Thermocouple types

The noble metal thermocouples, Types B, R, and S, are all platinum or platinum-rhodium thermocouples and hence share many of the same characteristics. Metallic vapor diffusion at high temperatures can readily change the platinum wire calibration, hence platinum wires should only be used inside a nonmetallic sheath such as high-purity alumina. 

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. 

Thermocouples. In several temperature ranges, the thermocouple is usually one of the most useful instruments for the accurate determination of temperature it is probably the most versatile temperature transducer and, because of its small thermal capacity and ready response to changing temperatures, it is especially suitable for equilibrium diagram work (see also 2.4.1). For this reason, and considering also the special conventional codes generally used to identify the various thermocouple types, a few more remarks will be reported here on this subject. 

Figure 6.7. Characteristics of commonly used and commercially available thermocouples e.m.f. data, with cold junction at 0°C (For the symbols of the various thermocouple types, see Table 6.1).

Temperature thermocouples (Type-K 1 mm) at ten vertical positions in the bed (Figure 10). 

BS 4937 Part 4 1973. British Standard 4937 (British Standards Institution, London). Nickel-chromium/nickel-aluminium thermocouples. Type K. 

The most common thermocouple type used in a laboratory situation is the Type J iron/constantan thermocouple. Low cost, high thermopower, and a use-... 

Figure 1. Supercritical flow reactor. Key (I) Mettler balance (2) flask with 1 0 (filtered and deaerated) (3) HPLC pump (4) bypass (three-way) valve (5) feed cylinder (6) weather balloon with feed solution (7) probe thermocouple (type K) (8) ceramic annulus (9) Hastelloy C-276 tube (10) entrance cooling jacket (11) entrance heater (12) furnace coils (13) quartz gold-plated IR mirror (14) window (no coils) (15) guard heater (16) outlet cooling jacket (17) ten-port dualloop sampling value (18) product accumulator (19) air compressor (20) back-pressure regulator and (21) outflow measuring assembly.

In several renewable energy processes, including the concentrating solar collectors, boilers, and combustion systems, the accurate measurement and control of high temperatures are required. These (over 1,000°C) temperatures are most often detected by thermocouples (types B, C, R, and S) and by optical and IR-radiation pyrometers. These devices are only briefly mentioned here, because they will be discussed in detail later. Here, the emphasis will be on some of the other high-temperature detectors such as sonic and ultrasonic sensors. 

The Seebeck coefficient gives the amount of voltage generated (in microvolts) by a 1°C change in temperature. The value of the Seeback coefficient varies not only with the thermocouple type but also with temperature. 

Table 2.2 shows various standardized thermocouple types commonly available. Each is optimal for a given set of conditions. For example, type K wire is used for lower temperature ( 1100°C max) furnaces and type S thermocouples for higher temperature furnaces ( 1500°C max). Type K is much less expensive than 5, has a higher (voltage) output, but is less refractory. The two alloys in type K can be distinguished since alumel is magnetic and chromel is not. The rhodium content of... 

Junction beads can be made for platinum-based thermocouples, such as types R, S, or B, by welding with a high-temperature flame (e.g. oxy-acetylene). Using a flame for junction formation in alloy-based (e.g. K- or 2 -type) thermocouples does not work well since the wires tend to oxidize rather than fuse. Beads are more effectively made by electric arc for these thermocouple types. 

Figure 2.7 Thermocouple output as a function of temperature for various thermocouple types.

There are seven common thermocouple types as identified by the American National Standards Institute (ANSI). They are identified by letter designation and are described in Table 2.33. There are four other thermocouple types that have letter designations however, these four are not official ANSI code designations because one or both of their paired leads are proprietary alloys. They are included at the end of Table 2.33. Although Table 2.33 lists the standard commercially available thermocouples, there are technically countless other potential thermocouples because all that is required for a thermocouple is two dissimilar wires. 

Be advised that the upper range of temperatures cited (within thermocouple catalogs) for a given thermocouple are related to the larger wire sizes. Thus, a small-wire thermocouple is likely to fail at the upper temperature ranges for that particular thermocouple type. 

A variety of thermocouples are available (Table 10.8), suitable for different measuring applications, and they are usually selected for the temperature range and sensitivity needed, for chemical inertness, and for magnetic properties (usually undesirable). They have been given alphabetic identifiers that have been accepted industry-wide. The B-, R-, and S-type thermocouples have low sensitivities and low resolutions the S type thermocouple is partially sheathed with a tube of fused alumina. The thermocouple types... 

Three kinds of sample holders are available for DTA and DSC (Fig. 4.8.6). Type 1 holders are commonly used for a DTA apparatus. In this type, the sample and reference holders are placed on the same metal block and heated by the same heater the temperature difference between the two holders is indicated by a thermocouple. Type 2 holders are generally used in a quantitative DTA (heat-flux DSC) apparatus. Both sample and reference holders are maintained at the same temperature by two individual internal heaters, which, in turn, are heated by the same main heater. The temperature difference between the sample and the reference material is measured by a thermocouple placed outside of the holders. Type 3 holders are customarily used for a power-compensated DSC apparatus. This apparatus has separate heaters for heating the sample and reference holders thus maintaining the sample and the reference... 

If the temperature of the liquid or solution being cooled is critical, do not assume that the temperature of the liquid or solution is the same as that of the cooling bath place a thermometer in the flask and remember that for temperatures lower than —5 C you should use an alcohol thermometer (red thread) or a thermocouple-type thermometer (after checking that the probe will not react with the contents of the flask). 

The temperature inside the reactor is measured with seven thermocouples (Type K). To avoid channelling, the thermocouples are placed only 5 mm into the bed. The temperature inside the manifold is also measured, as well as at several points at the outlet pipe. The pressure in the manifold and inside the reactor is also measured with the help of pressure transducers. In order to take gas samples, a sample line has been built (Fig. 4). It consists of a steel condenser and three glass bottles in an ice bath, a moisture filter (silica gel), a pump and a gas flowmeter. A sample of O.S ml is taken from the gas sample line with a syringe and inserted into a gas chromatograph from SRI Instruments equipped with a TCD detector and a Supelco column (Carboxen I0(X)). 

The large rig is of the same size as a domestic boiler. The design is chosen to ensure well-defined start- and boundary conditions for the fuel bed. The rig is equipped to measure airflow, weight loss and bed height. Temperatures can be measured upstream, in and downstream of the fuel bed and in the grate by shielded 1-mm thermocouples (type K), mounted both from the side (orthogonal to the movement of the ignition... 

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