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Temperature Basics Bill Bergquist, Sr. Applications Engineer Jeff Wigen, National Sales Manager Jeff Bill © Burns Engineering Temperature Basics Agenda 2 Temperature Defined Temperature Scales Types of Temperature Sensors Thermocouple Types Temperature ranges Lead wire colors Standards Temperature coefficient Construction Calibration Interchangeability Standards RTD Identification of Sensor Types Sensor Selection - The Four Ps Calibration Preventive Maintenance © Burns Engineering Temperature Basics Temperature 3 What is temperature? Measure of the average kinetic energy of particles in a substance. Temperature is the result of the motion of particles. Temperature increases as the energy of this motion increases. Physical quantity that is a measure of hotness and coldness on a numerical scale. © Burns Engineering Temperature Basics Temperature 4 Temperature of some things: ¾ Ice 32°F ¾ Dry ice −78.5°C (−109.3°F) at atmospheric pressure ¾ Liquid nitrogen -196°C ¾ “Fahrenheit 451” (Ray Bradbury) approximate auto-ignition point of some paper ¾ Sun 5700K (5430°C) ¾ Earth’s core 5700K (5430°C) (cooling at 100°C per billion years). The core may contain enough gold, platinum, and other siderophile elements that if extracted and poured onto the Earth's surface, would cover the entire Earth with a coating 0.45 m (1.5 feet) deep. ¾ Lightning bolt 50,000K ¾ Earth’s average surface temperature – good luck trying to figure out that one! ¾ In photography K (Kelvin) is used to denote color temperature. Match flame is about 1700K, sunshine is about 5400K. © Burns Engineering Temperature Basics Temperature Scales 5 Celsius Swedish astronomer Anders Celsius (1701–1744) By international agreement the unit "degree Celsius" and the Celsius scale are currently defined by two different temperatures: absolute zero, and the triple point of VSMOW (Vienna Standard Mean Ocean Water). Triple point of water is defined as 273.16K or 0.01°C, the point at which water exists as a vapor, solid, and liquid. A degree Celsius (or a Kelvin) is what you get when divide the thermodynamic range between absolute zero and the triple point of water into 273.16 equal parts. In 1948, it was renamed Celsius because centigrade had other meanings in Spanish and French. © Burns Engineering Temperature Basics Temperature Scales 6 Kelvin One of the seven base units in the SI system of units named after the Belfast-born, Glasgow University engineer and physicist William Thomson, 1st Baron Kelvin (1824–1907) Based on absolute zero which is 0K, or −273.15°C or −459.67°F. Theoretical point at which all thermal motion stops. Fahrenheit Temperature scale based on one proposed in 1724 by the physicist Daniel Gabriel Fahrenheit (1686–1736) freezing of water into ice is defined at 32 degrees, while the boiling point of water is defined to be 212 degrees — on Fahrenheit's original scale the freezing point of brine was zero degrees. © Burns Engineering The Celsius scale is widely used in industry with the Fahrenheit scale a close second. Kelvin is almost exclusively used for scientific and laboratory measurements. Temperature Basics A Kelvin or K does not have a degree symbol. It is used by itself such as 273K. On calibration reports you will see the calibration uncertainty expressed in millikelvin or mK so 1mK is 0.001K. Various Temperature Sensors • • • • • • 7 There are other types of sensors that have little practical use or are still in development such as the Johnson Noise Thermometer, and a device that uses a laser as a measure. Liquid displacement thermometers Dial thermometers Thermistors Optical (DTS, IR) Thermocouples RTD’s RTDs and Thermocouples Dial © Burns Engineering Thermistors Infrared Bimetal coil Temperature Basics Thermometers 8 Liquid Displacement Dial Both have . . . Limited temperature ranges Limited accuracy Industrial integrity Introduce human error © Burns Engineering Temperature Basics Thermistors • • • • • • • 9 Very precise Ceramic, silicon, or polymer resistor High resistance values Very limited temperature range Often have drift error Self-heating errors Low price © Burns Engineering Mercury-in-glass is probably the most well known liquid displacement type of thermometer. Other types of fluid are used with some used to deflect a needle that indicates temperature. Temperature Basics As temperature increases the resistance increases and can be correlated to a temperature. Thermocouples 10 As temperature increases, the measured voltage increases and does so in a predictable manner. The thermocouple standards define the voltage vs. temperature relationship for several types