Thermocouple wires materials

For the connection of a thermocouple to the measurement instrument, the best way is to use leads made of the same materials (extension leads) these avoid lead junction errors. For economic reasons, however, cheaper alloys having similar e.m.f. output, at least over a limited temperature range close to room temperature, are often used. These compensating leads are often supplied by the same producers of the thermocouple wires. 

Thermocouples. Their protection. In the various, often hostile, environments suitable sheathing materials must be used to protect the thermocouple wires. 

The traditional source in IR absorption spectroscopy is a glowing rod or wire heated by the passage of an electric current the hot body emits radiation over a continuous frequency range. The radiation is dispersed using a prism NaCl, which is transparent over much of the IR region, is commonly used for IR prisms and windows. The sample may be a solid, liquid, or gas. Various detectors are used the most common are thermocouples, photoconductive materials such as PbS, bolometers (which are temperature-dependent resistors), and the Golay cell (which uses the thermal expansion of a gas contained in a chamber). 

Solid solution alloys of W and Re in the range of 2-25 wt % Re play an important role as rotating anodes in X-ray tubes for diagnostic purposes and as wires for thermocouples. These materials are produced by powder metallurgical techniques. Inappropriate mixing of the two powder components may result in a partial a-phase formation, which is undesirable due to embrittlement. 

Fluoropolymers are used to insulate wire for critical aerospace and industrial applications where chemical and thermal resistance is essential. They are also materials of construction for connectors for high-frequency cables and for thermocouple wiring that must resist high temperatures. 

Thermoelectric Circuits. A typical circuit for a single thermocouple of materials A and B is shown in Fig. 16.17. The reference temperature (at which junctions b and d are maintained) is usually the ice point, 0°C. The connecting wires C are usually copper wires. Note that, according to Eq. 16.20, the connecting (copper) wires C should not affect the EMF EAB, which, for given materials A and B, is just a function of the temperature T. 

Fine wire thermocouples are of low cost and easy to use. They have relatively short response times and good spatial resolution. However, they are somewhat intrusive to the flame. Corrections are also required due to heat transfer effects at the junction bead. These include radiation from the bead to the environment and the heat conduction along the thermocouple wires. Often the junction must be coated to prevent catalytic reactions. There are material limitations, due to high temperature oxidizing and reducing conditions. They can only make so many point measurements over a span of time to get a temperature profile. 

The junctions may be formed with a variety of methods, including an oxyacetylene flame and spot welding. The choice of method depends on the oxidation resistance of the particular type of thermocouple wire used. Typically the actual thermocouple wires are made of higLpurity, reference-grade material. Wires with compositional variations may generate secondary voltages and produce erroneous readings. 

Corrosion resistance can also be taken care of by a thermowell made from a suitable construction material. Generally a proper sealing, weather-resistant cover protection should be providedfor the junction box ofthe thermowell, i.e., where cables are connected to the ends of the thermocouple wires (please see thermowells). 

The second experimental setup in Fig. 3.5 eliminates the need to carry the thermocouple wires to the measuring instrument by changing from thermocouple material to normal conductance copper in a reference ice bath. In this arrangement all subsequent Cu/Cu junction potentials cancel. Such an arrangement is particularly useful if the reference bath is automated or a controlled counter-emf is used. 

When certain materials are bonded together, electrons tend to transfer from one to the other. This is called the Volta effect. If two such materials are joined together with two junctions at the same temperature, the plus Volta emf at one junction will be balanced by a minus emf at the other and no current will flow. However, if the two junctions have different temperatures, a current will flow from one junction to the other. This is called the Seebeck effect, and is the basis of the thermocouple. Figure 11.1 (a) shows a thermocouple with a small voltmeter in series with two thermocouple wires (iron and 60 Cu-40 Ni constantan). The emf will be proportional to (T2 - Tj). Figure 11.1 (6) gives the calibration curve for an iron-constantan couple. Thermocouples are used to measure very high (furnace) temperatures, and when the upper range of an iron-constantan thermocouple is reached, a platinum-rhodiiun couple [also shown in Fig. 11.1(6)] may be employed. 

This method, which uses a cross-wire welded at the center, is given in ISO 8894-1 1987. One of the two wires is cormected to a power source used as a heating element. The other wire is a thermocouple wire. This cross-wire is sandwiched between two blocks of the refractory material. Power is fed into the heating element for a short time. The temperature rise in the blocks is measured. This temperature rise is related to the thermal conductivity of the material. Thermal conductivity values up to 2 W/mK can be measured by this method. 

