Practical Temperature Measurements

Practical Temperature Measurements
Agilent Technologies Classroom Series Practical Temperature Measurements A1 Welcome to the Agilent Technologies session on practical temperature measurements. If you have any questions or would like to look at some related topics, we suggest investigating subjects such as guarding, scanning, A/D converters and filters. Some related reading: Agilent Technologies Application Note 290 : Practical Temperature Measurements Agilent Technologies Application Note 123: Floating Measurements and Guarding. 001

Agenda Background, history Mechanical sensors Electrical sensors
Agilent Technologies Classroom Series Agenda Background, history Mechanical sensors Electrical sensors Optical Pyrometer RTD Thermistor, IC Thermocouple Summary & Examples Temperature is the most commonly measured physical parameter in industry. This lecture will give you some feel for the techniques required to make reasonably accurate measurements of temperature. For more information, consult the bibliography in Agilent’s Application Note 290: Practical Temperature Measurements. During this session, we will first cover the fundamentals of heat transfer and a little bit about the history of temperature measurement. We will then discuss in detail the most common mechanical and electrical sensors that are used to measure temperature. Finally, we will summarize by using some examples that take advantage of what we have learned during this session. 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 The first difficulty encountered when studying temperature is that "temperature" is not easily defined. Temperature is a quantity that determines the direction of heat flow between two bodies. It is a statistical measurement, as evidenced by the fact that it is impossible to determine the temperature of a single molecule. Whenever you try to measure temperature with some accuracy, you find very quickly that it is a measurement that is difficult to repeat, and one that is heavily influenced by variables such as thermal contact, response time and electrical noise. To gain a feel for the science, let's take a look at how heat is transferred. 002

How is heat transferred?
Agilent Technologies Classroom Series How is heat transferred? Conduction Metal coffee cup Convection 003 How is heat transferred? Anyone who has picked up a hot tin cup full of coffee or sat on a cold aluminum bleacher seat understands the heat transfer method known as "conduction". If you have ever been in a house that uses a blower to move air around from a forced air natural gas heater, you also understand the principal of convection. Convection is the movement of heat by a liquid or gas medium. Radiation, on the other hand, does not rely upon any contact between the heat source and the heated object. Solar radiation is the most obvious example. If you stand near a south facing window of a building while the sun is out, you can immediately feel the effects of solar radiation. Using a single sheet of paper in front of your face to block the sun, you can immediately feel a reduction in the heat transferred to your face. There is one device that minimizes the effects of all three methods of heat transfer. Radiation 003

The Dewar Glass is a poor conductor Gap reduces conduction
Agilent Technologies Classroom Series The Dewar Glass is a poor conductor Gap reduces conduction Metallization reflects radiation Vacuum reduces convection 004 That device is the Dewar flask. The common thermos bottle is the mass-produced version of the Dewar. The Dewar is typically constructed of glass, primarily because the glass envelope can be easily formed into difficult shapes. In this case, glass is formed in the shape of a double- walled cup. The gap produced between the inner and outer surfaces of the cup reduces conduction. Evacuating this gap eliminates convection currents that could also transfer heat. And finally, the surfaces are metallized to reflect any radiated energy that might infringe upon the center cup. The Dewar is such an effective insulator that it is used to store things like liquid nitrogen and liquid helium. Now let's talk about thermal mass. 004

Thermal Mass Sensor Don't let the measuring device change the temperature of what you're measuring. Response time = f{Thermal mass} f{Measuring device} 005 Everything has a thermal mass. One critical thing to remember when measuring temperature is to not let the sensor change the temperature of what's being measured. If the thermal mass of the sensor is comparable to the thermal mass of the measurand, then the measurement will be compromised. It's also possible for the thermal mass of the sensor to upset the response time of the temperature measurement. An example of this is the common thermometer that the doctor uses to check your temperature. 005

Temperature errors What is YOUR normal temperature?
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 006 In a simple test of body temperature, we can detect a number of possible errors in the measurement. For example, most common thermometers are marked with the temperature of 98.6oF or 37oC as "normal" temperature. However, it is widely know that 98.6oF is simply an average temperature for an average person. Individual "normal" temperatures vary from person to person. The accuracy of the measurement will depend upon the accuracy of the thermometer itself, the thermometer's resolution, the total time that the thermometer is in contact with the tongue, the thermal mass of both the thermometer and the tongue, and any human error that results in the reading. In order to gain more appreciation for the measurement of temperature, let's take a glance back in history. 006

History of temperature sensors
Agilent Technologies Classroom Series History of temperature sensors 1600 ad 1700 ad 96 12 1 Fahrenheit Instrument Maker 12*8=96 points Hg: Repeatable One standard scale 007 One of the most interesting aspects in tracing the history of science is noting who gets credit for the discovery. For example, Galileo invented the first temperature sensor, but you don't see his name anywhere in thermometry. His first thermometer was simply an inverted globe filled with liquid and gas, and when he heated the gas by placing his hands on the globe, gas bubbled through the liquid columns and caused the length of the column to fluctuate. This was an indication of temperature change, but it had some major drawbacks: It was sensitive to pressure and it was not repeatable. Following Galileo, there were some early thermometers made, but they were neither repeatable nor easy to calibrate. In the early 1700's, Gabriel Fahrenheit, an instrument maker, divided the twelve points of common Flemish thermometers into ninety-six points, giving them more resolution. He also used mercury for repeatability, then calibrated the thermometer at two known end points. Galileo: First temp. sensor pressure-sensitive not repeatable Early thermometers Not repeatable No good way to calibrate 007