of thermocouple junctions. Made by connecting two different metals to form a closed circuit. High durability Low initial cost Voltage change with temperature + - © Burns Engineering Temperature Basics Thermocouples 11 2500 2156 2000 1500 Temp 1600 1400 1200 1000 700 F 500 32 32 32 -450 -328 -328 Type T Type E Thermocouples © Burns Engineering -328 Industrial RTD Temperature Basics Thermocouples 12 Types Most common • • • • Type T: Copper-Constantan Type J: Iron-Constantan Type E: Chromel-Constantan Type K: Chromel-Alumel Red/Blue Red/White Red/Purple Red/Yellow High temperature types • R, S, B Platinum/Platinum-Rhodium • W3, W5 Tungsten/Tungsten-Rhenium 2640°F 4200°F Each type has a different temperature range and Voltage vs. Temperature relationship ASTM Standard E230 , ANSI MC 96.1, and IEC 584-3 © Burns Engineering Temperature Basics Thermocouples 13 Junctions can be grounded, ungrounded or exposed. © Burns Engineering Temperature Basics Thermocouples Advantages Very wide temperature Range [1.2K to 2300°C] Fast Response Time Available in small sheath sizes Low initial cost Durable © Burns Engineering Disadvantages Decreased accuracy vs. RTDs More susceptible to RFI/EMI Recalibration is difficult Requires expensive TC wire from sensor to recording device Difficult to verify Not as stable as RTDs Temperature Basics Resistance Temperature Detector RTDs 3 common element styles Coil in the hole Wire wound Thin film Resistor made from platinum, nickel, copper, or other metals © Burns Engineering Thermocouples are typically used in an application where high temperature or high vibration is present. Their durability is probably their best asset. Temperature Basics 15 The platinum resistance thermometer (PRT) is the most widely used sensor type in applications where highly accurate, repeatable and stable measurements are required. Other metals are used but none have the wide temperature capability of platinum. RTD 16 Basic Operation Resistance changes with temperature. As temperature increases, resistance increases in response. Small current is sent through the resistor element and electrical resistance is measured Performance defined by IEC 60751 and ASTM E1137 © Burns Engineering Temperature Basics RTD 17 Basic Operation Temperature coefficient • Also called the Temperature Coefficient of Resistance or alpha • Units are ohms/ohm/°C • The average change in resistance per unit change in temperature between 0 and 100°C • α = R100 - R0 / 100°C*R0 » R0 = resistance at 0°C » R100 = resistance at 100°C For the current industry standard 100 ohm RTD the alpha is .00385 which means at 100°C the nominal sensor resistance is 138.5 ohms © Burns Engineering Temperature Basics RTD 18 Most common coefficients • 0.00385 – ASTM E1137 or IEC 60751 • 0.003902 – American • 0.003916 – JIS • 0.003925 - SPRT, Secondary SPRT Must match your instrument to the proper temperature coefficient of your sensor © Burns Engineering Temperature Basics When specifying an RTD it is necessary to select the correct temperature coefficient to match the readout instrument or controller. Failure to do so will result in an inaccurate measurement. RTD 19 Coefficient example A temperature is being measured with a sensor having a temperature coefficient of .003916 (JIS) but due to a sensor failure it was replaced with a sensor having a temperature coefficient of .00385. If the transmitter/controller is not recalibrated, at 100°C it will read 1.7°C low. © Burns Engineering Temperature Basics Interchangeability 20 The two main standards in use today are the IEC 60751 and the ASTM E1137. Both have the same nominal R vs. T values but differ in defining some performance characteristics and the tolerances associated with the grades or classes of sensors. The interchangeability tolerances are the target that manufacturers shoot for when building the sensing elements. RTDs are manufactured to have 100 ohms at 0°C. Interchangeability refers to the “closeness of agreement” between an actual PRT R vs. T relationship and a predefined R vs. T relationship. Defined by ASTM E1137 and IEC 60751 Nominal R vs. T of ASTM and IEC standards are equivalent but tolerances are different. © Burns Engineering Temperature Basics Interchangeability 21 4 IEC Class B ASTM Grade B 3 2 IEC Class A ASTM Grade A Tolerance (±°C) 1 0 -300 -200 -100 0 100 200 300 400 500 600 700 ASTM Grade A -1 IEC Class A -2 ASTM Grade B -3 IEC Class B -4 Temperature (°C) © Burns Engineering Temperature Basics 800 Note that the ASTM standard has slightly tighter tolerances for the two grades of sensors. All RTDs are built with the tightest tolerance at 0°C and as the temperature diverges