Despite the delay and difficulties in derivation of a suitable theoretical treatment for differential thermal analysis, it is clear that such derivations have not only justified certain aspects that had been empirically established—such as the relationship between peak area and amount of reactant and the necessity for dilution of the specimen with reference material— but have also revealed aspects that were not fully appreciated—such as the fact that the proportionality relationship holds strictly only for a ATf curve and the importance of heat conduction along thermocouple wires. Further developments in quantitative application obviously depend on designing experimental conditions compatible with those demanded by theory Wilburn [1972]. 

Thermistors have been used in another attempt to avoid heating of thermocouple wires. Unfortunately, these devices are liable to be limited in the temperature ranges within which they can be used to obtain resu lts(22). Bosisio et al developed an alternative approach in which the material properties %iere monitored by a calibrated signal at a second, lower frequency. Changes in this second signal could then be used to provide an indication of temperature. Such a system may well find seme applications, though it is liable to be expensive. 

Rhenium is also used as an electrical contact material because it has good wear resistance and withstands arc corrosion. Thermocouples made of Re-W are used for measuring temperatures up to 2200C, and rhenium wire is used in photoflash lamps for photography. 

Fig. 5. Basic thermocouple circuit. A and B are wires of different materials.

Fig. 6. Rule 3 of thermocouples, where (a) represents a four-junction and (b) a two-junction thermocouple. A and B, the two legs of the thermocouple, are wires of dissimilar materials. Junctions are at temperatures T, and Ty Ey E, < ...

In industrial appHcations it is not uncommon that the thermocouple must be coupled to the readout instmment or controUer by a long length of wire, perhaps hundreds of feet. It is obvious from the differential nature of the thermocouple that, to avoid unwanted junctions, extension wine be of the same type, eg, for a J thermocouple the extension must be type J. Where the thermocouple is of a noble or exotic material, the cost of identical lead wine may be prohibitive manufacturers of extension wine may suggest compromises which are less costiy. Junctions between the thermocouple leads and the extension wine should be made in an isothermal environment. The wine and junctions must have the same electrical integrity as the thermocouple junction. Because the emf is low, enclosure in a shield or grounded conduit should be considered. 

Thermocouples are primarily based on the Seebeck effect In an open circuit, consisting of two wires of different materials joined together at one end, an electromotive force (voltage) is generated between the free wire ends when subject to a temperature gradient. Because the voltage is dependent on the temperature difference between the wires (measurement) junction and the free (reference) ends, the system can be used for temperature measurement. Before modern electronic developments, a real reference temperature, for example, a water-ice bath, was used for the reference end of the thermocouple circuit. This is not necessary today, as the reference can be obtained electronically. Thermocouple material pairs, their temperature-electromotive forces, and tolerances are standardized. The standards are close to each other but not identical. The most common base-metal pairs are iron-constantan (type J), chomel-alumel (type K), and copper-constantan (type T). Noble-metal thermocouples (types S, R, and B) are made of platinum and rhodium in different mixing ratios. 

Amounts of heat released in pyrolysis trials were small. Even heats measured at around 130 C, far above the 80"C margin, could adiabatically raise the temperature by less than one centigrade in an hour (6). With so little heat, inaccuracies of measurements could cause qualitative mistakes. Additionally, temperatures were usually determined with thermocouples, whose metallic wires conducted heat up to one thousand times faster than the pyrolzed material, and could again involve substantial errors, especially with small samples and steep temperature gradients. 

Saito with a fine wire thermocouple embedded at the surface [3]. The scatter in the results are most likely due to the decomposition variables and the accuracy of this difficult measurement. (Note that the surface temperature here is being measured with a thermocouple bead of finite size and having properties dissimilar to wood.) Likewise the properties k. p and c cannot be expected to be equal to values found in the literature for generic common materials since temperature variations in the least will make them change. We expect k and c to increase with temperature, and c to effectively increase due to decomposition, phase change and the evaporation of absorbed water. While we are not modeling all of these effects, we can still use the effective properties of Tig, k, p and c to explain the ignition behavior. For example,... 

Electric tube furnaces of appropriate dimensions are available from various manufacturers. A model RO 4/25 by Heraeus GmbH, Hanau, FRG is suitable. However, a very satisfactory furnace can be built by any well equipped laboratory workshop at little cost and effort. The material required consists of thin walled ceramic tubing, 3.5 cm i.d., nichrome resistance wire, heat resistant insulation, and ordinary hardware material. A technical drawing will be provided by the submitters upon request. The temperature of the furnace can be adjusted by an electronic temperature controller using a thermocouple sensor. A 1.5 kW-Variac transformer and any high temperature thermometer would do as well for the budget-minded chemist. 

Thermocouples are composed of two dissimilar materials, usually in the form of wires, that accomplish a net conversion of thermal eneigy into electrical energy with the occurrence of an electrical current. Unlike resistance thermometers, where the response is proportional to temperature, the response of thermocouples is proportional to the temperature difference between two junctions. Figure 5 illustrates such a circuit. 

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