03 Thermocouples Concept Overview
THERMOCOUPLES | CONCEPT OVERVIEW
The topic of THERMOCOUPLES can be referenced on page 124 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing.
CONCEPT INTRO: A THERMOCOUPLE (TC) is a device used to relate change in voltage to change in temperature. Thermocouples operate under the principle that a circuit made by connecting two dissimilar metals produces a measure voltage when a temperature different is imposed between one end and the other.
One junction, the COLD JUNCTION, is maintained at a known reference temperature, and the other junction also known as the HOT JUNCTION is attached to the object or system of interested to measure the temperature. Made with by Prepineer | Prepineer.com
For the purposes of resistivity, we can assume that each wire is in series with the other, such that the equivalent resistance is the sum of the resistances.
The formula for RESISTORS IN SERIES can be referenced under the topic of RESISTORS IN SERIES AND PARALLEL on page 201 of the NCEES Supplied Reference Handbook, 9.4 Version for Computer Based Testing. When resistors are in SERIES, the resistors act like a single resistor whose value is the sum of the resistors: π " = π $ + π & + π ' + π ( When the temperature difference is maintained across a given metal, the vibration and motion of electrons is affected so that a difference in electric potential exists across the metal. This potential difference is related to the fact that electrons in the hotter end of the material have more thermal energy than those in the cooler end, thus causing the electrons to migrate towards the cooler end. The table for the PROPERTIES OF THERMOCOUPLES can be referenced under the topic of MEASUREMENT on page 124 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing.
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Thermocouples are preferred as the instrument of choice for the measurement of temperature due to available ranges in various environments. As shown below, there 4 commonly used thermocouples that ANSI has recommended based on the desired temperature range and environment. Each thermocouple has characteristics that can be matched to environmental factors such as whether moisture is present, the likelihood for corrosion (oxidizing) or reducing, and whether the atmosphere is inert or in a vacuum. Maximum ANSI Code
Alloy Combination and Color
Outer Jacket Color
Thermocouple Temperature
Environment
Range + Lead
-
Lead
Thermocouple
Extension
Leads
Cable
CONSTANTAN J
IRON Fe (magnetic) White
Reducing, Vacuum, Inert.
COPPER-NICKEL Cu-Ni
Brown
Black
β346 π‘π 2,193Β°πΉ β210 π‘π 2,100 Β°πΆ
Red
K
NICKEL
NICKEL-
CHROMIUM
ALUMINUM
Ni-Cr
Ni-Al
Yellow
(magnetic)
Limited use in oxidizing at high temperatures. Not recommended for low temperatures
Brown
Yellow
β454 π‘π 2,501Β°πΉ β270 π‘π 1.372 Β°πΆ
Clean oxidizing and inert. Limited use in vacuum or reducing
Red COPPER
CONSTANTAN
Cu T
Blue
COPPER-NICKEL
Brown
Blue
Cu-Ni
β454 π‘π 752Β°πΉ β270 π‘π 400 Β°πΆ
Mild oxidizing, reducing vacuum or inert. Good where moisture is present
Red NICKEL CHROMIUM E
CONSTANTAN
Ni-Cr
COPPER-NICKEL
Purple
Cu-Ni
Brown
Purple
β454 π‘π 1,832Β°πΉ
Oxidizing or inert. Limited use
β270 π‘π 1.000 Β°πΆ
in vacuum or reducing
red
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The graph for the OUTPUT OF COMMON THERMOCOUPLES can be referenced under the topic of MEASUREMENT on page 124 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. As a thermocouple is comprised of two metals, both of which have unique physical properties, it is important to realize that each thermocouple will have varying outputs for a particular temperature based on how much of each metal is present in the thermocouple.
Thermocouple tables and graphs correlate temperature to a particular voltage output, as we as providing a reference junction or reference temperature to correlate the data. Usually the reference temperature or reference junction is 0Β°πΆ ππ 32Β°πΉ, as shown in the graph above, which all of the thermocouples use the cold junction (reference point).
