Difference of thermocouple and thermal resistance

Thermocouple and RTD are contactless temperature measurement of temperature measurement, although its the same, it is measuring the temperature of a body, but their principles and characteristics are different.
First of all, introduce the thermocouple. The temperature of thermocouple temperature measurement is the most widely used devices, whose main features are wide temperature range, stable performance, and simple structure, dynamic response, better able to remote 4-20mA signal, easy to control and centralized control. Thermocouple temperature measurement principle is based on the thermoelectric effect. Two different conductors or semiconductors into closed loop, when the temperature at two points at the same time, the loop will be produced in the thermoelectric power, a phenomenon known as the thermoelectric effect, also known as the seebeck effect.
Thermoelectric power in a closed circuit, there are two potential: the difference in electric potential and the contact potential. Temperature difference electromotive force is the same at both ends of the conductor electric potential arising from temperature, different conductors with different electron densities, so their potential is not the same and contact potential by definition refers to two different types of conductor contact, because of their Electron density is different, so there‘s a certain amount of electron diffusion, when they reach a certain balance formed by electric potential. Contact potential depends on the size of conductors of two different materials, as well as their contact point temperature. Currently international Shang application of thermocouple has a standard specification, international Shang provides thermocouple is divided into eight a different of points degrees, respectively for b, and r, and s, and k, and n, and e, and j and t, its measurement temperature of minimum can measuring lingxia270sheshidu, highest can up 1800 degrees Celsius, which b, and r, and s belongs to Platinum series of thermocouple, due to Platinum belongs to your heavy metal, so they and was called precious metals thermocouple. While the rest of the then known as cheap metal thermocouples.
Structure there are two kinds of thermocouple, ordinary and sheathing. Common thermocouple thermoelectric poles, insulating tubes, cannula and junction boxes, and other components, and sheathed thermocouple the thermocouple wire, insulating material and metallic cannula combinations of the three Assembly, after drawing a solid combination. But signals of thermocouple needs a special wire to pass the wires we call compensating wire. Requires different different thermocouple compensating conductor, its main role is to connect to the thermocouple, thermocouple reference junction away from the power supply, so that the reference junction temperature.
Compensating cables are divided into two types of compensation and extension, extension lead the same chemical composition and compensation of thermocouple, but in practice, extend the wire and thermocouple for is not the same as the metal, and thermocouple wire with the same Electron density is usually used instead. Compensation wire and thermocouple wires are generally simple, positive connection of thermocouple compensating wire red wire, while the anode is connected the rest of the colors. Most general material of compensating wire made of copper-nickel alloy.
Second, we introduce thermal resistance, thermal resistance are widely applied in industry, but due to the temperature range of the application subject to certain restrictions, RTD temperature measurement principle is based on the conductor or semiconductor characteristic of resistance change with temperature changes. Its advantages are many, can also transfer electrical signals, high sensitivity, strong stability, interchangeability, and accuracy is good, but need the power incentive, not instantaneous temperature changes. Industrial thermal resistance Pt100,Pt10,Cu50,Cu100, platinum resistance temperature measurement range is generally lingxia200-800sheshidu, copper heat resistance is lingxia40dao140sheshidu. The distinction between RTD and Thermocouple type, but he did not go to compensating cables, and cheaper than thermocouples.


Of course there are differences, is a temperature sensor of thermocouple and RTD temperature sensor, as is, but the main difference is that he and thermal resistance:
First, the nature of the signal, heat resistance was resistance, temperature changes, have a positive or negative value change in resistance and thermally coupled, is the induced voltage changes, he changes with changes in temperature.
Second, two sensor for detecting temperature range are not the same, General testing heat resistance 0-150 degree temperature range, maximum measuring range of up to 600 degrees (of course you can test negative temperature).
Thermal coupling temperature of 0-1000 degrees detection range (or even higher), the former is the low temperature testing, which is high temperature detection.
Third, from the material, the thermal resistance is a metallic material, with changes of temperaturesensitive metal, thermal coupling is bi-metals, both of two different metals, due to changes in temperature, electrical potential difference at two different ends of the wire.