The 1700's: Standardization
Agilent Technologies Classroom Series The 1700's: Standardization 1700 ad 1800 ad 100 100 Thomson effect Absolute zero 008 Later that century, Anders Celsius realized that it would be advantageous to use more common calibration references and to divide the scale into 100 increments instead of 96. He chose to use one hundred degrees as the freezing point and zero degrees as the boiling point of water. Later these two endpoints were reversed and the Centigrade scale was born. In the early 1800's, William Thomson ("Lord Kelvin"), discovered that a current could be induced in ring of metal by heating one end of the ring. He also postulated the existence of an absolute zero. Celsius: Common, repeatable calibration reference points "Centigrade" scale 008

d 1821: It was a very good year 1800 ad 1900 ad The Seebeck effect
Agilent Technologies Classroom Series 1821: It was a very good year 1800 ad 1900 ad The Seebeck effect Davy: The RTD 009 The 1800's saw some major advances in electrical sensors. In 1821 both the Seebeck effect and the RTD were discovered. Seebeck discovered that a current could be produced by unequally heating two junctions of two dissimilar metals-- the thermocouple effect. Seebeck assigned constants to each type of metal, and used these constants to compute total amount of current flowing. In the very same year, Sir Humphrey Davy discovered that metals all have a positive temperature coefficient, and that platinum could be used as an excellent temperature indicator. Pt @ O deg.C d 009

The 1900's: Electronic sensors
Agilent Technologies Classroom Series The 1900's: Electronic sensors 1900 ad 2000 ad 1 uA/K Thermistor IC sensor 010 The 1900's saw the discovery of semiconductor devices such as the thermistor and the integrated circuit sensor. The thermistor has a negative temperature coefficient and a very large change in resistance with temperature; hence, it is excellent at detecting very small temperature changes. The integrated circuit sensor does not require any external linearization techniques, since it is inherently a linear device. Also in the 1900's, Lord Kelvin was finally rewarded for his early work in temperature measurement. The increments of the Kelvin scale were changed from degrees to kelvins. Now we no longer say "One-hundred degrees kelvin;" we instead say "one-hundred kelvins". In this same century, the "centigrade" scale was changed to the "Celsius" scale, in honor of Anders Celsius. IPTS 1968 IPTS 1990 "Degree Kelvin">> "kelvins" "Centigrade">> " Celsius" 010

Temperature scales Freezing point H O Absolute zero Boiling point H O
Agilent Technologies Classroom Series Temperature scales Absolute zero Freezing point H O 2 Boiling point H O 2 Celsius 100 Kelvin 273.15 373.15 Fahrenheit 32 212 011 So that leaves us with the common temperature scales of Celsius, Kelvin, Fahrenheit and Rankine. The Rankine scale is rarely used, and simply references the Fahrenheit scale to absolute zero. Even the Fahrenheit scale is used rarely, except by a very few countries. The Celsius and Kelvin scales are by far the most popular scales for measuring temperature throughout the world. Rankine 427.67 671.67 "Standard" is "better": Reliable reference points Easy to understand 011

IPTS '90: More calibration points
Agilent Technologies Classroom Series IPTS '90: More calibration points 273.16: TP H2O : FP Cu : TP Hg : FP Au : FP Ag Large gap : FP Al : FP Zn 012 In 1990, another International Practical Temperature Scale meeting was held, and more calibration points were added to the standard calibration series for thermometry. You can see that there still exists a large gap with no calibration point between 83 kelvins and 234 kelvins. Note also that there is no equivalent of a "voltage divider" for temperature. The source itself is not changed... we rely upon several cardinal phenomena, such as the freezing point of mercury and the triple point of water. The interpolation is done by the sensor. The remarkable event that points out the inexactness of temperature measurement is that during the IPTS 1990 meeting, the temperature standard at about 900 kelvins was changed by nearly 0.1 degree C. That's an enormous jump for a "standard". : TP Ar : FP Sn : TP O2 : FP In : TP Ne 20.3: BP H2 17 Liq/vapor H2 : MP Ga TP H2 3 to 5: Vapor He 012

Agilent Technologies Classroom Series Agenda Background, history Mechanical sensors Electrical sensors Optical Pyrometer RTD Thermistor, IC Thermocouple Summary & Examples A2 That's enough history. Let's concentrate on understanding some simple mechanical sensors. A2

Bimetal thermometer Two dissimilar metals, tightly bonded
Agilent Technologies Classroom Series Bimetal thermometer 100 300 200 400 Two dissimilar metals, tightly bonded Forces due to thermal expansion Result Bimetallic thermometer Poor accuracy Hysteresis Thermal expansion causes big problems in other designs: IC bonds Mechanical interference 013 One of the simplest temperature indicators can be produced by joining two dissimilar metals tightly bonded together. Since dissimilar metals exhibit dissimilar thermal expansion coefficients, the result will be that the two metals tend to bend in an arc. A pointer placed at the end of this arc can be used as a temperature indicator. This principal is used in the common kitchen thermometer. The disadvantages of this type of indicator are that it has poor accuracy and it exhibits hysteresis. "Hysteresis" is a memory phenomenon. A device with a large hysteresis will not return to the same starting point. The same dissimilar expansion coefficients that are a benefit in the bimetal thermometer can create big problems in other designs. For example, when an integrated circuit gets bonded to a substrate, such as ceramic, the silicon integrated circuit must have the same thermal expansion coefficient as the substrate; otherwise, the resultant thermal stress can cause a failure. 013