from 0°C the tolerance increases. The vertical line on the graph represents 0°C and the tolerance on the y axis is expressed in ± °C from nominal. Interchangeability 22 Standard Tolerance Defining Equation¹ ASTM E1137 Grade A ± [ .13 + 0.0017 | t | ] ASTM E1137 Grade B ± [ .25 + 0.0042 | t | ] Class AA2 ± [ .1 + 0.0017 | t | ] IEC 607512 IEC 60751 Class A ± [ .15 + 0.002 | t | ] IEC 60751 Class B ± [ .3 + 0.005 | t | ] Class C2 ± [ .6 + 0.01 | t | ] IEC 607512 These equations can be used to calculate the interchangeability at any temperature. Note that the temperature t is an absolute value in °C. The resultant is the interchangeability in ± °C. Note 1: | t | = absolute value of temperature of interest in °C Note 2: These tolerance classes are included in a pending change to the IEC 60751 standard. © Burns Engineering Temperature Basics RTDs Advantages Very stable output Linear and predictable Easy to verify and recalibrate High accuracy No special wires required for installation © Burns Engineering Disadvantages More limited temperature range [-200°C to 500°C] High initial price Slower response time than a thermocouple Less durable than a thermocouple Temperature Basics Identification 24 RTD or Thermocouple? Lead wires • RTD has two, three, or four leads per sensing element • TC has two leads per junction • RTDs typically have red, white, green or black leads (not defined by the standards) • TC colors match thermocouple type – red (common), yellow, purple, blue, white Resistance check • RTD will have about 109 ohms between leads • TC typically less than 1 ohm Continuity check • TC grounded junction has path from leads to case Magnet test • RTD leads are not magnetic – usually copper • TC type J has one iron lead which is highly magnetic © Burns Engineering Temperature Basics Selection and Application 25 How do we decide which technology to use? Thermocouple Exhaust gas Injection molding Bearings Refinery RTD Pharmaceuticals Fuel custody transfer Chemical Tire /rubber © Burns Engineering Accuracy and stability are almost always preferable characteristics to have in a temperature sensor and for that reason I recommend that an RTD be selected unless the environment or process characteristics dictate another technology. Those factors are discussed on the following slides. Temperature Basics Selection 26 Factors to consider Placement Protection Performance Price Service life © Burns Engineering There are other factors to consider other than these but for most applications these will get you the measurement and service life from the sensor that you desire. Temperature Basics Placement 27 Two options Surface mount Immersion © Burns Engineering For a pipe there are two options, surface mounted or immersion. The two parts on the left are just two of a wide variety of surface mount sensors available and the two on the right are examples of the two most common styles of immersion sensors. Temperature Basics Placement 28 Surface mount © Burns Engineering Installation of a surface mount sensor can be accomplished with a hose clamp, tape, or an adhesive. Temperature Basics Placement 29 A few additional styles of surface mount sensors. Whether RTD or thermocouple they almost all look alike but are very different inside. © Burns Engineering Temperature Basics Placement 30 Surface mount - positives Easy installation No flow obstruction Low cost Surface mount - negatives Requires insulation for best accuracy Minimal protection from ambient conditions Difficult to calibrate Measures pipe surface © Burns Engineering Temperature Basics Installation is very easy to do and is low cost. Accuracy suffers though and measurement accuracy requirements may not be achieved. Placement 31 Immersion © Burns Engineering An immersion sensor overcomes the negatives of the surface mount and in most cases dramatically improves the measurement accuracy. Temperature Basics Placement 32 Immersion (no thermowell) Fast response Low cost Short immersion Direct immersion of a small diameter sensor gives an accurate measurement at low cost. It is not always possible or desirable though because of maintenance or durability considerations. Limitations Durability Maintenance Strength © Burns Engineering Temperature Basics Protection 33 Process connection These are just a few of the several styles available. Any piping connection either has been or could be adapted to a thermowell process connection. © Burns Engineering Temperature Basics Protection 34 Connection heads Attach extension wires Protect sensor from ambient conditions © Burns Engineering A connection head is the best method and provides a convenient place to attach lead wires