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sensors:temperature [2018/06/06 13:23]
admin [Devices]
sensors:temperature [2018/11/01 18:55]
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-====== Temperature ====== 
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-===== Summary ===== 
-==== Introduction ==== 
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-There are four main contact temperature-sensing devices available, divided in three families: thermocouples (self-generating sensors), resistance temperature detectors and thermistors (resistive sensors), and temperature-transducing ICs (PN or Semiconductive). These sensors translate the temperature into a reference voltage, resistance or current, which is then measured and processed and a numerical temperature value is computed. 
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-==== Types of temperature sensor ==== 
-=== Thermocouples === 
-Thermocouples are a physically simple sensor, though how they function is more complex. ​ Figure 1 shows the basic wiring of a thermocouple. ​ A thermocouple is comprised of two dissimilar alloys (wires A and B) joined at one end, called the "hot junction"​ (T1).  The other leads are connected to a voltmeter or other input device that measures the voltage (V1) across the '"​cold junction"​ (T2).  ​ 
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-The hot junction is the sensing element, and the cold junction is kept at a constant reference temperature. ​ A voltage is produced as the hot junction is heated, which is proportional to the temperature difference between the two junctions. ​  This principle, called the thermocouple effect was discovered by Thomas Seebeck in 1821 (Awtrey, 2001). ​ And so, the electromotive force (EMF) produced when the junctions of dissimilar alloys are maintained at different temperatures is known as the Seebeck EMF.  
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-{{:​sensors:​thermocouple.jpg|}} 
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-Fraden (1997) defines three laws for proper connection of thermoelectric materials : 
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-Law No.1 (A) - A thermoelectric current can not be established in a homogeneous circuit by heat alone. (LAW OF HOMOGENEOUS CIRCUITS) 
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-Law No.2 (B) - The algebraic sum of the thermoelectric forces in a circuit composed of any number and combination of dissimilar materials is zero if all junctions are at a uniform temperature. ​ (LAW OF INTERMEDIATE METALS) 
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-Law No.3 (C) - If two junctions at temperature T1 and T2 produce Seebeck voltage V2, and temperatures T2 and T3 produce voltage V1, then temperatures T1 and T3 will produce V3 = V1 + V2. (LAW OF INTERMEDIATE TEMPERATURES) ​ 
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-{{:​sensors:​thermocouples_laws.png|}} 
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-The theory behind the thermocouple and thermoelectric effect is based upon the atomic structure of the alloys and is beyond the scope of this report. ​ The voltage is also dependant upon the type of conductors used.  Different alloys produce distinct voltages; therefore standards have been established to facilitate reliability and repeatability. ​ There exist eight standardized alloy combinations,​ each referenced by a letter: B, E, J, K, R, S, T, and N.  The following table describes the main characteristics of these combinations (www.capgo.com) 
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-{{:​sensors:​table_thermocouples.png|}} 
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-The Seebeck EMF produced by a thermocouple is of such small scale that the voltage must be amplified and processed by a specialized thermocouple input module. 
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-Thermocouples are calibrated with a cold junction temperature of 0°C.  However, two problems arise when connecting thermocouples to their input device: ​ firstly, the input terminals, which are constructed with a different type of metal, create their own Seebeck voltage which alters the actual thermocouple voltage; and second, the device has to be recalibrated for an operational cold junction temperature. ​ With the advancements in technology over the past few decades, these input modules have been designed to be self-calibrating and self-compensating and are able to be configured for a variety of thermocouple types. 
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-===  Resistance temperature detectors (RTDs) ​ === 
-The same year Seebeck discovered the principle behind the thermocouple (1822), Humphrey Davy announced that the resistivity of metals were highly influenced by temperature. ​ Two major types of RTDs are available for industrial and commercial use : wire-wound and thin film.  (www.sensortips.com) 
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-{{:​sensors:​rtd_6.png|}} 
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-RTDs function on the principle that as the sensing element is heated, the resistance of the metal wire increases proportionally. ​ RTDs are commonly made with copper, nickel, or nickel-iron,​ but platinum RTDs are the most linear, repeatable, and stable. ​ The resistance is almost a linear function of temperature for very pure platinum, which is the primary reason for this metal’s pervasiveness in RTDs.  
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-RTDs are calibrated to exhibit a resistance of 100 Ω at 0°C.  Their resistance at other temperatures depends on the value of the mean slope of the metal’s resistance-temperature plot, known as the //constant alpha//​. ​ Alpha is dependant upon the platinum’s purity. ​ Although RTDs are fairly linear, advanced RTD input devices use software with curve fitting and software processing to increase their [[accuracy]] at higher temperatures. 