Four, corresponding to the PLC RTD and thermocouple input module is not the same, this sentence is not a problem, but in General PLC access 4~20mA signal directly, RTD and thermocouple transmitter to access PLC. If access transmitters for use in DCS, then you don’t have to! Thermal resistance is the RTD signal electric Europe is TC signal!
Five modules, the PLC also has thermal resistance and thermocouple modules can be directly input and thermocouple signals.
Six, a thermoelectric couple j, t, n, k, and s type, are more expensive than resistance, there are cheaper than resistance, but compensating cables are included, high cost thermocouples.
Thermal resistance is resistance signal, thermocouple voltage signal.
Seven, heat resistance temperature measurement principle is based on the conductor (or semiconductors) measured by the variation of resistance with temperature measurement range is negative 00~500du, commonly used are platinum resistance thermometer (Pt100, Pt10), copper Cu50 (50-150 degrees).
Thermocouple temperature principle is based on thermoelectric to measurement temperature of, common of has Platinum rhodium–Platinum (points degrees, s, measurement range 0~1300du), and nickel chrome–nickel Silicon (points degrees, k, measurement range 0~900du), and nickel chrome–Lala (points degrees, e, measurement range 0~600du), and Platinum rhodium 30–Platinum rhodium 6 (points degrees, b, measurement range 0~1600du).

Charge heat resistance also has two terminals to be screwed together two negative leads in thermal resistance a thermal resistance of only two terminals on the Terminal is not divided into positive and negative
Is there have been posts on this forum I turned
RTD and thermocouple measurement principle and difference
Thermocouples are one of the most commonly used temperature sensor in industry. Its advantages are:
high measurement accuracy. Thermocouple direct contact with the object being measured, not under the influence of medium.
wide measurement range. Commonly used heat measurements from -50~ 1600 c can be continued, some extraordinary thermocouple minimum measurable269 (gold in FE-NI-CR), up to a maximum of 2,800 (such as tungstenrhenium).
simple structure, easy to use. Thermocouples are usually made up of two different kinds of wire, and are not subject to size restrictions and opening, outer protective casing, it is very convenient to use.
1. Thermocouples principle
The two conductors or semiconductor a and b of different materials welded together to form a closed loop. When the conductor a and b two attachment points 1 and 2 when there is a temperature difference, between EMF and current in the circuit forming a size, a phenomenon known as the thermoelectric effect. Thermocouple is used this effect to work.


2. Thermocouple type and structure formation
(1) type of thermocouple
Common thermocouple thermocouple can be divided into standard and non standard thermocouple in two categories. Standard thermocouple is a national standard that is called its thermoelectric power and temperature relationship, agreed error, and a unified standard thermocouple indexing table, it has its matching display instrument for selection. Non standard thermocouple in use or on the orders of magnitude are not standardized thermocouple, in General, no uniform indexing table, mainly for the measurement of some extraordinary occasions. Standard thermocouple
From January 1, 1988, the thermocouple and thermal resistance according to IEC international standards of production, and specify s, b, e, k, r, j, t, seven kinds of standardizing the thermocouple in a unified design-type thermocouple.
(2) thermocouple‘s structure in order to ensure that the thermocouple is reliable and stable work, its structure is as follows:
two thermoelectric thermocouple composed of welding to be fixed;
Between the two heating each other should be well insulated to prevent short circuits;
compensating cables connect to the free end of the thermocouple to be convenient and reliable;
cannula should ensure full heat and harmful media isolation.
3. Temperature compensation for thermocouple cold junction
Because the thermocouple materials are generally more valuable (especially when using precious metals), and temperature measurement points to a very far distance from meter to save thermocouple materials, reduce costs, often using compensation wires of thermocouple cold end (end) extends to the relatively stable temperature control, connected to the instrument terminals. It must be noted that thermocouple compensating wire extends only thermoelectric, thermocouple cold-end to move to the control room on the instrument terminals, which itself does not eliminate the cold junction temperature changes affect the temperature, without compensation. Therefore, other amendments should be used to compensate for the cold end temperature t0 0 temperature effects.
When using a thermocouple compensating wire must focus on model matching, polarity cannot be wrong, compensation lead wire and thermocouple connections-side temperature should not exceed 100 degrees centigrade.
