Thermocouple
A thermocouple or thermocouple for its translation from English thermocouple, is a transducer formed by the union of two different metals that produces a difference of very small potential (of the order of millivolts) which is a function of the temperature difference between one of the ends called the «hot spot» or «hot junction» or «measurement» and the other called the «cold spot» or «junction cold" or "reference" (Seebeck effect).
Usually industrial thermocouples are made of a stainless steel tube or other material. At one end of the tube is the union, and at the other the electrical terminal of the cables, protected inside a round aluminum box (head).
In industrial instrumentation, thermocouples are used as temperature sensors. They are inexpensive, interchangeable, have standard connectors, and are capable of measuring a wide range of temperatures. Its main limitation is accuracy, since it is easy to obtain system errors when working with precisions below one degree Celsius.[citation required]
The group of thermocouples connected in series is called a thermopile. Both thermocouples and thermopiles are widely used in gas heating applications.
Linearization
In addition to dealing with cold junction compensation, the measuring instrument must also deal with the fact that the power generated by a thermocouple is not a linear function of temperature. This dependence can be approximated by a complex polynomial (of degree 5 to 9, depending on the type of thermocouple). Analog linearization methods are used in low cost thermocouple meters.
Thermocouple format
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Thermocouples are available in different formats, such as probes. The latter are ideal for various measurement applications, for example, in medical research, temperature sensors for food, in industry and other branches of science, in radioisotope thermoelectric generators, etc.
When selecting a probe of this type, the type of connector must be taken into account. The two types are the "standard" model, with round pins, and the "miniature" model with flat pins, the latter (contradictory to the name of the former) being the most popular.
Another important point in the selection is the type of thermocouple, the insulation and the construction of the probe. All these factors have an effect on the temperature range to be measured, precision and reliability of the readings.
Types
- Type K (cromel/alumel): with a wide variety of applications, it is available at a low cost and in a variety of probes. The cromel is a Ni-Cr alloy, and the alumel is a Ni-Al alloy. They have a temperature range of –200 °C to +1372 °C and approximately 41 μV/°C sensitivity. It has good oxidation resistance.
- Type E (cromel/constant [Cu-Ni alloy]): they are not magnetic and thanks to their sensitivity, they are ideal for use in low temperatures, in the cryogenic field. They have a sensitivity of 68 μV/°C.
- Type J (hero/constant): its range of use is –270/+1200 °C. Due to its characteristics it is recommended to use in inert, reducing or vacuum atmospheres, its continued use at 800 °C does not present problems, its main inconvenience is the rapid oxidation that suffers from iron above 550 °C; and below 0 °C it is necessary to take precautions because of the condensation of water vapor over iron.
- Type T (copper/constant): ideal for measurements between -200 and 260 °C. Wet, reducer and oxidant atmospheres exist and are applicable in cryogenia. The thermocouple type of T has a sensitivity of about 43 μV/°C.
- Type N (nicrosil [Ni-Cr-Si]/nisil [Ni-Si]): It is suitable for high temperature measurements thanks to its high stability and resistance to oxidation of high temperatures, and does not need the platinum used in types B, R and S, which are more expensive.
On the other hand, type B, R and S thermocouples are the most stable, but due to their low sensitivity (10 µV/°C approx.) they are generally used to measure high temperatures (above 300 °C).
- Type B (Pt-Rh): are suitable for measuring high temperatures above 1800 °C. Type B presents the same result at 0 °C and 42 °C due to its temperature/voltage curve, thus limiting its use at temperatures above 50 °C.
- Type R (Pt-Rh): suitable for temperature measurement up to 1600 °C. Its low sensitivity (10 μV/°C) and its high price remove its attractiveness.
- Type S (Pt/Rh): ideal for high temperature measurements up to 1600 °C, but its low sensitivity (10 μV/°C) and its high price make it an instrument not suitable for general use. Due to its high stability, type S is used for universal calibration of the gold melting point (1064.43 °C).
Thermocouples with low sensitivity, such as types B, R, and S, also have lower resolution. The selection of thermocouples is important to ensure that they cover the range of temperatures to be measured.
Precautions and considerations when using thermocouples
Most measurement problems and errors with thermocouples are due to a lack of understanding of how thermocouples work. Below is a brief list of the most common problems that should be taken into account.
Connection problems
Most measurement errors are caused by unintentional thermocouple binding. It should be noted that any contact between two dissimilar metals will create a bond. If it is desired to increase the length of the guides, the correct type of extension cable must be used. Thus, for example, type K corresponds to thermocouple K. When using another type, another thermocouple junction will be introduced (generating error in the measurement). Whichever connector is used must be made of the correct thermocouple material and its polarity must be correct. The most correct thing is to use commercial connectors of the same type as the thermocouple to avoid problems and measurement errors.
