1. With the aid of a sketch, explain how compensation is achieved in mercury in steel thermometers.
2. A resistance thermometer has a resistance of 117.254 sl at 18°C. If the temperature coefficient of resistance of the resistance thermometer wire is 0.003664 plot a graph of thermometer resistance to a base of temperature over the range 0°C to 100°C and determine :
(i) the fundamental interval of the thermometer
(ii) the resistance at 65°C.
3. With the aid of a circuit diagram(s) show how a manufacturer can avoid the balancing point of a bridge being affected by the high resistance associated with a dirty slidewire contact.
4. (a) The resistance thermometer bridge circuit shown in FIGURE 1 has a designed maximum temperature of 200°C, ignoring the effects of connecting wire resistance. If the connecting loop is 250 m determine the smallest gauge (swg) of copper wire which must be used if the indicated maximum temperature is to be less than 202°C.
The temperature coefficient of resistance of the thermometer is 0.0042 °C-I and the resistance of the thermometer is 112 C2 at 20°C. Assume the connecting leads are at 20°C.
(b) Using the gauge of copper wire calculated in part (a), calculate the maximum indicated temperature using a 3-wire system (as shown in FIGURE 2) over a distance of 125 m.
5. (a) Explain what is meant by 'the law of intermediate metals' relating to temperature measurement using thermocouples.
(b) With the aid of sketches give two industrial situations to which the law in (a) is applicable.
6. A long length of compensating cable is joined as shown in FIGURE 3. The conductors are not touching directly and the connection is made via the connectors themselves. What effect (if any) will the connection have on the accuracy of the system? Give reasons for your answer.
7. (a) With the aid of a sketch describe the construction and principle of operation of a total radiation pyrometer.
(b) Explain how the following parameters affect the total amount of radiation which is emitted from a hot object :
(i) object temperature
(ii) object surface area.
(c) Explain why the emissivity of a black body is equal to I.