Emf-measuring device

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 metallic wires if the two junctions are at different temperatures. The thermocouple may be represented diagrammatically as shown in Fig. 8-63. There A and B are the two metals, and I) and T2 are the temperatures of the junctions. Let 7) and T2 be the reference junction (cold junction) and the measuring junction, respectively. If the thermoelectric current i flows in the direction indicated in Fig. 8-63, metal A is customarily referred to as thermoelectrically positive to metal B. Metal pairs used for thermocouples include platinum-rhodium (the most popular and accurate), chromel-alumel, copper-constantan, and iron-constantan. The thermal emf is a measure of the difference in temperature between T2 and Ij. In control systems the reference junction is usually located at the emf-measuring device. The reference junction may be held at constant temperature such as in an ice bath or a thermostated oven, or it may be at ambient temperature but electrically compen-... 

Consider a basic thermocouple circuit with one measuring junction, as shown in Fig. 16.17. The two thermoelements A and B are joined at point c to form the measuring junction at temperature T. The thermoelements are connected to wires C at points b and d, both immersed in an ice bath (liquid water and ice in equilibrium) at T0. The two wires C are connected to the input of an EMF measuring device. The input ports, a and e, are maintained at temperature 7j. Applying Eq. 16.18 over the various legs of the circuit gives... 

Thermocouple Components and Fabrication. A thermocouple measurement assembly includes a sensing element assembly, extension wires (when used), reference junction, connecting wires, an EMF-measuring device (possibly with signal-processing equipment), and other hardware needed for applications in adverse environments such as protection tubes, connectors, adapters, and so on. Each of the above components will be discussed in the following paragraphs. 

By hooking the other end of joined dissimilar wires up to a voltmeter and measuring the emf (output), it is possible to determine temperature. Such a temperature-measuring device is called a thermocouple. 

Equation 16.18 is particularly useful in deriving the net EMF of a complex thermoelectric circuit. In many reference works, three laws—the law of homogeneous materials, the law of intermediate materials, and the law of intermediate temperature—are used to show how the EMF at a measuring device is affected by various lead wires from a thermocouple junction [33], These laws can be derived from Eq. 16.17 but are generally more difficult to apply in a complex circuit. 

The EMF measured by a readout device reflects the difference between the temperatures of the measuring junction and the reference junction thus, the temperature at the reference junction must be properly controlled. It is usually maintained at the freezing point of water (0°C) by using an ice bath (Fig. 16.23). The ice bath should be a mixture of crushed ice and pure water in a Dewar flask with considerably more ice than liquid. It can be very accurate but not too convenient in some applications, since frequent replacement of melted ice is required. With a thermoelectric refrigeration system [36], ice can be maintained by cooling the bath of water with thermoelectric cooling elements. 

A healthy battery for powering a Walkman or radio has a voltage of about 1.5 V. In the terminology of batteries, this value is called its open-circuit potential, but an electrochemist talking in terms of cells will call it the emf. This voltage is read on a voltmeter when we remove the battery from the device before measurement. But the voltage would be different if we had measured it while the battery was, for example, powering a torch. 

A reference electrode is defined as a constant-potential device . In other words, if we make a cell in which one half cell is the SHE and then measure the emf. we then simply employ the equation emf = Eright-hand side — ieft-hand side (equation (3.3)) to determine the other half-cell potential. 

Figure 15-10. a) Electrochemical device for the determination of chemical diffusion in mixed conductors (Ag2+aS). b) Course in time of 5 (resp. emf of Ag/Ag[ probes), measured at probe n. 6 = starting composition. 

Thermocouples consist of two dissimilar electrical conductors which are joined to form a measuring junction, with the free ends of the wires constituting the reference junction. When a temperature difference exists between the measuring and reference junctions, an emf is produced between the free ends of the device. This emf, which is a function of the temperature difference, can be used to determine the temperature at the measuring junction if the reference junction temperature is known. A schematic of a typical thermocouple circuit is shown in Fig. 9.12. 

One device of this type was discussed by Hickam and Witkowski(23). In the diffusion limited devices that have been discussed, the flow rate of the gas is not normally an important factor. In the Hickam device, however, the gas flow is of paramount importance and new possibilities or complications arise. The structure consists of pump (upstream) and sensor (downstream) cells cylindrically surrounding a flowing stream of gas containing oxygen. The sensor cell EMF is fedback to the pump so that oxygen is either added to or subtracted from the stream in the amount required to keep the sensor EMF at a constant value. For a calibrated device, the amount of pump current required measures the oxygen content of the gas at the inlet of the structure provided the flow rate is held constant. Alternately, if a gas of constant composition were employed, the structure could be used to measure flow rate. 

A common device for measuring a cell s emf is the potentiometer, which can match the voltage of the cell precisely without actually draining any current from it. 

The temperature drop across the thermal resistance is usually measured with a multijunction thermocouple (thermopile) to increase the sensitivity of the device. The sensitivity of a heat flux sensor depends on the slab material and slab thickness, which essentially determine Rik, and the number of junctions in the thermopile, which determines the output EMF as a function of A T. 

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