Liquid thermometer; Paints
Agilent Technologies Classroom Series Liquid thermometer; Paints 100 Thermally-sensitive paints Irreversible change Low resolution Useful in hard-to-measure areas 014 As you have already seen, the liquid-filled thermometer has been around since the 1700's. It is accurate over a small range, but its accuracy and resolution are a function of the thermometer's length. It is slow, fragile and has a large thermal mass, and its range is limited by the liquid inside. For measuring temperature in difficult-to-reach places, or places where a safety hazard is a concern, thermally sensitive paints offer a unique opportunity. Paint them on a surface, and they undergo an irreversible color change when they pass a temperature threshold. While their resolution is not very good, they are quite inexpensive. Liquid-filled thermometer 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 Now let's talk about some electrical sensors. The first one on our list is the optical pyrometer. A3

Optical Pyrometer Infrared Radiation-sensitive
Agilent Technologies Classroom Series Optical Pyrometer Infrared Radiation-sensitive Photodiode or photoresistor Accuracy= f{emissivity} very high temperatures Non-contacting Very expensive Not very accurate 015 The optical pyrometer uses an infrared radiation-sensitive sensor, e.g. a photodiode or a photoresistor, to compare the radiation from the unknown with that of the radiation from an internal incandescent source. The accuracy of the optical pyrometer is very much a function of the emissivity of the device that is radiating the heat. The obvious advantage in using an optical pyrometer at very high temperatures is that the measurement is non-contacting. This approach is very expensive, and due to the variability in emissivity of many physical bodies, it is not very accurate. However, for making non-contact measurements on very high temperature bodies such as molten glass and molten steel, the optical pyrometer excels. 015

Agilent Technologies Classroom Series Agenda Background, history Mechanical sensors Electrical sensors Optical Pyrometer RTD Thermistor, IC Thermocouple Summary & Examples A4 Now let's get to a more common temperature measuring device: The RTD. "RTD" stands for "resistance temperature detector". A4

Resistance Temperature Detector
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 The Platinum RTD is the most accurate and stable temperature detector from zero to about 500o C. It can measure temperatures up to 800o C. The resistance of the RTD changes as a function of absolute temperature, so we categorize it as one of the absolute temperature devices. (In contrast, the thermocouple cannot measure absolute temperature; it can only measure relative temperature.) One disadvantage of the RTD is that it self-heats. To measure an RTD, you have to put a current through it, and that causes I2R heating in the device. The result is an error in the measurement of the unknown temperature. Another disadvantage is that the RTD has fairly low resistance, so small changes in lead resistance can cause a big error. It is also a nonlinear device, which means that the result must be interpreted by some algorithm. 016

RTD Equation R= 100 Ohms @ O C Callendar-Van Deusen Equation:
Agilent Technologies Classroom Series RTD Equation R= 100 O C Callendar-Van Deusen Equation: R=Ro(1+aT) - Ro(ad(.01T)(.01T-1)) Ro= @ O C a= / - C d= 1.49 For T>OC: for Pt R 017 At 0o C, a platinum RTD has a resistance of 100 ohms and a temperature coefficient of about ohms per ohm degrees C; that is, a 100 ohm thermometer will have: 100 x = ohms/ oC change at 0 oC. These non-linearities are described in the Callendar-Van Deusen equation. This equation consists of both a linear portion and a non-linear portion. 300 200 100 Nonlinearity T 017

Measuring an RTD: 2-wire method
Agilent Technologies Classroom Series Measuring an RTD: 2-wire method Rlead Rx + 100 d V Rlead I ref= 5 mA Pt - R= Iref*(Rx + 2* Rlead) Error= 2 /.385= more than 5 degrees C for 1 ohm Rlead! d Self-heating: For 0.5 V signal, I= 5mA; P=.5*.005=2.5 mwatts @ 1 mW/deg C, Error = 2.5 deg C! 018 Since the RTD has a low resistance, we need to look very carefully at how we are going to measure it. Let's take a simple two-wire ohms measurement with 5 mA reference current being pumped through the unknown RTD. In the two-wire measurement, the lead resistance becomes part of the unknown. In the case of the 100 ohm platinum RTD, a total of two ohms lead resistance can result in more than five degrees C error! Let's also look at the error due to self heating. For a 5 milliamp signal, the power in this device is about 2.5 milliwatts. At 1 milliwatt per degree C, that is an error of 2.5 degrees C. The lesson that we can learn from this is to always use a 4-wire measurement, not a 2-wire measurement. If you must use a 2-wire measurement, be sure to null out the lead resistance and minimize the measurement source current. Now let's take a look at the 4-wire technique. Moral: Minimize Iref; Use 4-wire method If you must use 2-wire, NULL out the lead resistance 018

d d The 4-Wire technique Rx + 100 V Rlead=1 I ref= 5 mA - R= Iref * Rx
Agilent Technologies Classroom Series The 4-Wire technique Rx + 100 d V Rlead=1 I ref= 5 mA d - 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 With the 4-wire technique, the voltage is sensed not at the voltmeter input terminals, but at the terminals of the RTD. That means that error is no longer a function of the lead resistance. It also means that there is no error due to changes in lead resistance, as there would be in the 2-wire measurement. The disadvantage of the 4-wire technique is that is requires twice as much wire and twice as many scanner channels. It is also typically slower than a 2-wire method. 019

d Offset compensation Voffset + 100 V I ref (switched) -
Agilent Technologies Classroom Series Offset compensation Voffset + 100 d V I ref (switched) - Eliminates thermal voltages Measure V without I applied Measure V I applied 020 Whenever a wire is heated, there exists the possibility of a thermal potential being generated. This thermal voltage can be eliminated through an offset-compensation technique. In offset compensation, we switch the reference currents off and on while measuring the voltage across the unknown. First measure the voltage without the current applied, then measure the voltage with the current applied. The result, delta V over delta I, is the unknown resistance. With V R= I 020

