JNTU B.Tech II BIOTRANSDUCERS AND APPLICATIONS Previous Question Paper (November/December - 2020 - R13)

Quasi-state effects refer to the phenomenon where a transducer exhibits a time-dependent response that deviates from its ideal static characteristics due to the presence of internal energy storage elements or thermal/mechanical inertia. In many physical systems, the output is not strictly a function of the instantaneous input but is influenced by the history of the input or the rate of change of the input.

When the frequency of the input signal is low enough, the transducer behaves in a quasi-static manner, meaning it follows the static calibration curve closely. However, as the frequency increases, the quasi-state effect becomes significant, leading to phase shifts, amplitude attenuation, and hysteresis. This is primarily caused by factors such as the mass of the sensing element (mechanical inertia), the thermal capacity of the probe (thermal lag), or the dielectric relaxation time in capacitive sensors. To minimize these effects, transducers are designed with low mass, high stiffness, or high thermal conductivity to ensure the dynamic response remains as close to the static response as possible within the required operating frequency range.

Thermoresistive transducers, such as RTDs and thermistors, operate on the principle that the electrical resistance of a material changes with temperature. For metals like platinum, resistance increases linearly with temperature, while for thermistors (semiconductors), resistance changes non-linearly, typically decreasing as temperature rises. The sensing element is placed in a Wheatstone bridge circuit to convert the resistance change into a measurable voltage.

Thermoelectric transducers, or thermocouples, operate on the Seebeck effect. When two dissimilar metal wires are joined at two junctions maintained at different temperatures, a small electromotive force (EMF) is generated proportional to the temperature difference between the hot (measuring) junction and the cold (reference) junction. The output voltage is typically in the millivolt range and requires amplification.

Diagram description: A thermoresistive setup consists of a sensing resistor connected in a bridge configuration with a constant voltage source and a galvanometer or voltmeter. A thermoelectric setup consists of two dissimilar metal wires (e.g., Chromel-Alumel) joined at both ends, with one junction exposed to the process temperature and the other held at a known reference temperature, connected to a high-impedance voltmeter to measure the generated potential difference.