or to house a local transmitter. Temperature Basics Protection 35 Numerous styles and materials from plastic to aluminum are available. Some carry ratings for use in hazardous atmospheres. © Burns Engineering Temperature Basics Protection 36 Transmitters © Burns Engineering Heads provide protection for transmitters and local indicators. Temperature Basics Protection 37 Hazardous atmosphere © Burns Engineering Hazardous atmospheres require an RTD and connection head assembly that carry an appropriate rating. A word of caution -- just the addition of a rated head to any sensor does not make the whole assembly rated. The entire assembly must be tagged and identified as having the desired rating. Temperature Basics Protection 38 Options Nipple Nipple - coupling Union • Calibration • Replacement © Burns Engineering Head is attached to the thermowell and sensor typically with a pipe nipple. The most versatile is the union connection. It allows easy removal of the sensor from a thermowell. Temperature Basics Performance Consideration 39 Thermocouple RTD Accuracy at 32°F Standard limits: ± 4°F* Special limits: ± 2°F* Grade B: ± 0.54°F (± .3°C) Grade A: ± 0.27°F (± .15°C) Calibration Limited to in-situ calibration - Easily recalibrated for longer service life and traceability - Matching transmitter improves performance Stability Dependent on wire homogeneity and process conditions Average drift is ± 0.06°C after 1000 hours at 400°C. Repeatability Highly dependent on process characteristics Less than ± 0.04% change in ice point resistance after 10 cycles -200 to 500°C. *Types J and K. Types T and E special limits are ± 0.9°F © Burns Engineering Temperature Basics Here are some general performance specifications for RTDs and thermocouples. As you can see the RTD has quite a large accuracy advantage over the thermocouple. Performance 40 High accuracy insures product quality and efficient use of your energy dollar. Cost of inaccuracy • • • • Process Fluid: Flow Rate: Control Temperature: Energy Cost: Water 25 GPM 100 °F 2.9¢ / KW-hour Annual Cost of Energy Per °F Error $923 / year © Burns Engineering Temperature Basics Performance 41 Time response TIME RESPONSE Direct Immersion RTDs 2.5 seconds A tapered thermowell will have a 3 to 4 times slower response than the ¼” diameter direct immersion sensor. This can be a big factor in accuracy for processes that are changing temperature rapidly. The sensor needs to be fast enough to keep up with the process. DIAMETERS 1/4” - 1/8” 4 to 6 seconds 1/4” 6 to 8 seconds 1/2” - 1/4” Thermowells 22 seconds Stepped 26 seconds Tapered © Burns Engineering Temperature Basics Performance 42 Time response – surface mount more dependent on housing style and mounting surface than immersion types Surface Mount RTDs Ranges from 8 to 67 seconds Thermowells Surface Mount TCs 8 to 67 seconds depending on junction type © Burns Engineering Temperature Basics Surface mount sensors will have a slower time response that an immersion style. They are affected more by the surface they are mounted to rather than by the sensor design. Performance 43 Transmitters Converts resistance or voltage to a variable current of 4 to 20 ma Lead wire > 250 feet (+0.16°F/100 ft) Accuracy Matching Lead wire Robust signal RFI/EMI Local display © Burns Engineering Adding a transmitter can improve accuracy when a long run of lead wire is required. They also provide a more robust signal that is less susceptible to interference from electromagnetic or radio frequency interference. Temperature Basics Selection Guide 44 Situation Thermocouple Wire Wound RTD Accuracy/Stability X Mod. Temp (-50 to 200°C) X High Temp (-200 to 500°C) X Higher Temp (over 500°C) X Time Response (< 6 sec.) X Long-term Stability X X X Extra High Vibration, Shock X X X Critical Temp. Application Temperature Basics What is Calibration? 45 • Calibration is performed to verify sensor/instrument performance. • Calibration is the process used to insure that a sensor/instrument maintains specification over time and changing ambient conditions. • Calibration is the process used to maintain traceability of parameters with reference to national/international standards. © Burns Engineering X X High Vibration (g level) © Burns Engineering Thin Film RTD Temperature Basics Why 46 Initial Calibration New plant or equipment commissioning Verify vendor data – shipping and installation damage • Insure accuracy of measurements Recording data Calibration should be performed when starting up a new facility or if a new piece of equipment is added. This insures that the instruments have not been damaged during shipping or installation and provides a baseline