Cold junction

Emf values are given vs the reference-grade platinum of NIST (NBS Pt27) with the cold junction at the ice point. 

The cold-junction temperature can be fixed by immersing the cold junctions into some known thermal environment an ice bath or a properly maintained water triple-point cell. A temperature-controlled oven at a temperature above ambient may be used. 

The cold junction may be attached to or contained within an isothermal mass, eg, a large block of copper, adequately insulated from ambient, whose temperature is measured by some independent means, eg, an embedded thermistor or PRT. 

Cold-junction compensation can be provided by a network which includes a constant voltage source and a temperature-sensitive bridge to provide an offset voltage which is proportional to the temperature sensitivity of the thermocouple and of opposite sign. 

Thermocouples Temperature measurements using thermocouples are based on the discovery by Seebeck in 1821 that an electric current flows in a continuous circuit of two different metalhc wires if the two junctions are at different temperatures. The thermocouple may be represented diagrammaticaUy as shown in Fig. 8-60. A and B are the two metals, and T and To are the temperatures of the junctions. Let T and To be the reference junction (cold junction) and the measuring junc tion, respectively. If the thermoelectric current i flows in the direc tion indicated in Fig. 8-60, metal A is customarily referred to as thermoelectricaUy positive to metal B. Metal pairs used for thermocouples include platinum-rhodium (the most popular and accurate), cmromel-alumel, copper-constantan, and iron-constantan. The thermal emf is a measure of the difference in temperature between To and T. In control systems the reference junction is usually located at... 

If the instrument is to be direct reading, the second (or cold ) junction must be kept at a constant reference temperature. If high temperatures are to be measured then the terminals of the detector can be used as the cold junction without an unacceptable loss of accuracy. 

The commercial units have a very low thermal capacity and very high response speeds. Some are available with several independent channels and a common cold junction. Each channel is scanned in turn by the instrument, and the readings either displayed or stored for future recovery. Accuracies of better than 0.2 per cent are possible. Thermocouples are available to cover a very wide range of temperatures, their cost is low and they have a small mass, so minimizing the intrusive effect on the surface at the point where the temperature is being measured. The output characteristics (output voltage versus temperature) are reasonably linear but the measurement accuracy is not particularly high. 

As examples of properties of systems satisfying the conditions of definiteness at a particular temperature and of reversion, we may refer to the electrical resistance of a metal wire the electromotive force of a thermocouple with a fixed temperature at the cold junction the volume of a homogeneous gaseous, liquid, or... 

We first assume that the Peltier effects are the only reversible heat effects in the circuit. Then if 7Ti, 7t2 are the Peltier effects at the hot and cold junctions ... 

The production of a current of electricity by heating a junction formed by two dissimilar metals. For temperature measurement the metals are usually in the form of wires see Thermocouple) and the circuit has two junctions, the hot junction which is exposed to the temperature to be measured and the cold junction which is kept at a standard temperature. The thermo-electric effect is also termed the Seebeck Effect after its discoverer. 

Thermopiles are considered temperature sensors and are fabricated incorporating a number of thermocouples. Each thermocouple is formed by a couple of different materials (Metall-Metal2, Metal-Semiconductor, Semiconductor-Semiconductor) and responds to a temperature difference localized between the two junctions (cold junction and warm junction ), see fig. 11. One of the two junctions can be considered the reference one. 

During operation the voltage developed at the thermopile output is proportional to the thermoelectric power of each of the two different materials and to the temperature difference between the warm and cold junction (Seebeck effect). 

A thermopile can also be used as a chemical sensor if one of the two materials is a catalytic metal for a given volatile compound. In this case it is necessary to keep the warm and cold junctions at constant temperature. During absorption of the volatile compound on behalf of the catalytic material the thermoelectric power may change, giving rise to an output voltage which can be related to the concentration of the volatile compound. A typical example is the thermopile as hydrogen sensor, where one of the two materials is palladium, a standard hydrogen catalyzer. 

It is important to know the temperature of the cold junction for evaluation of the exact temperature in a measured curve. In the case where the temperature is... 

C, no correction is required. For higher temperatures at the cold junction corresponding corrections have to be made. 

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).

FIGURE 3.17 Typical device (Eppley Laboratories Model 8-48) used to measure solar irradiance. The detector consists of a differential thermopile with the hot junction receivers blackened with flat black coating and the cold junction receivers whitened with BaS04 (photo supplied courtesy of G. L. Kirk, Eppley Laboratories). 

TTeat absorbed at the cold junction is pumped to the hot junction at a rate proportional to earner current passing through die circuit and the number... 

Power supply capabilities for thermoelectric coolers range from the simple open-loop direct current supply with a switch to sophisticated feedback systems with close temperature regulation and fast response. The only limitation on the supply is that ripple be maintained at a point lower than 10 to 15%. Open-loop systems will generally contain a transformer, rectifiers, choke, and chassis with heat sink for the rectifiers. In feedback systems, a thermistor is used to sense temperature at the cold junction. This signal is compared with the desired temperature setting to obtain an error signal. 

Thermocouples operate on the principle of thermoelectricity (Seebeck effect). When the junction of two dissimilar electronic conductors - typically two metals, two semimetals, or semiconductors - is held at temperature T (the so-called cold junction ), a measurable voltage develops between it and the hot (Tz) junction. This thermoelectric voltage (V) of the two conductors can be measured according to (3.9). 

Thermocouples are based on the thermoelectric Seebeck effect, which generates a voltage at the junction between two metallic conductors, which depends on temperature [13]. Thus, in the measuring circuit, two junctions are created, namely, a sensitive (or hot) junction at the point where temperature has to be measured and a nonsensitive (cold) junction, kept at a constant known temperature, where the voltage established between the conductors can be easily measured [19]. Different typologies of thermocouples exist for application in a wide range of conditions they essentially differ by the materials, the most common being J (iron/constantan), K (chromel/alumel), T (copper/constantan), and E (chromel/constantan). 

Thermistors have the desirable characteristics of small size, narrow spans, fast response (their time constant can be under 1 second), and a very high sensitivity. They do not need a cold-junction compensation, errors due to contact or lead-wire resistance are insignificant, and they are well suited for remote temperature sensing. They are inexpensive, their stability increases with age, and they are the most sensitive differential temperature detectors available. 

The most often used temperature detectors in renewable energy and most other processes are the thermocouples (TCs). Their operation is based on the principle known as the Seebeck effect. T. J. Seebeck discovered that heating the junction of dissimilar metals generates a small, continuous electromotive force (EMF). The name is a combination of thermo and couple denoting heat and two junctions, respectively. The dissimilar TC wires are joined at the hot (or measurement) end and also at the cold junction (reference end),... 

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