JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III INSTRUMENTATION AND PROCESS CONTROL Previous Question Paper (August - 2022 - R18)

A thermocouple is a temperature-measuring device consisting of two dissimilar metal wires joined at one end to form a sensing junction, known as the hot junction. The other ends are connected to a measuring instrument, forming the reference or cold junction. When the two junctions are at different temperatures, a small voltage is generated, known as the Seebeck effect.

The construction involves two wires, such as Chromel and Alumel, twisted or welded together. The magnitude of the generated electromotive force (EMF) is proportional to the temperature difference between the junctions. The relationship is given by:

where E is the EMF, T₁ is the hot junction temperature, T₂ is the cold junction temperature, and α and β are Seebeck coefficients specific to the materials used. In practice, the cold junction is often maintained at a constant reference temperature or compensated electronically to ensure accurate readings.

Spectrophotometers measure the interaction of light with matter to determine chemical composition. Emission and absorption types operate on distinct principles.

Absorption Spectrophotometry: This technique measures the amount of light absorbed by a sample as a function of wavelength. A light source emits a continuous spectrum, which passes through a monochromator to isolate a specific wavelength. This monochromatic light passes through the sample, and a detector measures the intensity of transmitted light. The concentration is determined using the Beer-Lambert Law:

where A is absorbance, ε is molar absorptivity, c is concentration, and l is path length.

Emission Spectrophotometry: This technique measures the light emitted by a sample after it has been excited by an external energy source, such as heat or electricity. The sample absorbs energy, causing electrons to move to higher energy states. As they return to their ground state, they emit light at characteristic wavelengths. The intensity of this emitted light is proportional to the concentration of the analyte. The system consists of an excitation source, a sample holder, a monochromator to select the emission wavelength, and a detector to quantify the light intensity.