Bridge method 100 d 1000 d V 100 d 1000 d High resolution (DMM stays on most sensitive range) Nonlinear output Bridge resistors too close to heat source 021 You could also use a bridge to measure an RTD. The advantage of the bridge is that the voltmeter always stays on its most sensitive range. The disadvantage shown here is that the bridge resistors are too close to the heat source, and hence we will get an incorrect reading due to the temperature coefficient of the bridge resistor. This can be solved by moving the RTD away from the bridge with an extra wire. 021

3-Wire bridge d d V 3-Wire PRTD d d Keeps bridge away from heat source
Agilent Technologies Classroom Series 3-Wire bridge 1000 100 d d Rlead 1 V 3-Wire PRTD Sense wire 1000 d Rlead 2 100 d Keeps bridge away from heat source Break DMM lead (dashed line); connect to RTD through 3rd "sense" wire If Rlead 1= Rlead 2, sense wire makes error small Series resistance of sense wire causes no error 021 You can move the RTD a good distance away from the bridge and add a third sense wire directly connected to the voltmeter. This has the advantage of keeping the bridge away from the heat source and it also has the advantage of having the lead resistances compensate for each other. However, any mismatch in lead resistances will cause an error. While this method of measurement does save one wire, it is typically not quite as accurate as a 4-wire technique. 022

Agilent Technologies Classroom Series Agenda Background, history Mechanical sensors Electrical sensors Optical Pyrometer RTD Thermistor, IC Thermocouple Summary & Examples A5 Now let's talk about the thermistor and integrated circuit sensor. A5

Electrical sensors: Thermistor
Agilent Technologies Classroom Series Electrical sensors: Thermistor Rlead=1 d + 5k V d I= 0.1 mA Rlead=1 d - Hi-Z; Sensitive: 5 k @ 25C; R = 4%/deg C d Limited range 023 The thermistor has three major advantages: first, it is high impedance, so we only need to use a 2-wire technique. Second, it exhibits a very large change in resistance with a small change in temperature. That means high resolution, and also means that lead resistance is not a problem. For example, the 2-ohm lead resistance results in only 5 milli-degrees of error. Third, the thermistor can be built with a very small thermal mass, meaning that it will not cause thermal loading on the device being measured. While low thermal mass can be a benefit, it can also be a detriment, since low thermal mass means high self heating. Since the thermistor is very non-linear, we will have to supply some sort of linearization algorithm to get the final answer. 2-Wire method: R= I * (Rthmr + 2*Rlead) Lead R Error= /400= degrees C Low thermal mass: High self-heating Very nonlinear d 023

I.C. Sensor I= 1 uA/K + High output Very linear
Agilent Technologies Classroom Series I.C. Sensor AD590 I= 1 uA/K + High output Very linear room ambient Limited range Cheap 100 5V - d V = 1mV/K 024 The integrated circuit sensor gets around the linearity problem. It also has a very high output that makes it easy to use. It is accurate at room temperature and is relatively inexpensive, but it operates only over limited range and it does require a power source. The I.C. sensor is available in either voltage output or current output modes. The I.C. sensor heralds a trend that is sweeping the transducer industry. "Smart Sensors" are devices that contain on-board intelligence. For example, an accelerometer can now be built by micro-machining the surface of an IC so that it resembles the interlocking tines of two forks. When acceleration is applied, the tines try to come closer together, but are held apart by a force-balance feedback loop that sends a signal to an on-board amplifier. This signal is linearized and passed to the 2-wire connection for remote readout on a computer. Other "Smart Sensors" can detect specific ions and chemicals, such as the oxygen content in an automotive exhaust system. 960 d 024

Summary: Absolute T devices
Agilent Technologies Classroom Series Summary: Absolute T devices Expensive Slow Needs I source Self-heating 4-wire meas. RTD Most accurate Most stable Fairly linear Thermistor High output Fast 2-wire meas. Very nonlinear Limited range Needs I source Self-heating Fragile AD590 I.C. High output Most linear Inexpensive Limited variety Limited range Needs V source Self-heating 025 So let's summarize all of these absolute temperature devices. The RTD, or the Resistance Temperature Detector, is by far the most accurate of the three types of devices. It is the most stable, yet is fairly linear and operates over a very wide temperature range. Its disadvantages are that it is expensive, slow and requires a 4-wire measurement. Like all the absolute temperature devices, it exhibits some self-heating. The thermistor has the highest output for the smallest change in temperature. Because we can make it quite small, it is very fast to respond to a temperature change. It only requires a 2-wire measurement. Its disadvantages are a limited temperature range, significant self-heating, and the fragility of the device. The integrated circuit sensor is the most linear of all three devices, and has a very high output. It is inexpensive, but comes in only a limited variety of devices with limited temperature ranges. 025

Agilent Technologies Classroom Series Agenda Background, history Mechanical sensors Electrical sensors Optical Pyrometer RTD Thermistor, IC Thermocouple Summary & Examples A6 Now let's take a look at the most common temperature sensor: the thermocouple. A6

