Thermocouple Theory
By Caleb Streur
Overview
A thermocouple is a commonly used type of sensor that is used to measure temperature. Thermocouples are popular in industrial control applications because of their relatively low cost and wide measurement ranges. In particular, thermocouples excel at measuring high temperatures where other common sensor types cannot function. Try operating an integrated circuit (LM35, AD 590, etc.) at 800C.
Thermocouples are fabricated from two electrical conductors made of two different metal alloys. The conductors are typically built into a cable having a heat-resistant sheath, often with an integral shield conductor. At one end of the cable, the two conductors are electrically shorted together by crimping, welding, etc. This end of the thermocouple--the hot junction--is thermally attached to the object to be measured. The other end--the cold junction, sometimes called reference junction--is connected to a measurement system. The objective, of course, is to determine the temperature near the hot junction.
It should be noted that the "hot" junction, which is somewhat of a misnomer, may in fact be at a temperature lower than that of the reference junction if low temperatures are being measured.
Reference Junction Compensation Thermocouples generate an open-circuit voltage, called the Seebeck voltage, that is proportional to the temperature difference between the hot and reference junctions :
Vs = V(Thot-Tref)
Since thermocouple voltage is a function of the temperature difference between junctions, it is necessary to know both voltage and reference junction temperature in order to determine the temperature at the hot junction. Consequently, a thermocouple measurement system must either measure the reference junction temperature or control it to maintain it at a fixed, known temperature.
There is a misconception of how thermocouples operate. The misconception is that the hot junction is the source of the output voltage. This is wrong. The voltage is generated across the length of the wire. Hence, if the entire wire length is at the same temperature no voltage would be generated. If this were not true we connect a resistive load to a uniformly heated thermocouple inside an oven and use additional heat from the resistor to make a perpetual motion machine of the first kind.
The erroneous model also claims that junction voltages are generated at the cold end between the special thermocouple wire and the copper circuit, hence, a cold junction temperature measurement is required. This concept is wrong. The cold -end temperature is the reference point for measuring the temperature difference across the length of the thermocouple circuit.
Most industrial thermocouple measurement systems opt to measure, rather than control, the reference junction temperature. This is due to the fact that it is almost always less expensive to simply add a reference junction sensor to an existing measurement system than to add on a full-blown temperature controller. Sensoray Smart A/D's measure the thermocouple reference junction temperature by means of a dedicated analog input channel. Dedicating a special channel to this function serves two purposes: no application channels are consumed by the reference junction sensor, and the dedicated channel is automatically pre-configured for this function without requiring host processor support. This special channel is designed for direct connection to the reference junction sensor that is standard on many Sensoray termination boards.
Linearization Within the "useable" temperature range of any thermocouple, there is a proportional relationship between thermocouple voltage and temperature. This relationship, however, is by no means a linear relationship. In fact, most thermocouples are extremely non-linear over their operating ranges. In order to obtain temperature data from a thermocouple, it is necessary to convert the non-linear thermocouple voltage to temperature units. This process is called "linearization."
Several methods are commonly used to linearize thermocouples. At the low-cost end of the solution spectrum, one can restrict thermocouple operating range such that the thermocouple is nearly linear to within the measurement resolution. At the opposite end of the spectrum, special thermocouple interface components (integrated circuits or modules) are available to perform both linearization and reference junction compensation in the analog domain. In general, neither of these methods is well-suited for cost-effective, multipoint data acquisition systems.
In addition to linearizing thermocouples in the analog domain, it is possible to perform such linearizations in the digital domain. This is accomplished by means of either piecewise linear approximations (using look-up tables) or arithmetic approximations, or in some cases a hybrid of these two methods.
The Linearization Process Sensoray’s Smart A/D’s employ a thermocouple measurement and linearization process that is designed to hold costs to a practical level without sacrificing performance.
First, both the thermocouple and reference junction sensor signals are digitized to obtain thermocouple voltage Vt and reference junction temperature Tref. The thermocouple signal is digitized at a higher rate than the reference junction because it is assumed that the reference junction is relatively stable compared to the hot junction. Reference junction measurements are transparently interleaved between thermocouple measurements without host processor intervention.
