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

A pressure spring thermometer is a filled-system temperature measuring device consisting of a sensing bulb, a capillary tube, and a pressure-sensitive element (usually a Bourdon tube). The system is filled with a fluid—liquid, gas, or vapor—that expands or exerts pressure as temperature changes.

Construction: The bulb is placed in the medium to be measured. It is connected to the Bourdon tube via a capillary tube. The Bourdon tube is linked to a mechanical pointer on a calibrated scale. As the bulb temperature rises, the fluid inside expands or increases in pressure, causing the Bourdon tube to uncoil, which moves the pointer.

Working: In liquid-filled systems, expansion of the liquid drives the movement. In gas-filled systems, pressure increases according to the ideal gas law. In vapor-pressure systems, the pressure is determined by the vapor pressure of the liquid at the bulb temperature, which is a non-linear function of temperature. The device is robust and suitable for remote temperature indication without requiring external power.

A thermal conductivity gas analyzer operates on the principle that different gases have different thermal conductivities. It typically uses a Wheatstone bridge circuit where two arms contain heated filaments (sensing elements).

Working: One filament is exposed to a reference gas (often air or a known inert gas), and the other is exposed to the sample gas mixture. When current flows, the filaments heat up. The heat dissipated from each filament depends on the thermal conductivity of the surrounding gas. If the sample gas has a different thermal conductivity than the reference gas, the temperature of the sensing filament changes, altering its electrical resistance.

This resistance change creates an imbalance in the Wheatstone bridge, producing an output voltage proportional to the concentration of the specific gas component being measured. This method is highly effective for detecting gases like hydrogen or carbon dioxide in mixtures where the thermal conductivity of the target gas significantly differs from the background gas.