Thermocouples The Gradient Theory
Agilent Technologies Classroom Series Thermocouples The Gradient Theory Tx Ta V The WIRE is the sensor, not the junction The Seebeck coefficient (e) is a function of temperature 026 Let's start out by looking at a piece of wire by itself, not a pair of wires, but simply a single wire. There is a theory called the "gradient theory" that states that the wire is the sensor, not the junction. This theory can be used to explain several phenomena associated with thermocouples. The gradient theory says that if you heat one end of the wire, the wire will produce a voltage that is a function of the temperature difference from one end of the wire to the other. The coefficient of that voltage will be a function of the type of wire we use. That coefficient is called the "Seebeck Coefficient". This is a non-linear function: The Seebeck coefficient actually varies as a function of temperature. V= e(T) dT Ta Tx 026

Making a thermocouple Ta Tx B V Two wires make a thermocouple A
Agilent Technologies Classroom Series Making a thermocouple Tx Ta V A B Two wires make a thermocouple Voltage output is nonzero if metals are not the same 027 Now by simply putting two dissimilar wires together, we can make a thermocouple. The voltage at the output of this thermocouple will be a function of the Seebeck coefficient of the two wires and a function of the total temperature difference across the wires. V= e dT Ta Tx Ta e dT B A Tx 027

Gradient theory also says...
Agilent Technologies Classroom Series Gradient theory also says... Tx Ta V A If wires are the same type, or if there is one wire, and both ends are at the same temperature, output= Zero. 028 Notice that if the two wires are of the same type, or if there is only one wire and both ends of the same wire are at the same temperature, the output will be zero volts. V= e dT Ta Tx Ta e dT = 0 A A Tx 028

= Now try to measure it: Con a Tx Fe b Theoretically, Vab= f{Tx-Tab}
Agilent Technologies Classroom Series Now try to measure it: Con a Tx Fe b Theoretically, Vab= f{Tx-Tab} But, try to measure it with a DMM: Tx Con Fe V Cu = Cu Con Fe Tx V 029 Now let's use this theory to make a measurement. Suppose we have one iron wire and one constantan wire (that is a "type J" "thermocouple") joined together to measure temperature. Theoretically, according to the gradient theory, the voltage from point A to point B is a function of the temperature Tx-Ta, and Tx-Tb (where Tx is the unknown temperature). But, when we try to measure the thermocouple wire with a voltmeter, we create two new junctions caused by the copper wires of the voltmeter being connected to the thermocouple. The result is three unequal junctions all at unknown temperatures. Result: 3 unequal junctions, all at unknown temperatures 029

Solution: Reference Thermocouple
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 Cu V Fe Tref Con Tx Add Cu V Fe Tref = 0 C Con Tx o 030 So the problem is: we have three different thermocouples and three unknown temperatures. One solution could be to add an opposing thermocouple and use a known reference temperature, as we've shown here. Now we can at least analyze the circuit. We have added a constantan-iron thermocouple at the bottom of the circuit, and the result is now a copper-iron thermocouple at the top and a copper-iron thermocouple at the bottom of the circuit. If we can put both of these thermocouples on an isothermal block, their voltages will cancel each other and we will not have to consider them in the circuit analysis. The resulting voltage should be proportional to the difference between the unknown temperature and the reference temperature. Now, if we hold the reference temperature at oC, we will have a voltage that is proportional only to the unknown temperature. 030

The Classical Method Cu Fe
Agilent Technologies Classroom Series The Classical Method 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} Tx V Con Tref = 0 C o 031 Indeed, this is the classical method of measuring thermocouples. If both copper junctions are at the same temperature, the two "batteries" cancel each other, and the only temperature left to measure is the unknown. We will fix the reference temperature using an ice bath or "electronic ice point". Since all thermocouple tables are reference to an ice bath temperature, we now know that the voltage will be a function of Tx minus the reference temperature, which is easily determined from a thermocouple table. As a side point, there is no one thermocouple that matches exactly the thermocouple tables published by NIST, simply because the standard thermocouple tables were derived by averaging many manufacturers' products. In a practical situation, we would like to eliminate the ice bath from this circuit. Cu Fe Question: How can we eliminate the ice bath? 031

Eliminating the ice bath
Agilent Technologies Classroom Series Eliminating the ice bath Don't force Tref to icepoint, just measure it Compensate for Tref mathematically: V=f{ Tx Tref } If we know Tref , we can compute Tx. Cu V Fe Con Tx Tref Tice Tice 032 So how do we totally eliminate the ice bath? The easiest way is to simply measure the reference temperature rather than forcing it to be exactly 0 oC. Then we compensate for the reference temperature mathematically. We know that the voltage will be a function of: (unknown temperature relative to an ice point) - ( reference temperature relative to an ice point). So if we know the reference temperature, we can compute the unknown. Tice 032

Eliminating the second T/C
Agilent Technologies Classroom Series Eliminating the second T/C Cu V Fe Con Tx Extend the isothermal block If isothermal, V1-V2=0 2 Cu Fe Tref Tx 033 In our last example, we still needed a reference thermocouple to measure the unknown thermocouple. It would be great if we could eliminate that reference thermocouple altogether. Suppose we were to extend the isothermal block to encompass the reference thermocouple. According to the gradient theory, that means that the iron wire going from point 1 to point 2 has no gradient across it, and that means that its equivalent voltage output is zero. This is a significant advantage for a system measurement, since it means that we can use copper wires directly connecting a voltmeter to an isothermal block, and that block can be directly connected to individual thermocouples without the use of a reference thermocouple. V Tref Con 1 2 Cu 1 033