Guide resistance
To minimize thermal drift and improve response times, thermocouples are embedded with thin wires. This can cause thermocouples to have a high resistance, which can make them sensitive to noise and can also cause errors due to the resistance of the measuring instrument. A typical exposed thermocouple junction with 0.25mm will have a resistance of about 15 ohms per meter. If thermocouples with thin leads or long leads are needed, it is best to keep the leads short and then use the extension lead, which is thicker (meaning less resistance) located between the thermocouple and the measuring instrument. It is recommended to measure the resistance of the thermocouple before using it.
Mismatch
Mismatch is the process of accidentally altering the conformation of the thermocouple wire. The most common cause is the diffusion of atmospheric particles into the metal at the extremes of the operating temperature. Other causes are impurities and chemicals from the insulation diffusing into the thermocouple wire. If operating at high temperatures, the probe insulation specifications should be reviewed. Keep in mind that one of the criteria for calibrating a measuring instrument is that the standard must be at least 10 times more accurate than the instrument to be calibrated.
Noise
The output of a thermocouple is a small signal, so it is susceptible to error due to electrical noise. Most measurement instruments reject any mode noise (signals that are on the same or both leads) so noise can be minimized by twisting the leads to ensure that they both pick up the same noise signal. If operating in an extremely noisy environment (for example near a large motor), it is necessary to consider using a shielded extension cord. If noise reception is suspected, all suspected equipment should first be turned off and the readings checked for change. However, the most logical solution is to design a low-pass filter (resistor and capacitor in series) since it is likely that the frequency of the noise (for example from a motor) is much higher than the frequency with which the temperature oscillates. Or put a repeater after the thermocouple so that the signal in the cable is greater and that the receiving equipment is compensated to be able to connect that repeater.
Voltage in common mode
These voltages can be caused either by inductive reception (a problem when measuring the temperature of motor parts and transformers) or by junctions to ground connections. A typical example of bonding to ground would be the measurement of a hot water pipe with an uninsulated thermocouple. If there is any ground connection there may be some volts between the tube and the ground of the measuring instrument. These signals are once again in common mode (the same on both thermocouple leads) so they won't cause any problems with most instruments as long as they aren't too large. Common mode voltages can be minimized by using the same wiring precautions established for noise, and also by using insulated thermocouples.
Noise in serial mode
If the sensor is exposed to high voltage cables, a voltage may appear that appears in only one of its lines, this noise can be reduced by transmitting the signal in current.
Thermal drift
By heating the mass of the thermocouples, energy is extracted that will affect the temperature to be determined. Consider, for example, measuring the temperature of a liquid in a test tube: there are two potential problems. The first is that the heat energy will travel up the thermocouple wire and dissipate into the atmosphere thus reducing the temperature of the liquid around the wires. A similar problem can occur if a thermocouple is not sufficiently immersed in the liquid, due to a cooler air temperature environment in the wires, thermal conduction can cause the thermocouple junction to be at a different temperature than the liquid itself. In this example, a thermocouple with thinner wires may be useful, as it will cause a steeper temperature gradient along the thermocouple wire at the junction between the liquid and the ambient air. If thermocouples with thin wires are used, attention must be paid to the resistance of the lead. Using a thermocouple with thin leads connected to a much thicker lead thermocouple often gives the best result.
Laws
Studies carried out on the behavior of thermocouples have allowed us to establish three fundamental laws:
- Law of the homogeneous circuit: in a homogeneous metallic conductor the circulation of an electric current cannot be sustained by the exclusive application of heat.
- Law on intermediate metals: if in a circuit of several conductors the temperature is uniform from a welding point 'A' to another 'B', the algebraic sum of all electromotric forces is totally independent of intermediate metal conductors and is the same as if they were put in direct contact 'A' and 'B'.
- Act on successive temperatures: The f.e.m. generated by a thermocouple with its unions at temperatures T1 and T3 is the algebraic sum of the f.e.m. of thermocouple with its connections to T1 and T2 and f.e.m. of the same thermocouple with its connections at temperatures T2 and T3.
By these laws it becomes evident that a small continuous voltage is developed in the circuit proportional to the temperature of the measurement junction, whenever there is a temperature difference with the reference junction. The values of this f.e.m. they are tabulated in conversion tables with the reference junction at 0 °C.