An onboard processor then performs linearization and reference junction compensation in the digital domain. Depending on the thermocouple type being used, an appropriate "correction voltage" is computed by mapping reference junction temperature into equivalent thermocouple voltage: Vc=V(Tref). This correction voltage is added to the measured thermocouple voltage to obtain the "corrected" thermocouple voltage:Vtc=Vt+Vc. Finally, the corrected thermocouple voltage is linearized by mapping it into temperature units: T=T(Vtc). Sensoray Smart Sensor Processors utilize look-up tables for determination of both correction voltage and thermocouple temperature. Although there are many advantages to this approach, the most important is this: high measurement throughputs can be achieved without the need for a high-speed DSP. Consequently, Smart A/D’s offer superior thermocouple measurement performance at a low cost and low power-consumption..
Article Source: _http://EzineArticles.com/?expert=Caleb_Streur
IPTS-68 REFERENCE TEMPERATURES
EQUILIBRIUM POINT K C
Triple Point of Hydrogen 13.81 -259.34
Liquid/Vapor Phase of Hydrogen 17.042 -256.108
at 25/76 Std. Atmosphere
Boiling Point of Hydrogen 20.28 -252.87
Boiling Point of Neon 27.102 -246.048
Triple Point of Oxygen 54.361 -218.789
Boiling Point of Oxygen 90.188 -182.962
Triple Point of Water 273.16 0.01
Boiling Point of Water 373.15 100
Freezing Point of Zinc 692.73 419.58
Freezing Point of Silver 1235.08 961.93
Freezing Point of Gold 1337.58 1064.43
Showing posts with label Thermocouples. Show all posts
Showing posts with label Thermocouples. Show all posts
Sunday, April 26, 2009
Saturday, April 25, 2009
Basic Thermocouples
How Does A Thermocouple Work?
By Joe Crew
Thermocouples
Measurement and control of temperature is one of the most common requirements of industrial instrumentation and the thermocouple is by far the most widely used temperature sensor. Its characteristics include good inherent accuracy, suitability over a broad temperature range, fast thermal response, ruggedness, high reliability and low cost.
How does a thermocouple work?
T.J Seebeck discovered in the 1820s that an electric current flows in a closed circuit of two dissimilar metals when one of the two junctions is heated with respect to the other. In a thermocouple circuit the current continues to flow as long as the two junctions are at different temperatures. The magnitude and direction of the current depends on the temperature difference between the junctions and the properties of the metals used in the circuit. This is known as the Seebeck effect. Click here to see an example of the circuit.
If the circuit is broken at the center, the net open circuit voltage (the Seebeck voltage) is a function of the junction temperature and the composition of the two metals.
If the hot and cold junctions are reversed, current will flow in the opposite direction. Any two dissimilar metals can be used and the thermocouple circuit will generate a low voltage output that is almost (but not exactly) proportional to the temperature difference between the hot junction and the cold junction. The voltage output is between 15 and 40µV per degree C, dependant on the thermocouple conductor metals used. The actual metals used in industrial thermocouples depend on the application and temperature measurement range required.
Thermocouple failure prediction
Like any other metal object, thermocouples are subject to metal fatigue wear and tear; they have a finite life. Many users of thermocouples are not aware of thermocouple deterioration until the sensor breaks, often causing an expensive interruption of a process. Removing a thermocouple from a furnace when at operating temperature can be difficult and dangerous. In fact the thermocouple, a simple and generally inexpensive sensor, can cause inaccurate readings for some time before any errors are detected. The errors usually cause low readings due to the thermocouple wires becoming thinner.
Impurities induced by any handling during manufacture or installation can accelerate chemical deterioration of the thermocouple. For base metal thermocouples, deterioration occurs slowly due to contact with the atmosphere, which in turn causes oxidation. As the surface of the thermocouple wires oxidises the current carrying cross sectional area is reduced. Nobel metal thermocouple deterioration is also well documented.
In "Principals and Method of Temperature Measurement", Thomas D McGee explains that the usual result of deterioration is the gradual reduction in the Seebeck voltage, often extended over several weeks and not frequently detected. If the Seebeck voltage is low, the measured temperature will also be low, so the actual process temperature will be increased to produce the required Seebeck voltage. The net result will be excessive temperature generation with resulting damage to material and processes. Those who use thermocouples should be aware of the possibilities of slow deterioration and its consequences.
