JNTU B.Tech III INSTRUMENTATION AND PROCESS CONTROL Previous Question Paper (March - 2021 - R16/R18)

The laws of thermoelectricity are fundamental principles governing the operation of thermocouples:

1. Law of Homogeneous Circuit: A thermoelectric current cannot be sustained in a circuit consisting of a single homogeneous material, regardless of temperature variations along the conductor.
2. Law of Intermediate Metals: The insertion of an intermediate metal into a thermocouple circuit does not alter the net electromotive force (EMF), provided the two new junctions are at the same temperature.
3. Law of Intermediate Temperatures: The EMF generated by a thermocouple with junctions at temperatures T1 and T3 is the sum of the EMFs of two thermocouples, one operating between T1 and T2, and the other between T2 and T3.

An industrial thermocouple thermometer consists of two dissimilar metal wires joined at one end to form the hot junction (measuring junction) and connected at the other end to a measuring instrument to form the cold junction (reference junction). The construction typically involves a protective thermowell or sheath made of stainless steel or ceramic to withstand harsh industrial environments, with the wires insulated from each other and the sheath using ceramic beads or mineral insulation.

The working principle is based on the Seebeck effect, where a temperature difference between the hot and cold junctions creates a potential difference (EMF). This EMF is proportional to the temperature difference and is measured by a millivoltmeter or a digital indicator calibrated to display temperature directly. In industrial applications, the cold junction is often maintained at a constant reference temperature or compensated electronically to ensure accuracy.

Spectroscopic analysis is a technique used to identify or quantify the chemical composition of a sample by measuring the interaction between electromagnetic radiation and matter. The two primary modes are absorption and emission spectroscopy.

Absorption Spectroscopy: This method measures the amount of light absorbed by a sample as a function of wavelength. When a beam of light passes through a sample, specific wavelengths are absorbed by atoms or molecules, causing transitions to higher energy states. The amount of light absorbed is proportional to the concentration of the analyte, as described by the Beer-Lambert Law: A = εlc.

Block Diagram of Absorption Spectrometer:

The radiation source provides a continuous spectrum. The monochromator selects a specific wavelength. The sample cell holds the analyte. The detector measures the intensity of transmitted light, which is then processed to determine absorbance.

Emission Spectroscopy: This method measures the light emitted by a sample after it has been excited by an external energy source, such as heat, electricity, or light. As the excited atoms or molecules return to their ground state, they release energy in the form of photons at characteristic wavelengths.

Block Diagram of Emission Spectrometer:

In emission spectroscopy, the excitation source (e.g., plasma or arc) provides the energy to elevate the sample to an excited state. The emitted light is collected and passed through a monochromator to isolate specific spectral lines, which are then detected and converted into an electrical signal representing the concentration of the element present.

The fundamental difference lies in the source and the measurement: absorption measures the depletion of incident radiation, while emission measures the radiation generated by the sample itself.