for comparison to subsequent calibrations. Ongoing Calibration Minimize and control random and systematic errors Compare and complement the quality and reliability of measurements by comparison to international standards • Provide traceability to national standards, (e.g. NIST) • Meet Regulatory Requirements (FDA, NRC) Quality System requirements • Ensure consistent product quality • Safety Cost • Poor accuracy = wasted $$ © Burns Engineering Temperature Basics When 47 RTD has drifted How do you know? Frequency Process dictates the calibration cycle Probe drift • Vibration • Shock • Temperature • Cycling Product value Complete 5 cycles w/o shift then double the interval © Burns Engineering Temperature Basics When? Is one of the most frequently asked questions about calibration. There is no generic answer for when; it all depends on the process and comfort level of the invested parties. Process conditions that affect the probe drift rate and the product value are the two main considerations. Consult with the manufacturer(s) of your system and they should be able to help you estimate the long term accuracy of their equipment based on your process conditions. After that it’s up to you to pick a frequency that meets your comfort level and that of any 3rd party watchdogs that oversee your production facility. You may decide because of product value that a calibration is performed before and after each batch. Then if you desire, after 5 cycles are completed without a significant shift, the calibration cycle could be doubled. Terminology • • • • • • • • • • • • 48 ITS-90 = International Temperature Scale of 1990 IPTS-68 = International Practical Temperature Scale TPW – triple point of water 0.01°C or 273.16 K R0 = resistance at 0°C SPRT = standard platinum resistance thermometer Dewar = insulated container IR = insulation resistance K – Kelvin temperature scale (used for ITS-90) mK or milliK = .001 K NIST – National Institute of Standards and Technology NVLAP – National Voluntary Laboratory Accreditation Program A2LA – American Association for Laboratory Accreditation © Burns Engineering Temperature Basics How - Temperature Scales Evolution of standard temperature scales IPTS-27 IPTS-48 IPTS-68 49 Beginning in 1927 the International Bureau of Weights and Measures decided that a better standard was required for temperature and the International Practical Temperature Scale was born. Since then about every 20 years the scale has been refined to improve accuracy. In 1990 the name changed to International Temperature Scale and the equations defining the R vs. T relationship became more accurate. ITS-90 ITS-90 (International Temperature Scale) Released in 1990 The official international scale In better agreement with thermodynamic values than the IPTS-68 ITS-90 vs. IPTS-68 ITS-90 • Uses TPW • Most accurate • Complex equations IPTS-68 • Simpler equations • Less accurate • Callendar-Van Dusen equation © Burns Engineering Temperature Basics How - Calibration Options Factory Calibration Options Matched Calibration • Matched with other probes • Matched to a transmitter Multiple Point Calibration • -196, -38, 0, 100, 200, 300, and 420 °C Matched to a Temperature Readout (meters) © Burns Engineering Temperature Basics 50 Calibrating an RTD and adjusting the readout or transmitter accordingly is a cost effective method to improve measurement system accuracy. This eliminates most of the RTD interchangeability tolerance and can also minimize other instrument errors inherent in the system. Calibration 51 Ice Bath • Easy to Produce • Uncertainty ±.002°C © Burns Engineering The ice bath is the easiest and most accurate method of checking an RTD. Addition of a stirring motor insures even temperature throughout the insulated Dewar. Ice is made from pure water, crushed, and packed into the Dewar. Purified water is added to fill in the gaps. Too much water and the ice will float which is not desirable. Temperature Basics Calibration 52 Triple-Point-of-Water Cell © Burns Engineering The triple point of water (TPW) cell may be the most commonly used type of fixed point and is used in ITS-90 calibrations. Water can exist as a solid, liquid, and vapor at 0.01°C and this device creates this temperature. Temperature Basics Comparison Calibration 53 Most common method Comparison of unknown to known sensors Multiple sensors can be calibrated at the same time Equipment Meter, Standard PRT, Recorder, etc. (system) • All add to uncertainty level The standard PRT should have an accuracy at least four times greater than the unit under test © Burns Engineering Temperature Basics Comparison Calibration 54 More practical and less