JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year MECHANICAL MEASUREMENTS AND CONTROL SYSTEMS Previous Question Paper (February - 2022 - R18)

A thermocouple consists of two dissimilar metal wires joined at both ends to form two junctions: the measuring (hot) junction and the reference (cold) junction. When a temperature difference exists between these junctions, a small electromotive force (EMF) is generated, known as the Seebeck effect, which is proportional to the temperature gradient.

The thermo-electric laws governing thermocouple operation include:

• Law of Homogeneous Circuit: A thermoelectric current cannot be sustained in a circuit consisting of a single homogeneous material, regardless of temperature variations.
• Law of Intermediate Metals: The insertion of a third metal into a thermocouple circuit does not affect the output EMF, provided the two new junctions are at the same temperature.
• Law of Intermediate Temperatures: The EMF generated by a thermocouple with junctions at temperatures T1 and T3 is equal to the sum of the EMFs generated by the same thermocouple with junctions at T1-T2 and T2-T3.

These laws are applied in industrial temperature measurement, allowing for the use of extension wires, the insertion of measuring instruments (like voltmeters) into the circuit, and the calibration of thermocouples against standard reference temperatures.

Dynamic performance characteristics describe how a measuring instrument responds to inputs that change with time. These are typically analyzed by subjecting the instrument to standard inputs like step, ramp, or sinusoidal signals.

Key dynamic characteristics include:

• Speed of Response: The rapidity with which an instrument responds to changes in the measured quantity.
• Dynamic Error: The difference between the true value of the quantity changing with time and the value indicated by the instrument, assuming static error is zero.
• Lag: The delay in the response of an instrument to a change in the measured variable. It is categorized into retardation lag (immediate response but slow) and time delay (finite time interval before response begins).
• Fidelity: The ability of the system to reproduce the output in the same form as the input, without dynamic distortion.

Instruments are mathematically modeled as first-order or second-order systems. A first-order system is characterized by a time constant, while a second-order system is characterized by natural frequency and damping ratio, which determine the overshoot and settling time of the measurement.