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RESISTANCE TEMPERATURE DETECTOR (RTD): A RESISTANCE TEMPERATURE DETECTOR (RTD), also known as a RESISTANCE THERMOMETER is a device used to measure the temperature of an object or system correlating changes in resistance to change in temperature. A RTD is typically manufactured from platinum, nickel, or copper metals. The formula for the CONTROLLING EQUATION OF A RESISTANCE TEMPERATURE DETECTOR (RTD) can be referenced under the topic of MEASUREMENT on page 124 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. The variation of resistance π has linear characteristics that correlate with the change in temperature for most metallic metals, and can be represented by the following equation: π = = π > [1 + πΌ π β π> ] Where β’ π = is the resistance of the RTD in units of ohms (πΊ) at a specified temperature (π) given in units of Β°πΆ β’ π > is the resistance of the RTD in units of ohms (πΊ) at the reference temperature (π> ) given in units of Β°πΆ β’ πΌ is the temperature coefficient of the RTD given in units of 1/Β°πΆ The formula for THERMOCOUPLE SENSITIVITY is not provided in the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. We must
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memorize this formula and understand its application independent of the NCEES Supplied Reference Handbook. The THERMOCOUPLE SENSITIVITY is the ratio of change in electrical signal magnitude to change in temperature.
The thermocouple sensitivity defines the
relationship of how much the voltage output will increase or decreases depending on the magnitude of the difference between the reference cold junction temperature and the specified hot junction temperature.
ππππ ππ‘ππ£ππ‘π¦=NOPQRSRTUVO =
πΈππππ‘πππππ ππππππ π = πΆβππππ ππ ππππππππ‘π’ππ π β π>
Where: β’ π is the voltage output given in units of volts (V) β’ π is the specified hot junction temperature or temperature of interest given in units of Β°πΆ ππ Β°πΉ β’ π> is the specified cold junction temperature or reference temperature given in units of Β°πΆ ππ Β°πΉ The graph for the TOLERANCE VALUES OF THERMOCOUPLES can be referenced under the topic of MEASUREMENT on page 124 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing.
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A RTD relies on the predictable behavior of the electrical resistance of materials changing with temperature as defined the temperature coefficient, πΌ. The following graph shows tolerance values as a function of temperature for 100 β πΊ π ππ·π.
To measure the resistance across an RTD, apply a constant current, measure the resulting voltage, and determine the RTD resistance. We then use a resistance vs. temperature plot to determine the temperature of the surrounding medium. RTDs exhibit fairly linear resistance to temperature curves over their operating regions, and any nonlinearities are highly predictable and repeatable.
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CONCEPT EXAMPLE: A manufacturer states that a resistance temperature detector is calibrated with a defined temperature coefficient of 3.9 Γ 10c' Β°πΆ c$ using a reference temperature of 500Β°πΆ and reference resistance of 500 πΊ. If the resistance measured at an unknown temperature is 1247 πΊ, what is the value of the unknown temperature closest too: A. 104 B. 249 C. 883 D. 947
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SOLUTION: The formula for the CONTROLLING EQUATION OF A RESISTANCE TEMPERATURE DETECTOR (RTD) can be referenced under the topic of MEASUREMENT on page 124 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. A RESISTANCE TEMPERATURE DETECTOR (RTD) is a device used to related change in resistance to change in temperature. A RTD is typically manufactured from platinum, nickel, or copper metals. The variation of resistance π with the change in temperature for most metallic metals can be represented by the following equation: π = = π > [1 + πΌ(π β π> ) Where β’ π = is the resistance of the RTD at a specified temperature (π) given in units of Β°πΆ β’ π > is the resistance of the RTD at the reference temperature (π> ) given in units of Β°πΆ β’ πΌ is the temperature coefficient of the RTD However, the temperature the RTD indicates is not the actual temperature of the 500Β°πΆ, so re-arranging the RTD equation to solve for the temperature that the RTD indicates yields: 1247 πΊ = 500 πΊ [1 + (3.9 Γ 10c' Β°πΆ c$ )(π β 500Β°πΆ)
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Solving for the temperature: π = 883.1Β°πΆ
Therefore, the correct answer choice is C. πππ
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The topic of THERMOCOUPLES can be referenced on page 124 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing.
