High output Fast Most stable 2-wire meas. Fairly linear Expensive Very nonlinear Slow Limited range Needs I source Needs I source Self-heating Self-heating 4-wire meas. Fragile I.C. Thermocouple High output Most linear Cheap Limited variety Limited range Needs V source Self-heating Wide variety Cheap Wide T. range No self-heating Hard to measure Relative T. only Nonlinear Special connectors Absolute temperature sensors 052 Agilent Technologies Classroom Series Summary Innovation by itself is not enough... you must develop standards Temperature is a very difficult, mostly empirical measurement Careful attention to detail is required 053 Agilent Technologies Classroom Series Examples Measurement Photochemical process Sensor RTD (most accurate) control: Thermistor Flower petal: Molten glass: Induction furnace: 100 degree Heat aging oven: (lowest thermal mass) Optical pyrometer (hi temp, no contact) RTD (if <800C); or T/C (Beware magnetic I noise) Any of the 4 sensors 054