The Algorithm for one T/C
Agilent Technologies Classroom Series The Algorithm for one T/C Cu V Con Fe Tx Tref Measure Tref: RTD, IC or thermistor Tref ==> Vref @ O C for Type J(Fe-C) Know V, Know Vref: Compute Vx Solve for using Vx o Tx 034 So how do you actually make the measurement? Well, first you measure the reference temperature of the isothermal block using an absolute temperature device such as an RTD, IC or thermistor. You then convert that reference temperature to its equivalent voltage, referenced to zero degrees C, for the type for thermocouple you intend to measure. Knowing the reference voltage and knowing the voltage at the voltmeter itself, you can compute the unknown voltage. Then solve for the unknown temperature using the computed voltage. For example, in the graph shown, use the thermocouple curve to convert the reference temperature, which we know from an absolute temperature measurement, to its equivalent voltage reference, Vref. Then measure the total voltage V with a voltmeter and add this to the voltage reference to establish the unknown, Vx. You then go back to the thermocouple curve for the intercept to determine the unknown temperature, Tx. Compute Vx=V+Vref Vx V Vref o Tref Tx 034

Linearization V Small sectors Tref Tx T
Agilent Technologies Classroom Series Linearization V Small sectors o Tref Tx T 2 035 Thermocouples are quite non-linear, and it can take a very large polynomial to describe all the hills and valleys of their temperature plot. But computing a high-order polynomial can be quite slow, so there are other techniques that we use. The first technique is simply to nest the polynomial. This method runs somewhat faster than a standard polynomial solution. The next method simply takes small chunks out of the linearization curve and operates only over a small range. This is a much faster way to compute temperature. The fastest way is to use a look-up cable, but this of course requires a great deal of memory in a computer or inside a data acquisition system. Real-life algorithms use combinations of the above methods. 3 9 Polynomial: T=a +a V +a V +a V a V Nested (faster): T=a +V(a +V(a +V(a ))))))))) Small sectors (faster): T=T +bV+cV Lookup table: Fastest, most memory 1 2 3 9 1 2 3 2 035

Common Thermocouples mV deg C E R N K J S T Platinum T/Cs
Agilent Technologies Classroom Series Common Thermocouples mV deg C 20 40 60 E R N K J S T Platinum T/Cs Base Metal T/Cs 036 Probably the biggest advantage of using thermocouples lies in the huge variety of available metals that can be used to withstand hostile surroundings. Thermocouples can be divided into two main groups: Base metal thermocouples and noble metal thermocouples. The noble metal thermocouples, such as platinum, are obviously very expensive. They also exhibit a very small Seebeck coefficient, which also makes them difficult to measure. The base metal thermocouples come in all types and varieties. All have Seebeck coefficients in MICROvolts/deg.C 036

Common Thermocouples Seebeck Coeff: uV/C Type Metals J K T S E N
Agilent Technologies Classroom Series Common Thermocouples Seebeck Coeff: uV/C Type Metals J K T S E N 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 Type J, K, T, E and N are all base-metal thermocouples. Notice their high output signals, compared to the signal from a type S (noble-metal) thermocouple. Even though the output of the base-metal thermocouples is high compared with noble metal thermocouple, the magnitude of either is still quite small. The outputs are measured in microvolts per degrees C, which tends to create serious problems in a noisy factory environment. The nicrosil/nisil thermocouple is only a few years old, but it has gained rapid acceptance in replacing type K, because it is more rugged yet still inexpensive. 037

Extension Wires Possible problem here Large extension wires
Agilent Technologies Classroom Series Extension Wires Possible problem here Large extension wires Small diameter measurement wires 038 Sometimes it is necessary to extend the length of the thermocouple wire. In a factory environment it is impractical to try to pull a very small diameter thermocouple wire through a conduit, so a thermocouple extension wire is used in its place. The extension wire exhibits a thermocouple curve similar to that of the measurement thermocouple wire; however, it is also a possible source of error. Since the extension wire cannot exactly match the characteristic curve of the thermocouple wire, it is best to keep the extension/thermocouple junction near room temperature. Remember, according to the gradient theory, the bulk of the voltage signal will be generated in the circuit where the temperature gradient occurs. 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 Normal Mode: In series with input
Agilent Technologies Classroom Series Noise: DMM Glossary DMM Input Resistance Normal Mode dc SIGNAL ac NOISE Common Mode HI LO Normal Mode: In series with input Common Mode: Both HI and LO terminals driven equally 039 Since noise is a limiting factor in any thermocouple measurement, let's spend a little time talking about how to define and eliminate noise. First, let's look at a key definition concerning a digital voltmeter. A "normal mode" signal is any signal in series with the input. That means that normal mode "noise" is in series with the normal mode signal (the input signal) being measured. A "common mode" signal is one that is applied equally to both high and low terminals of the multimeter. A common mode noise is a noise that is applied equally to both high and low terminals. 039

Generating noise Large surface area, high Rlead: Max. static coupling
Agilent Technologies Classroom Series Generating noise DMM Input Resistance dc SIGNAL HI LO Electrostatic Noise Magnetic Common Mode ac source R lead R leak Common Mode Current Normal Mode Large surface area, high Rlead: Max. static coupling Large loop area: Max. magnetic coupling 040 How is noise generated? Normal mode noise can be induced by a magnetic current when the current cuts through the loop area of the signal. Electrostatic noise can be produced with coupling from a static source to the conductor or elements along the conductor. Common mode noise is typically created by an offset voltage between two grounds. A common mode current is generated which flows through any lead resistance in series with the unknown. This common mode current creates a voltage drop across the lead resistance, resulting in a "normal mode" noise signal. Large R lead, small R leak: Max. common mode noise 040

