Electrical Engineering ⇒ Topic : Thermocouple Instruments
The hot-wire instruments are obsolete and have been superseded by thermocouple instruments. A thermocouple instrument consists of (i) a permanent-magnet moving coil instrument and (ii) a thermoelement i.e., an evacuated glass bulb containing a heater wire and a *thermocouple whose active junction J is in contact with the heater as shown in Fig. (a). When operating current flows through the heater, the heat produced is applied to the active junction J of the thermocouple. Due to thermo-electric effect, a direct voltage (directly proportional to the heat) appears across the cool ends 1 and 2 of the thermocouple. The permanent-magnet moving coil instrument connected across ends 1 and 2 will give the indication of the current flowing in the line. Since the heating effect in a resistance (i.e., heater in this case) is independent of current direction, thermocouple instruments can be used for both d.c. and a.c. measurements.
As the amount of direct voltage (called thermo-e.m.f.) appearing across the cold ends of the thermocouple is directly proportional to the heating effect (I2R) at the active junction J, the deflection is directly proportional to the square of current i.e.,
Scale. It may be seen that scale of such an instrument is of square-law type i.e., crowded in the beginning and open near the end of the scale. The scale can be modified to a uniform type by changing the shape of the pole pieces of the moving-coil meter as shown is Fig.(b). When the moving coil is in its low-scale position (i.e., pointer is to the left), it is cutting across stronger portion of the magnetic field as indicated by the concentration of the magnetic lines. Consequently, the deflecting torque *increases which makes the meter more sensitive during the initial portion of the scale. When the coil is in its high-scale position (i.e., pointer is to the right), it is in the weaker portion of the field. This decreases the deflecting torque and hence the sensitivity of the meter for that portion of the scale. The effect of this arrangement is that the scale of the meter tends to change from square-law relation to the linear one
They are based on the Seebeck effect that an e.m.f. is generated across a junction formed of two dissimilar metals. It was later on discovered by Peltier and Thomson that the generation of e.m.f. due to contact of dissimilar metals (Thomson effect) is very much smaller than the generation of e.m.f. due to the difference of junction temperatures (Peltier effect).
The dissimilar metals and the e.m.f. generated, by keeping one junction at 0°C (32°F) are tabulated in Table (1).
Table (1) Dissimilar metals and e.m.f. generated
The basic equation for generation of e.m.f. is:
VT = KAB(TM - TR) = VM - VR
where KAB is the thermosensitivity of the couple AIB depending on the temperature, KAB = f(T),TM is the temperature of the measurement of hot junction, and TR is temperature of the reference or cold junction, usually at 0°C, Vm is the e.m.f. in hot junction, and VR is the e.m.f. in cold junction.
If the two junctions are at the same temperature then no e.m.f. is generated. The two junctions are soldered or brazed. Two methods of connecting the PMMC meter for reading the generated e.m.f. are shown in Figure (2).
figure (2) Methods of connecting PMMC meter
By breaking one metal and connecting a meter with different metal leads does not effect the e.m.f. generated provided the break points are at the same temperature. In Figure 2(a) the break points are at environmental temperature and in Figure 2 (b) at the temperature of the cold junction. The cold junction is an ice bath like Dewar's jar. This helps in keeping the measuring instruments away from the thermocouple (TC). However, for greater accuracy, it is preferable to have connected leads of the same metal as the metal of thermocouple disjointed.
A number of thermocouples connected in series is called a thermopile. Assuming identical thermocouples, the output e.m.f. will be n times of each thermocouple for a pile of n thermocouples.
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