CONCEPT INTRO: A THERMOCOUPLE (TC) is a device used to relate change in voltage to change in temperature. Thermocouples operate under the principle that a circuit made by connecting two dissimilar metals produces a measure voltage when a temperature different is imposed between one end and the other.
One junction, the COLD JUNCTION, is maintained at a known reference temperature, and the other junction also known as the HOT JUNCTION is attached to the object or system of interested to measure the temperature. Made with by Prepineer | Prepineer.com
For the purposes of resistivity, we can assume that each wire is in series with the other, such that the equivalent resistance is the sum of the resistances.
The formula for RESISTORS IN SERIES can be referenced under the topic of RESISTORS IN SERIES AND PARALLEL on page 201 of the NCEES Supplied Reference Handbook, 9.4 Version for Computer Based Testing. When resistors are in SERIES, the resistors act like a single resistor whose value is the sum of the resistors: π " = π $ + π & + π ' + π ( When the temperature difference is maintained across a given metal, the vibration and motion of electrons is affected so that a difference in electric potential exists across the metal. This potential difference is related to the fact that electrons in the hotter end of the material have more thermal energy than those in the cooler end, thus causing the electrons to migrate towards the cooler end. The table for the PROPERTIES OF THERMOCOUPLES can be referenced under the topic of MEASUREMENT on page 124 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing.
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Thermocouples are preferred as the instrument of choice for the measurement of temperature due to available ranges in various environments. As shown below, there 4 commonly used thermocouples that ANSI has recommended based on the desired temperature range and environment. Each thermocouple has characteristics that can be matched to environmental factors such as whether moisture is present, the likelihood for corrosion (oxidizing) or reducing, and whether the atmosphere is inert or in a vacuum. Maximum ANSI Code
Alloy Combination and Color
Outer Jacket Color
Thermocouple Temperature
Environment
Range + Lead
-
Lead
Thermocouple
Extension
Leads
Cable
CONSTANTAN J
IRON Fe (magnetic) White
Reducing, Vacuum, Inert.
COPPER-NICKEL Cu-Ni
Brown
Black
β346 π‘π 2,193Β°πΉ β210 π‘π 2,100 Β°πΆ
Red
K
NICKEL
NICKEL-
CHROMIUM
ALUMINUM
Ni-Cr
Ni-Al
Yellow
(magnetic)
Limited use in oxidizing at high temperatures. Not recommended for low temperatures
Brown
Yellow
β454 π‘π 2,501Β°πΉ β270 π‘π 1.372 Β°πΆ
Clean oxidizing and inert. Limited use in vacuum or reducing
Red COPPER
CONSTANTAN
Cu T
Blue
COPPER-NICKEL
Brown
Blue
Cu-Ni
β454 π‘π 752Β°πΉ β270 π‘π 400 Β°πΆ
Mild oxidizing, reducing vacuum or inert. Good where moisture is present
Red NICKEL CHROMIUM E
CONSTANTAN
Ni-Cr
COPPER-NICKEL
Purple
Cu-Ni
Brown
Purple
β454 π‘π 1,832Β°πΉ
Oxidizing or inert. Limited use
β270 π‘π 1.000 Β°πΆ
in vacuum or reducing
red
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The graph for the OUTPUT OF COMMON THERMOCOUPLES can be referenced under the topic of MEASUREMENT on page 124 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. As a thermocouple is comprised of two metals, both of which have unique physical properties, it is important to realize that each thermocouple will have varying outputs for a particular temperature based on how much of each metal is present in the thermocouple.
Thermocouple tables and graphs correlate temperature to a particular voltage output, as we as providing a reference junction or reference temperature to correlate the data. Usually the reference temperature or reference junction is 0Β°πΆ ππ 32Β°πΉ, as shown in the graph above, which all of the thermocouples use the cold junction (reference point).