Eliminating noise DMM Input R Input R HI LO Electrostatic Noise Magnetic Common Mode ac source R leak Common Mode Current Normal Mode dc SIGNAL Filter, shielding, small loop area (Caution: filter slows down the measurement) 041 So how do you eliminate these noise sources? The easiest way to eliminate electrostatic noise is through the use of a shield. You can also use an input filter on the voltmeter, but this filter will of course increase the response time of the signal. Magnetic noise can be reduced simply by making the test lead loop area very small. Common mode currents can be reduced by making the leakage resistance from low to earth ground as close to infinity as possible. Another common method is to use a "guard", which can effectively increase the leakage resistance by several orders of magnitude. Make R leak close to 041

Magnetic Noise Magnetic coupling Induced I Minimize area Twist leads
Agilent Technologies Classroom Series Magnetic Noise Magnetic coupling DMM Input Resistance Induced I 042 So reduce magnetic coupling, minimize the area of the loop and also twist the leads of the thermocouple. Obviously, moving away from strong fields will also help reduce this noise. Minimize area Twist leads Move away from strong fields 042

Reducing Magnetic Noise
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 Twisting the leads will create equal and opposite induced currents in the thermocouple wire. The end result is a greatly minimized coupling of magnetic noise. Even when using twisted-pair, you should still try to minimize the area whenever possible and move the thermocouple wire as far away from the strong magnetic field as you can. 043

Electrostatic noise Stray capacitance causes I noise
Agilent Technologies Classroom Series AC Noise source Electrostatic noise Stray capacitances DMM Input Resistance Inoise 044 Electrostatic noise can be caused by capacitive coupling from an AC noise source such as a power line. Again, the resistance to ground from the voltmeter is very important. Increasing this resistance will reduce the amount of current caused by electrostatic coupling. Stray resistances Stray capacitance causes I noise DMM resistance to ground is important 044

Reducing Electrostatic Coupling
Agilent Technologies Classroom Series Reducing Electrostatic Coupling DMM Input Resistance Shield shunts stray current For noise coupled to the tip, Rleak is still important AC Noise source HI LO Rleak 045 The obvious way to reduce this electrostatic coupling is to use a shield. ONLY one end of the shield is grounded, and the electrostatic noise couples through the shield rather than through the measurement wire. The shield does not eliminate noise coupled directly to the tip of the thermocouple. For noise coupled directly to the tip, the leakage resistance on the voltmeter must be kept as high as possible. This can effectively be done by incorporating a guard on the voltmeter and connecting the guard lead to the tip of the thermocouple. 045

A scanning system for T/Cs
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 046 Typically, you need to measure more than one single channel of temperature. Here is a way to measure multiple channels attached to various types of thermocouples: One thermistor measures the temperature of the reference isothermal block. Each thermocouple is connected in sequence ('Multiplexed') to an isolation amplifier and an integrating A/D converter. The integrating converter eliminates some of the AC line-related noise associated with thermocouple measurement. To keep the leakage resistance as high as possible, an isolation method is used between the analog measuring circuitry and any grounded microprocessor or computer circuitry. The microprocessor uses a nonlinear algorithm to convert the measurements to actual temperature, for any type of thermocouple. Tell the computer the Reference Temperature and the type of thermocouple being used, and it can then perform the conversion. Good shielding techniques and twisted pair should also be used. Let's look at where some of the errors might occur in this system. OHMs Conv. Isolators I/O (HP-IB, RS-232) HI uP uP To Computer LO Integrating A/D ROM Lookup Floating Circuitry Grounded Circuitry 046

Errors in the system uP uP OHMs Conv. Ref. Block Thermal gradient
Agilent Technologies Classroom Series Errors in the system Ref. Block Thermal gradient T/C Calibration & Wire errors Thermal emf Ref. Thermistor cal, linearity Reference Thermistor Ohms measurement Extension wire junction error Linearization algorithm 047 Let's start at the top of the circuit. The relays used to switch the thermocouples can create thermal emf's on their own. It is not uncommon for a single reed switch to generate several microvolts. Five microvolts from the reed switch corresponds to an error of 0.1 oC for a base-metal thermocouple or 0.5 oC for a noble-metal thermocouple. The other source of error is the reference block thermal gradient. If there is a temperature difference across the reference "isothermal" block from one end to the other, it will be directly translated into a temperature measurement error. The wire error and calibration error of the thermocouple also directly impact the temperature measurement. It is not unusual to buy thermocouple wire that is specified to an error greater than 1 oC. The reference thermistor itself can be uncalibrated, or it can suffer from conversion error. If extension wire is used, there is a possible error at the junction between the extension wire and the actual thermocouple wire. Now let's look at the measurement itself. The measurement of the reference thermistor can add directly to the measurement error of the thermocouple. That means that the ohms converter must be accurate. Also, any offset or linearity errors in the digital multimeter will add to the thermocouple error. Finally, the conversion algorithm must be accurate and must be correct for the type of thermocouple that we are using. You can see from this list that accurately measuring temperature is very difficult. OHMs Conv. Isolators I/O (HP-IB, RS-232) HI uP uP LO Integrating A/D ROM Lookup Floating Circuitry Grounded Circuitry 047 DMM offset, linearity, thermal emf, noise