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-===  Thermistors ​ === 
-Thermistors,​ like RTDs, vary their resistance as the ambient temperature is changed. ​ Unlike RTDs, which use pure metals, the material used in a thermistor is generally a ceramic or polymer. ​ Positive temperature coefficient (PTC) thermistors will show an increase of resistance with increasing temperature,​ while negative temperature coefficient (NTC) thermistors will show a decrease of resistance with increasing of temperature. ​ Thermistors typically achieve a higher precision than RTD within a limited temperature range (usually −90 °C to 130 °C).  (www.sensorsmag.com) 
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-{{:​sensors:​graphntc.png|}} 
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-Comprised of a metal oxide ceramic semiconductor sensing element, thermistors are notorious for their non-linearity,​ which engineers often dampen by implementing pairs of offsetting thermistors,​ providing a more linear output. ​ These temperature dependent resistors are highly sensitive to temperature change. ​ Thermistors vary their resistance about -4.4% at 25°C when heated by one degree Celsius (Ogden, 2000). ​ Since thermistors are resistive devices, in operation an electrical current is passed through the sensor. ​ Some of this electricity is converted into heat, which may cause slightly higher than ambient temperature readings. ​ Thermistors can operate without significant error with long lead wires, because of their high base resistance. ​ Thus they can be installed at long distances, upwards of one hundred metres, from the input module. ​ Thermistor resistances are non-standardized and vary from 100 to 1,000,000 Ω at 25°C (Ogden, 2000). 
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-===  Temperature-transducer ICs  === 
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-Semiconductor temperature sensors are produced in the form of ICs. Their design results from the fact that semiconductor diodes have temperature-sensitive voltage vs. current characteristics. ​ When two identical transistors are operated at a constant ratio of collector current densities, the difference in base-emitter voltages is directly proportional to the absolute temperature. ​ 
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-The use of IC temperature sensors is limited to applications where the temperature is within a –55° to 150°C range. The measurement range of IC temperature sensors may be small compared to that of thermocouples and RTDs, but they have several advantages: they are small, accurate, and inexpensive. ​ 
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-Temperature sensing ICs are available either in analog form, which output a voltage or current which is proportional to the temperature,​ or digital, which communicate temperature over a digital communication line, such as one-wire PWM, two-wire I2C, or a multiple wire SPI connection. 
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-=== Projects That Use Temperature Sensors === 
-The [[http://​www.tii.se/​reform/​projects/​pps/​soniccity/​index.html\|Sonic City]] project developed a wearable system that creates music based on data from sensors measuring bodily and environmental factors. ​ This includes environmental temperature measurements. ​ A video summarising the project is linked to in the media section below, and [[http://​www.viktoria.se/​fal/​projects/​soniccity/​prototype.html\|Viktoria site]] for the project has a more detailed description. 
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-[[https://​ccrma.stanford.edu/​~hiroko/​250/​index.htm|Sound Kitchen]] includes temperature sensors and uses voltage changes in liquids to create music. ​ The liquids include wine, soda and other items you might find in a kitchen, and the over all aesthetic connects strongly with cooking. 
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-====  Comparison of temperature sensor types  ==== 
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-The following table offers a comparison of the different characteristics of the various temperature sensor types. 
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-^Characteristic ​ ^Platinum RTD  ^Thermistor ​ ^Thermocouple ​ ^Temperature IC  ^ 
-^Active Material ​ |Platinum Wire  |Metal Oxide Ceramic ​ |Two Dissimilar Metals ​ |Silicon Transistors ​ | 
-^Changing Parameter ​ |Resistance ​ |Resistance ​ |Voltage ​ |Voltage or Current ​ | 
-^Temperature Range  |-200°C to 500°C ​ |-40°C to 260°C ​ |-270°C to 1750°C ​ |-55°C to 150°C ​ | 
-^Sensitivity ​ |2 mv/​°C ​ |40 mV/​°C ​ |0.05 mV/​°C ​ |~1 mv/°C or ~1 uA/​°C ​ | 
-^Accuracy ​ |-45 to 100°C: ±0.5°C; 100 to 500°C: ±1.5°C; 500 to 1200°C: ±3°C ​ |-45 to 100°C: ±0.5°C; degrades rapidly over 100°C ​ |0 to 275°C: ±1.5 °C to ±4°C; 275 to 1260°C: ±0.5 to ±0.75% ​ |±2 °C  | 
-^Linearity ​ |Excellent ​ |Logarithmic,​ Poor  |Moderate ​ |Excellent ​ | 
-^Response Time  |2-5 s  |1-2 s  |2-5 s  | 
-^Stability ​ |Excellent ​ |Moderate ​ |Poor  |Excellent ​ | 
-^Base Value  |100 Ω to 2 kΩ  |1 kΩ to 1 MΩ  |< 10 mV  |Various ​ | 
-^Noise Susceptibility ​ |Low  |Low  |High  |High  | 
-^Drift ​ |+/- 0.01% for 5 years  |+/- 0.2 to 0.5°F per year  |1 to 2°F per year  |0.1°C per month  | 
-^Special Requirements ​ |Lead Compensation ​ |Linearization ​ |Reference Junction ​ |None  | 
-^Device Cost  |$60 - $215  |$10 - $350  |$20 - $235  |$5 - $50  | 
-^Relative System Cost  |Moderate ​ |Low to Moderate ​ |Moderate ​ |Low  | 
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-=====  Devices ​ ===== 
- 
-{{template>​device 
-|company=Analog Devices 
-|model=AD592AN 
-|sources=[[http://​www.digikey.ca\|Digikey]] CAN$ 5.40 
-|description=Low Cost, Precision IC Temperature Transducer. 
