Agilent Technologies Classroom Series Practical Temperature Measurements 001

Agilent Technologies Classroom Series Practical Temperature Measurements 001

Agenda _Background, history _Mechanical sensors _Electrical sensors _ Optical Pyrometer _ RTD _ Thermistor, IC _ Thermocouple _Summary & Examples A 1

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

How is heat transferred? _Conduction _ Metal coffee cup _Convection _Radiation 003

The Dewar _Glass is a poor conductor _Gap reduces conduction _Metallization reflects radiation _Vacuum reduces convection 004

Thermal Mass _Don't let the measuring Sensor device change the temperature of what you're measuring. _Response time = _ f{Thermal mass} _ f{Measuring device} Sensor 005

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

History of temperature sensors _1600 ad _1700 12 1 ad 96 _Fahrenheit _ 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

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

1821: It was a very good year _1800 ad _1900 ad _The Seebeck effect _Davy: The RTD _Pt 100 d @ O deg. C 009

The 1900's: Electronic sensors _1900 ad _Thermistor _2000 ad _1 u. A/K _IC sensor _IPTS 1968 _IPTS 1990 _"Degree Kelvin">> "kelvins" _"Centigrade">> " Celsius" 010

Absolute zero Temperature scales Freezing point H 2 O _Celsius 0 100 _Kelvin 273. 15 32 212 -273. 15 0 _Fahrenheit -459. 67 0 Boiling point H O 2 _Rankine 427. 67 671. 67 _"Standard" is "better": _ _ Reliable reference points Easy to understand 011

IPTS '90: More calibration points – – 273. 16: TP H 2 O 234. 3156: TP Hg Large gap – – – 83. 8058: TP Ar 54. 3584: TP O 2 24. 5561: TP 20. 3: BP Ne 17 Liq/vapor H 2 13. 81 TP H 2 3 to 5: Vapor H 2 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

Agenda _Background, history _Mechanical sensors _Electrical sensors _ Optical Pyrometer _ RTD _ Thermistor, IC _ Thermocouple _Summary & Examples A 2

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

100 0 Liquid thermometer; Paints _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

Agenda _Background, history _Mechanical sensors _Electrical sensors _ Optical Pyrometer _ RTD _ Thermistor, IC _ Thermocouple _Summary & Examples A 3

Optical Pyrometer _Infrared Radiation-sensitive _Photodiode or photoresistor _Accuracy= f{emissivity} _Useful @ very high temperatures _Non-contacting _Very expensive _Not very accurate 015

Agenda _Background, history _Mechanical sensors _Electrical sensors _ Optical Pyrometer _ RTD _ Thermistor, IC _ Thermocouple _Summary & Examples A 4

Resistance Temperature Detector _Most accurate & stable _Good to 800 degrees Celsius _Resistance= f{Absolute T} _Self-heating a problem _Low resistance _Nonlinear 016

RTD Equation _R= 100 Ohms @ O C _Callendar-Van Deusen Equation: For T>OC: _R=Ro(1+a. T) - Ro(ad(. 01 T)(. 01 T-1)) _ Ro=100 @ O C _ a= 0. 00385 / - C for Pt _ d= 1. 49 R 300 200 100 _0 Nonlinearity 200 400 600 800 T 017

Measuring an RTD: 2 -wire method 100 d Pt Rx Rlead + - V I ref= 5 m. A _R= Iref*(Rx + 2* Rlead) _ Error= 2 d /. 385= more than 5 degrees C for 1 ohm Rlead! _Self-heating: _ For 0. 5 V signal, I= 5 m. A; P=. 5*. 005=2. 5 mwatts _ @ 1 m. W/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

The 4 -Wire technique 100 d Rx Rlead=1 d + - V I ref= 5 m. A _ 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

Offset compensation Voffset + 100 d - V I ref (switched) _Eliminates thermal voltages _ Measure V without I applied _ Measure V With I applied R= V I 020

Bridge method 1000 d V 1000 d 100 d _High resolution (DMM stays on most sensitive range) _Nonlinear output _Bridge resistors too close to heat source 021

3 -Wire bridge 100 d 1000 d Rlead 1 V 1000 d Sense wire 3 -Wire PRTD Rlead 2 _Keeps bridge away from heat source _Break DMM lead (dashed line); connect 100 d to RTD through 3 rd "sense" wire _If Rlead 1= Rlead 2, sense wire makes error small _Series resistance of sense wire causes no error 022