Physical errors Shorts, shunt impedance Galvanic action Decalibration
Agilent Technologies Classroom Series Physical errors Shorts, shunt impedance Galvanic action Decalibration Sensor accuracy Thermal contact Thermal shunting 048 Those are the electrical errors that can take place, now let's look at the physical errors. 048

Physical Errors Hot spot causes shunt Z, meter shows the WRONG temperature Water droplets cause galvanic action; huge offsets 049 Any the droplets of water that seep into the thermocouple sleeve can cause galvanic action which means large voltage offsets. Alternatively, a "hot spot" which causes the thermocouple wires to accidentally touch each other, or causes resistive coupling between the wires can not only indicate an error, but can actually indicate the temperature at the wrong location on the thermocouple wire. Anytime the thermocouple typical range is exceeded, there can be a permanent offset created. For example, if you expose a type J thermocouple to greater than 760 oC, a permanent magnetic shift will occur inside the structure of the thermocouple. Since real thermocouples have an absolute accuracy that is not very good, you should calibrate them often and use extra precautions. Exceeding the T/C's range can cause permanent offset Real T/C's have absolute accuracy of 1 deg 25C: Calibrate often and take care 049

Physical error: Thermal contact
Agilent Technologies Classroom Series Physical error: Thermal contact Surface probe 050 As we mentioned before, make sure that the thermal mass of the device you are measuring is much larger than the thermal mass of the sensor being used to measure it. Allow sufficient time for both the sensor and the measured object to reach their final temperature. Use the proper type of probe, for example a surface probe, to measure surface temperature. Make sure thermal mass is much smaller than that of object being measured 050

Physical errors: Decalibration
Agilent Technologies Classroom Series Physical errors: Decalibration 350 C 300 C 975 C 200 C 1000 C 100 C 051 Constant temperature cycling can cause work hardening of the metal in the thermocouple and destroy its calibration. For example, in this autoclave the temperature is 1000 oC. The section of the wire that undergoes the gradient is not at the tip. The section of the wire that undergoes the major gradient is at the wall of the autoclave. If this thermocouple should become decalibrated, it would do no good to replace only the tip of the thermocouple. You must replace the entire section of the thermocouple wire that goes all the way back through the wall. This section produces the ENTIRE signal Don't exceed Tmax of T/C Temp. cycling causes work-hardening, 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 Now let's summarize what we have learned, and look at a few examples. A7

The basic 4 temperature sensors
Agilent Technologies Classroom Series The basic 4 temperature sensors AD590 Expensive Slow Needs I source Self-heating 4-wire meas. RTD Most accurate Most stable Fairly linear Thermistor High output Fast 2-wire meas. Very nonlinear Limited range Fragile I.C. Most linear Cheap Limited variety Needs V source Absolute temperature sensors Thermocouple Wide variety Cheap Wide T. range No self-heating 052 So we have already looked at the three types of absolute measuring devices: the RTD, the thermistor and the IC. Let's see how the thermocouple stacks up against these three. The thermocouple is useful in more types of atmospheres and over wider temperature ranges. Since you do not have to put current through the thermocouple to measure it, it has no self-heating and this can be extremely important. Because it has a very small output voltage, the thermocouple is hard to measure and is susceptible to noise, both magnetic and electrostatic. It only measures relative temperature, not absolute temperature. It is non-linear, and it requires either special connectors that are built with the same metal as the thermocouple itself, or a special "isothermal" junction. Hard to measure Relative T. only Nonlinear Special connectors 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 054 So far we have learned that temperature is a very difficult, mostly empirical measurement and requires careful attention to detail in order get accurate results. It is interesting to note that the names "Galileo" and "Fahrenheit" are rarely used in modern temperature thermometry. The moral: innovation in and of itself is not enough. We must develop standards that are easily repeatable and accurate, and develop measuring devices that everyone can use. 053

Examples Measurement Sensor Photochemical process control:
Agilent Technologies Classroom Series Examples Photochemical process control: Flower petal: Molten glass: Induction furnace: 100 degree Heat aging oven: Measurement Sensor RTD (most accurate) Thermistor (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 In closing, let's try to determine what type of sensor might be best to make a typical measurement. Suppose you work for Kodak, Fuji or Polaroid and you want to control the chemical process used to develop film. What type of sensor would you use? Probably an RTD, since it gives the most accurate answers. Now suppose you want to measure the temperature of the flower petal. A flower petal has an extremely low thermal mass, so we would probably use a thermistor. The thermistor is also appropriate since the required temperature range is small. Let's try to look into a furnace and measure the temperature of some molten glass. The proper tool for this is probably an optical pyrometer. The pyrometer is excellent at measuring very high temperatures and does not require any contact with the device being measured. The one thing we will have to watch out for is the emissivity of the glass. The proper emissivity must be used to calibrate the pyrometer. Now let's measure some molten aluminum inside an induction furnace. This is a tough one, since almost any sensor that we use that is an electrical sensor will be susceptible to electrostatic and magnetic noise. We would probably choose an RTD, but we would have to be very careful to filter it, to twist the leads, and do anything that we could do to reduce magnetic noise. The Optical Pyrometer could also suffice. Suppose you worked for an electronics manufacturer, and you want to plot the heat distribution inside a heat-aging oven that runs about 100 degrees C. Almost any of the sensors would work. If you have access to a data acquisition system, you might choose the thermocouple. If your only test equipment was a voltmeter with no internal linearization, you would probably choose an integrated circuit sensor, but this would of course mean connecting a voltage supply in order to get an answer. 054

Practical Temperature Measurements

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Practical Temperature Measurements

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