-|datasheet=[[http://​rocky.digikey.com/​WebLib/​Analog%20Devices/​Web%20Data/​AD592.pdf\|pdf]] 
-|resources= 
-|notes=Changes output current proportional to absolute temperature. 
-|variants=AD592CN,​ AD592BN 
-}} 
- 
-{{template>​device 
-|company=BC Components 
-|model=BC1566KR 
-|sources=[[http://​www.digikey.ca\|Digikey]] CAN$ 33.60 
-|description=THERMISTOR 10 OHM 245V SMT PTC 
-|datasheet=[[http://​rocky.digikey.com/​WebLib/​BC%20Components/​Web%20Data/​2322%20661%2097...%20SMT%20PTC%20Therm.pdf\|pdf]] 
-|resources= 
-|notes= 
-|variants=BC1560DKR,​ BC1561DKR 
-}} 
- 
-{{template>​device 
-|company=Omega 
-|model=F3105 
-|sources=[[http://​www.omega.com\|Omega]] US$19.00 
-|description=Thin Film RTD Element 
-|datasheet=[[http://​www.omega.com/​ppt/​pptsc.asp?​ref=F3105_3100_W2100_2200&​Nav=temc13\|datasheet]] 
-|resources= 
-|notes=Temperature Range -50 to 600°C 
-|variants=W2100,​ W2200 
-}} 
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-=====  Media  ===== 
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-[[http://​www.viktoria.se/​fal/​projects/​soniccity/​index/​soniccity_video.mov|Sonic City Video]] 
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-===== External Links & References ===== 
-  * Hollinger, Avrum, 2002. {{:​sensors:​workterm_report_2002.pdf|Workterm Report - Temperature Sensors}} 
-  * Awtrey, Dan (Dallas Semiconductor),​ 2001.  //​[[http://​www.sensorsmag.com/​articles/​0102/​29/​main.shtml|The 1-Wire Thermocouple]]//,​ Sensors Online. 
-  * Ogden Manufacturing,​ 2000. //​[[http://​www.ogden-mfg.com/​specs.htm|Input Specifications]]//,​ Technical Specifications Document. 
-  * O'​Keefe,​ Claire and Black, Brian (Analog Devices). ​ //​[[http://​www.electronicproducts.com/​ShowPage.asp?​SECTION=3700&​PRIMID=&​FileName=ADI.feb2004.html|Choosing an IC temperature sensor]]//, Online Article, Wilmington MA. 
-  * Omega Engineering,​ 1999. //​[[http://​www.omega.com/​temperature/​Z/​pdf/​z013-015.pdf|Practical Guidelines for Temperature Measurement]]//,​ Online Technical Report. 
-  * [[http://​www.watlow.com/​reference/​refdata/​0310.cfm|Thermocouple Types]] (Watlow) 
-  * Lecture Series on Industrial Instrumentation by Prof.Alok Barua, Department of Electrical Engineering,​IIT Kharagpur [[http://​www.youtube.com/​watch?​v=5_MpULxwijg&​feature=channel|Lecture 7 - Thermistors]],​ [[http://​www.temperatures.com/​education-training/​nptel-lecture-8-thermocouples|Lecture 8 - Thermocouples]],​ [[http://​www.youtube.com/​watch?​v=85gm5aqeY44&​feature=channel|Lecture 9 - Resistance Temperature Detector]] 
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-{{tag>​Sensor Temperature Thermocouple Resistance}} 
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