Agenda _Background, history _Mechanical sensors _Electrical sensors _ Optical Pyrometer _ RTD _ Thermistor, IC _ Thermocouple _Summary & Examples A 5

Electrical sensors: Thermistor Rlead=1 5 k d d Rlead=1 d + - _Hi-Z; Sensitive: 5 k V d I= 0. 1 m. A @ 25 C; 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

I. C. Sensor AD 590 + 5 V - I= 1 u. A/K 100 d V = 1 m. V/K 960 d _High output _Very linear _Accurate @ room ambient _Limited range _Cheap 024

Summary: Absolute T devices RTD Thermistor AD 590 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

Agenda _Background, history _Mechanical sensors _Electrical sensors _ Optical Pyrometer _ RTD _ Thermistor, IC _ Thermocouple _Summary & Examples A 6

Thermocouples The Gradient Theory Ta Tx V= e(T) d. T Ta _The WIRE is the sensor, not the junction _The Seebeck coefficient (e) is a function of temperature 026

Making a thermocouple Ta Tx B V _Two wires make a A Ta thermocouple V= e Ta A _Voltage output is Ta Tx d. T + e B d. T nonzero if metals are not the same Tx 027

Gradient theory also says. . . Ta Tx A V A Ta Ta Tx V= e Ta _If wires are the A d. T + e A d. T = 0 same type, or if there is one wire, and both ends are at the same temperature, output= Zero. Tx 028

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

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 = 0 o. C Cu Fe 030

The Classical Method Cu Fe Tx V Con Cu Fe Tref o =0 C _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

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 Tice , we can compute Tx. 032

Eliminating the second T/C Cu Fe Tx V _Extend the isothermal block _If isothermal, V 1 -V 2=0 2 Con Cu Fe Tref Cu Fe 1 Tx V Tref 2 Con Cu 1 033

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(Fe Con -C) _Know V, Know Vref: Compute Vx _Solve for Tx using Vx Compute Vx=V+Vref Vx Vref 0 o Tref V Tx 034

V Linearization Small sectors 0 o Tref Tx T 2 3 V +. . a 9 V _Polynomial: T=a +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 +b. V+c. V _Lookup table: Fastest, most memory 035

Common Thermocouples m. V 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

Common Thermocouples Type J K T S E N Metals Seebeck Coeff: u. V/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

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 this circuit? 038

Noise: DMM Glossary DMM Input Resistance HI LO HI DMM Input Resistance LO Normal Mode ac NOISE Normal Mode dc SIGNAL _Normal Mode: In series with input _Common Mode: Both HI and LO terminals driven equally Common Mode ac NOISE 039

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 HI_Large 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

Eliminating noise Electrostati c Noise DM M Inpu R t Magnetic Noise HI Normal Mode dc SIGNAL LO _Filter, shielding, small loop area (Caution: filter slows down the measurement) HI DM M Input R _Make R leak close to LO - + R leak Common Mode Current Common Mode ac source 041

Magnetic Noise _Magnetic coupling DMM Input Resistance Induced I _Minimize area _Twist leads _Move away from strong fields 042

Reducing Magnetic Noise _Equal and opposite induced currents DMM Input Resistance _Even with twisted pair: _ Minimize area _ Move away from strong fields 043

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

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

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. HI LO Isolators u. P Integrating A/D Floating Circuitry ROM Lookup u. P I/O (HP-IB, RS-232) To Computer Grounded Circuitry 046

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 HI LO Linearization algorithm Isolators u. P Integrating A/D Floating Circuitry ROM Lookup DMM offset, linearity, thermal emf, noise u. P Extension wire junction error I/O (HP-IB, RS-232) Grounded Circuitry 047

Physical errors _Shorts, shunt impedance _Galvanic action _Decalibration _Sensor accuracy _Thermal contact _Thermal shunting 048

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 @ 25 C: Calibrate often and take care 049

Physical error: Thermal contact Surface probe _Make sure thermal mass is much smaller than that of object being measured 050

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

Agenda _Background, history _Mechanical sensors _Electrical sensors _ Optical Pyrometer _ RTD _ Thermistor, IC _ Thermocouple _Summary & Examples A 7

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 _Self-heating _4 -wire meas. _Fragile AD 590 I. C. Thermocouple _High output _Most linear _Cheap _Wide variety _Cheap _Wide T. range _No self-heating _Limited variety _Limited range _Hard to measure _Needs V _Relative T. only source _Nonlinear _Self-heating _Special connectors Absolute temperature sensors 052

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

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 <800 C); or T/C (Beware magnetic I noise) _Any of the 4 sensors 054