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RESISTANCE TEMPERATURE DETECTOR (RTD): A RESISTANCE TEMPERATURE DETECTOR (RTD), also known as a RESISTANCE THERMOMETER is a device used to measure the temperature of an object or system correlating changes in resistance to change in temperature. A RTD is typically manufactured from platinum, nickel, or copper metals. The formula for the CONTROLLING EQUATION OF A RESISTANCE TEMPERATURE DETECTOR (RTD) can be referenced under the topic of MEASUREMENT on page 124 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. The variation of resistance π has linear characteristics that correlate with the change in temperature for most metallic metals, and can be represented by the following equation: π = = π > [1 + πΌ π β π> ] Where β’ π = is the resistance of the RTD in units of ohms (πΊ) at a specified temperature (π) given in units of Β°πΆ β’ π > is the resistance of the RTD in units of ohms (πΊ) at the reference temperature (π> ) given in units of Β°πΆ β’ πΌ is the temperature coefficient of the RTD given in units of 1/Β°πΆ The formula for THERMOCOUPLE SENSITIVITY is not provided in the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. We must
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memorize this formula and understand its application independent of the NCEES Supplied Reference Handbook. The THERMOCOUPLE SENSITIVITY is the ratio of change in electrical signal magnitude to change in temperature.
The thermocouple sensitivity defines the
relationship of how much the voltage output will increase or decreases depending on the magnitude of the difference between the reference cold junction temperature and the specified hot junction temperature.
ππππ ππ‘ππ£ππ‘π¦=NOPQRSRTUVO =
πΈππππ‘πππππ ππππππ π = πΆβππππ ππ ππππππππ‘π’ππ π β π>
Where: β’ π is the voltage output given in units of volts (V) β’ π is the specified hot junction temperature or temperature of interest given in units of Β°πΆ ππ Β°πΉ β’ π> is the specified cold junction temperature or reference temperature given in units of Β°πΆ ππ Β°πΉ The graph for the TOLERANCE VALUES OF THERMOCOUPLES can be referenced under the topic of MEASUREMENT on page 124 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing.
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A RTD relies on the predictable behavior of the electrical resistance of materials changing with temperature as defined the temperature coefficient, πΌ. The following graph shows tolerance values as a function of temperature for 100 β πΊ π ππ·π.
To measure the resistance across an RTD, apply a constant current, measure the resulting voltage, and determine the RTD resistance. We then use a resistance vs. temperature plot to determine the temperature of the surrounding medium. RTDs exhibit fairly linear resistance to temperature curves over their operating regions, and any nonlinearities are highly predictable and repeatable.
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CONCEPT EXAMPLE: A manufacturer states that a resistance temperature detector is calibrated with a defined temperature coefficient of 3.9 Γ 10c' Β°πΆ c$ using a reference temperature of 500Β°πΆ and reference resistance of 500 πΊ. If the resistance measured at an unknown temperature is 1247 πΊ, what is the value of the unknown temperature closest too: A. 104 B. 249 C. 883 D. 947
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SOLUTION: The formula for the CONTROLLING EQUATION OF A RESISTANCE TEMPERATURE DETECTOR (RTD) can be referenced under the topic of MEASUREMENT on page 124 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. A RESISTANCE TEMPERATURE DETECTOR (RTD) is a device used to related change in resistance to change in temperature. A RTD is typically manufactured from platinum, nickel, or copper metals. The variation of resistance π with the change in temperature for most metallic metals can be represented by the following equation: π = = π > [1 + πΌ(π β π> ) Where β’ π = is the resistance of the RTD at a specified temperature (π) given in units of Β°πΆ β’ π > is the resistance of the RTD at the reference temperature (π> ) given in units of Β°πΆ β’ πΌ is the temperature coefficient of the RTD However, the temperature the RTD indicates is not the actual temperature of the 500Β°πΆ, so re-arranging the RTD equation to solve for the temperature that the RTD indicates yields: 1247 πΊ = 500 πΊ [1 + (3.9 Γ 10c' Β°πΆ c$ )(π β 500Β°πΆ)
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Solving for the temperature: π = 883.1Β°πΆ
Therefore, the correct answer choice is C. πππ
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