expensive than fixed point temperature calibration Laboratory Typical uncertainty: 0.001°C to 0.01°C Very high accuracy reference resistance bridge, standard PRT, calibration baths, etc. • Uses some fixed point temperatures Field Typical uncertainty: 0.05 to 0.5°C Accurate reference meters, secondary PRTs, baths or dry-wells Instruments are field compatible © Burns Engineering Temperature Basics Standard PRTs 55 Specifications Very fragile Use mainly in laboratory environments Highest accuracy, high repeatability, low drift -328 to 1983°F (-200 to 1084°C), accurate to ±.0018°F (±.001°C) © Burns Engineering Comparison calibrations can be performed in a laboratory or in the field. High accuracy can be obtained with careful selection of equipment. Durability is as important as accuracy when used for field calibrations. Equipment that cannot stand up to field use will drift quickly and not give the expected measurement uncertainties. This is NOT the type of device to use for field calibrations. It is extremely fragile and very expensive, about $10k with calibration. Temperature Basics Anatomy of an SPRT 56 Photo of an SPRT element inside its quartz sheath. © Burns Engineering Temperature Basics Anatomy of an SPRT 57 A little closer look. © Burns Engineering Temperature Basics Anatomy of an SPRT 58 Oooops Yes, the quartz sheath does break very easily and is one of the few things duct tape won’t fix! -$10K © Burns Engineering Temperature Basics Preventive Maintenance 59 Corroded terminals can cause high resistance in the leads 3-wire circuits are susceptible – accuracy depends on each conductor having exactly the same resistance Terminals clean and tight Terminal block clean and dry, secured to head Wires are tinned, or terminated with spade lugs Connector pins connect firmly and are clean Use gold plated pins in a high quality connector 4-wire circuits also compensate for some poor maintenance Compensate fully for all lead wire resistance in the circuit © Burns Engineering Temperature Basics Preventive Maintenance The good © Burns Engineering 60 and bad The connection head shown in the lower right corner had a lot of dust in it that caused some electrical leakage between the terminals resulting in a bad temperature reading. Temperature Basics Preventive Maintenance 61 RTD Fan blowing on sensor location Radiated heating or cooling from nearby equipment Insulation covering the external portions of sensor Sunlight - solar heating right where you don’t want it Thermowell Bore cleaning Heat transfer compound Product buildup on wetted portion Cracks in flange weld or leaky gasket RTD bottoms in well and spring loads Controller RTD temperature coefficient is set correctly in controller 3 or 4 wire circuit connected correctly with correct wire type Transmitter Wires connected securely Check output at zero and span © Burns Engineering Temperature Basics Preventive Maintenance RTD Check insulation resistance 50 VDC > 100 megohms at 25°C Check ice point resistance 100 ±0.12 ohms or ±0.06 ohms RTD and transmitter matching Frequency of checks – process dictates the intervals Connection head Wire insulation Shielding General condition, corrosion, discoloration, threads, cracks Corrosion on terminal connections Water inside the connection head Conduit seal for hazardous atmospheres Transmitter Wires connected securely Check output at zero and span © Burns Engineering Something as innocuous as a fan blowing can cause a measurement error if it is directed on the external portions of the temperature assembly. Temperature Basics 62 Replacement Checklist 63 RTD Choose the correct temperature coefficient. Most common is a .00385 conforming to IEC 60751 or ASTM E1137 Interchangeability – choose class A for better accuracy 3 or 4 wire – 4 wire provides better accuracy Choose correct length to match thermowell or provide significant immersion to avoid stem conduction – for a direct immersion probe minimum immersion = 10x probe diameter + sensitive length Thermowell selection Corrosion Erosion Wake frequency and strength Time response Immersion length © Burns Engineering Temperature Basics Summary There are many types of temperature sensors. RTD and thermocouple are the most widely used in industry. RTD is the most accurate and TC is the most durable When selecting a sensor remember the 4 Ps Performance Placement Protection Price Periodic calibration is necessary to maintain measurement accuracy © Burns Engineering Temperature Basics BE educated Watch for upcoming RTDology® events View presentation notes from previous sessions on our website at: www.burnsengineering.com/RTDology © Burns Engineering Temperature Basics