A temperature controller, for example, would actually compensate for the thermocouple's loss of thermoelectric power by putting more heat into the process with all the energy, environmental and process plant costs that would be incurred. Fortunately, while Mr Thomas Johann Seebeck was experimenting with his wires in the 1820s, his contemporary and fellow countryman, Mr Georg Ohm, was also conducting his own experiments. Fortuitously because as the thermocouple conductors become thinner, their resistance changes as described in "Practical Temperature Measurement" by Peter R. N. Childs.
"The loop resistance of a thermocouple depends on its length, type and diameter of the thermocouple wire, the length type and diameter of extension wires, temperatures along the circuit and the contact resistance at any connections. If on installation, and at regular intervals in use, a measurement is made of this loop resistance, then a change in this value can be used to indicate wire thinning due to chemical attack, loose or corroded connections, contact resistance due to broken but touching wires or electrical shunting due to loss of insulation at some location along the wire."
Regular measurements of the thermocouple loop can indicate that the sensor should be replaced for reasons of accuracy and can also be used to predict its complete failure (sensor break). As thermocouple conductors oxidise they become brittle, making them more susceptible to breakage due to bending or vibration. Replacing thermocouples during a planned maintenance period is easier and more cost effecting than replacing thermocouples while the plant is running.
Joe Crew is the Product Manager at Data Track Process Instruments Ltd. Data Track manufactures digital panel meters, large number displays, PID controllers, signal conditioners and remote data acquisition systems for the process and control industry. Data Track can also supply HMI touchscreen operator panels and SCADA software. In their new line of PID controllers, Data Track has developed a solution to this common thermocouple problem. The Tracker 331 and Tracker 332 have, as standard, the ability to continually measure the condition of the connected thermocouple and prompt for its replacement before it starts to affect the process and/or fails completely.
Article Source: _http://EzineArticles.com/?expert=Joe_Crew
The Seebeck effect
The discovery of thermoelectricity dates back to Seebeck [1] (1770-1831). Thomas Johann Seebeck was born in Revel (now Tallinn), the capital of Estonia which at that time was part of East Prussia. Seebeck was a member of a prominent merchant family with ancestral roots in Sweden. He studied medicine in Germany and qualified as a doctor in 1802. Seebeck spent most of his life involved in scientific research. In 1821 he discovered that a compass needle deflected when placed in the vicinity of a closed loop formed from two dissimilar metal conductors if the junctions were maintained at different temperatures. He also observed that the magnitude of the deflection was proportional to the temperature difference and depended on the type of conducting material, and does not depend on the temperature distribution along the conductors. Seebeck tested a wide range of materials, including the naturally found semiconductors ZnSb and PbS. It is interesting to note that if these materials had been used at that time to construct a thermoelectric generator, it could have had an efficiency of around 3% - similar to that of contemporary steam engines.
The Seebeck coefficient is defined as the open circuit voltage produced between two points on a conductor, where a uniform temperature difference of 1K exists between those points
Source
http://www.thermoelectrics.com/introduction.htm
By Joe Crew
Thermocouples
Measurement and control of temperature is one of the most common requirements of industrial instrumentation and the thermocouple is by far the most widely used temperature sensor. Its characteristics include good inherent accuracy, suitability over a broad temperature range, fast thermal response, ruggedness, high reliability and low cost.
How does a thermocouple work?
T.J Seebeck discovered in the 1820s that an electric current flows in a closed circuit of two dissimilar metals when one of the two junctions is heated with respect to the other. In a thermocouple circuit the current continues to flow as long as the two junctions are at different temperatures. The magnitude and direction of the current depends on the temperature difference between the junctions and the properties of the metals used in the circuit. This is known as the Seebeck effect. Click here to see an example of the circuit.
If the circuit is broken at the center, the net open circuit voltage (the Seebeck voltage) is a function of the junction temperature and the composition of the two metals.
If the hot and cold junctions are reversed, current will flow in the opposite direction. Any two dissimilar metals can be used and the thermocouple circuit will generate a low voltage output that is almost (but not exactly) proportional to the temperature difference between the hot junction and the cold junction. The voltage output is between 15 and 40µV per degree C, dependant on the thermocouple conductor metals used. The actual metals used in industrial thermocouples depend on the application and temperature measurement range required.
