JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III MECHANICAL MEASUREMENTS AND CONTROL SYSTEMS Previous Question Paper (March - 2021 - R16)

Temperature measurement principles are categorized based on the physical property changes induced by heat. The primary methods include:

• Expansion Thermometry: Utilizes the thermal expansion of solids (bimetallic strips) or liquids (mercury/alcohol in glass). These are simple and inexpensive but limited in range and response time.
• Electrical Resistance Thermometry: Uses the change in electrical resistance of a conductor (RTD) or semiconductor (Thermistor). RTDs offer high linearity and stability, while thermistors provide high sensitivity over narrow ranges.
• Thermoelectric Effect (Thermocouples): Based on the Seebeck effect, where two dissimilar metals joined at two junctions generate an electromotive force proportional to the temperature difference. They are robust, have a wide range, and require no external power.
• Radiation Pyrometry: Measures the intensity of electromagnetic radiation emitted by a body (Stefan-Boltzmann law). This is a non-contact method suitable for extremely high temperatures or moving objects where physical contact is impossible.

A photoelectric transducer converts light energy into electrical signals based on the photoelectric effect. It consists of a cathode coated with a photosensitive material and an anode enclosed in an evacuated or gas-filled glass bulb.

Construction: The cathode is typically a concave metal plate coated with an alkali metal (like cesium or potassium) that emits electrons when exposed to light. The anode is a thin wire placed in front of the cathode to collect the emitted electrons.

Working Principle: When light photons strike the photosensitive cathode, they transfer energy to the electrons. If the photon energy exceeds the work function of the material, electrons are ejected from the surface (photoemission). When a potential difference is applied between the cathode and anode, these photoelectrons are attracted to the anode, creating a photocurrent in the external circuit. The magnitude of this current is directly proportional to the intensity of the incident light.

Applications: These transducers are widely used in light intensity measurement, industrial counting, automatic door openers, and flame detection systems.