Classification of temperature measuring instruments
Temperature measurement instruments can be classified by temperature measurement methodscontact and noncontact two categories. Generally contactless measuring instrument instruments are simple and reliable, high precision but because of temperature measurement component fully with measured medium needs to be hot to make King Kong, need time to reach thermal equilibrium, there are temperature delays, as well as by the limit of high temperature resistant material, cannot be applied to high temperature measurements. Non-contact type instrument temperature is through hot radiation principle to measurement temperature of, temperature components not needed and was measuring media contact, temperature range wide, not by temperature ceiling of limit, also not damage was measuring objects of temperature field, reaction speed General also compared fast; but by objects of launches rate, and measurement distance, and dust and water, outside factors of effect, its measurement errors larger.
Thermal resistance of applied principles
Thermal resistance is most commonly used as a low-temperature zone temperature detector. Its main feature is a high precision, stable performance. Platinum is the resistance of the measuring accuracy is the highest, it is widely used in industrial temperature measurement, and was made the standard reference instrument.
1. Heatresistance temperature measurement principle and material
Heat resistance temperature measurement is based on the metal conductor resistance value and increases with the temperature of this feature for temperature measurement. Thermal resistance are made from pure, Platinum and copper is the largest at present and, in addition, has now begun using meadow, NI, MN and RH material thermal resistance.
2. Thermal resistance of structure
(1) master heat resistance
From the principle of heat resistance, measured changes in temperature are measured directly through the thermal resistance changes, therefore, RTD lead wire and other wire resistance changes affect temperature measurements. To suppress lead wire resistance effects of using three-wire or fourwire,
(2) armoured thermal resistance
Armoured thermal resistance is determined by the temperature of the components (resistors), lead, insulation material, combination of the solid body of the stainless steel casing diameter for PHI 2~ 8mm, minimum diameter of mm.
Compared with ordinary thermal resistance, which has the following advantages: small size, internal airgap, on the thermal inertia, measure lag small; second, the mechanical properties and good resistance to vibration, impact c can bend, facilitating the installation II and long service life.
(3) end of thermal resistance
Surface heat resistance temperature element processed by the extraordinary resistance of wires around the system, against the thermometer end, its structure is shown in Figure 2-18. It compared with the axial thermal resistance, to reflect more accurately and quickly end the actual temperature being measured, suitable for measuring the surface temperature of bearing and other parts.
(4) flameresistance
Flameproof thermal resistance junction box through extraordinary structure, explosive gas mixture inside the housing due to spark or arc and explosion effects and limitations within the junction box, site not hyper-explosion. Flameproof thermal resistance can be used in Bla~B3c-level explosion hazardous area temperature measurement within the area.
3. Heat resistance temperature measurement system for
Heat resistance temperature measurement system by thermal resistors, connected components, such as wiring and display instruments. Must focus on the following two points: thermal resistance index and display instrument must be consistent
in order to eliminate the influence of wire resistance changes, you must use three-wire bonding method
(2) armoured thermal resistance
Armoured thermal resistance is determined by the temperature of the components (resistors), lead, insulation materials, a combination of stainless steel casing of a solid body, it is generally the outside diameter of φ 2~ PHI 8mm, minimum diameter of mm.
Compared with ordinary thermal resistance, which has the following advantages: small size, internal airgap, on the thermal inertia, measure lag small; second, the mechanical properties and good resistance to vibration, impact c can bend, facilitating the installation II and long service life.
(3) end of thermal resistance
Surface heat resistance temperature element processed by the extraordinary resistance of wires around the system, against the thermometer end, compared with axial thermal resistance, to reflect more accurately and quickly end the actual temperature being measured, suitable for measuring the surface temperature of bearing and other parts.
(4) flameflameproof electric barrier thermal resistance junction box through extraordinary structure, explosive gas mixture inside the housing due to spark or arc shadow resistance of circuit breaker repair it is necessary to change the length of the resistance wire affect resistance, for replacement of the resistor as well, if the use of welding repairs, after checking compliance before use
Add, thermocouple need to use cables, thermal resistance common control cable is OK for general use (must select fire-resistant high temperature type)
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