Thermocouple failure prediction
Like any other metal object, thermocouples are subject to metal fatigue wear and tear; they have a finite life. Many users of thermocouples are not aware of thermocouple deterioration until the sensor breaks, often causing an expensive interruption of a process. Removing a thermocouple from a furnace when at operating temperature can be difficult and dangerous. In fact the thermocouple, a simple and generally inexpensive sensor, can cause inaccurate readings for some time before any errors are detected. The errors usually cause low readings due to the thermocouple wires becoming thinner.
Impurities induced by any handling during manufacture or installation can accelerate chemical deterioration of the thermocouple. For base metal thermocouples, deterioration occurs slowly due to contact with the atmosphere, which in turn causes oxidation. As the surface of the thermocouple wires oxidises the current carrying cross sectional area is reduced. Nobel metal thermocouple deterioration is also well documented.
In "Principals and Method of Temperature Measurement", Thomas D McGee explains that the usual result of deterioration is the gradual reduction in the Seebeck voltage, often extended over several weeks and not frequently detected. If the Seebeck voltage is low, the measured temperature will also be low, so the actual process temperature will be increased to produce the required Seebeck voltage. The net result will be excessive temperature generation with resulting damage to material and processes. Those who use thermocouples should be aware of the possibilities of slow deterioration and its consequences.
A temperature controller, for example, would actually compensate for the thermocouple's loss of thermoelectric power by putting more heat into the process with all the energy, environmental and process plant costs that would be incurred. Fortunately, while Mr Thomas Johann Seebeck was experimenting with his wires in the 1820s, his contemporary and fellow countryman, Mr Georg Ohm, was also conducting his own experiments. Fortuitously because as the thermocouple conductors become thinner, their resistance changes as described in "Practical Temperature Measurement" by Peter R. N. Childs.
"The loop resistance of a thermocouple depends on its length, type and diameter of the thermocouple wire, the length type and diameter of extension wires, temperatures along the circuit and the contact resistance at any connections. If on installation, and at regular intervals in use, a measurement is made of this loop resistance, then a change in this value can be used to indicate wire thinning due to chemical attack, loose or corroded connections, contact resistance due to broken but touching wires or electrical shunting due to loss of insulation at some location along the wire."
Regular measurements of the thermocouple loop can indicate that the sensor should be replaced for reasons of accuracy and can also be used to predict its complete failure (sensor break). As thermocouple conductors oxidise they become brittle, making them more susceptible to breakage due to bending or vibration. Replacing thermocouples during a planned maintenance period is easier and more cost effecting than replacing thermocouples while the plant is running.
Joe Crew is the Product Manager at Data Track Process Instruments Ltd. Data Track manufactures digital panel meters, large number displays, PID controllers, signal conditioners and remote data acquisition systems for the process and control industry. Data Track can also supply HMI touchscreen operator panels and SCADA software. In their new line of PID controllers, Data Track has developed a solution to this common thermocouple problem. The Tracker 331 and Tracker 332 have, as standard, the ability to continually measure the condition of the connected thermocouple and prompt for its replacement before it starts to affect the process and/or fails completely.
Article Source: _http://EzineArticles.com/?expert=Joe_Crew
The Seebeck effect
The discovery of thermoelectricity dates back to Seebeck [1] (1770-1831). Thomas Johann Seebeck was born in Revel (now Tallinn), the capital of Estonia which at that time was part of East Prussia. Seebeck was a member of a prominent merchant family with ancestral roots in Sweden. He studied medicine in Germany and qualified as a doctor in 1802. Seebeck spent most of his life involved in scientific research. In 1821 he discovered that a compass needle deflected when placed in the vicinity of a closed loop formed from two dissimilar metal conductors if the junctions were maintained at different temperatures. He also observed that the magnitude of the deflection was proportional to the temperature difference and depended on the type of conducting material, and does not depend on the temperature distribution along the conductors. Seebeck tested a wide range of materials, including the naturally found semiconductors ZnSb and PbS. It is interesting to note that if these materials had been used at that time to construct a thermoelectric generator, it could have had an efficiency of around 3% - similar to that of contemporary steam engines.
The Seebeck coefficient is defined as the open circuit voltage produced between two points on a conductor, where a uniform temperature difference of 1K exists between those points
Source
http://www.thermoelectrics.com/introduction.htm
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