Jawaharlal Nehru Technological University Hyderabad B.Tech IV ELECTRONIC MEASUREMENTS AND INSTRUMENTATION Previous Question Paper (October/November - 2020 - R15)

Performance characteristics of measuring systems are classified into static and dynamic characteristics. Static characteristics describe the performance of instruments when the measured quantity is constant or varies slowly over time, while dynamic characteristics describe the response to rapidly changing inputs.

Static Characteristics:

• Accuracy: The closeness with which an instrument reading approaches the true value of the quantity being measured.
• Precision: A measure of the reproducibility of the measurements, indicating how closely multiple readings of the same quantity agree.
• Sensitivity: The ratio of the change in output of the instrument to the change in input.
• Resolution: The smallest change in the input signal that can be detected by the instrument.
• Linearity: The ability of the instrument to produce an output that is directly proportional to the input over a specified range.
• Hysteresis: The difference in output for the same input when approached from increasing versus decreasing directions.
• Drift: A gradual shift in the instrument's reading over time while the input remains constant.

Dynamic Characteristics: These include speed of response, fidelity (the ability to reproduce the input signal without distortion), and dynamic error (the difference between the true value and the measured value during transient conditions).

A DC voltmeter is primarily based on the PMMC (Permanent Magnet Moving Coil) movement, which acts as a current-sensitive device. To measure voltage, a high resistance called a multiplier is connected in series with the PMMC meter movement.

The multiplier limits the current through the meter coil so that the full-scale deflection current (I_fsd) is not exceeded. If the internal resistance of the meter is R_m and the desired full-scale voltage range is V, the required multiplier resistance R_s is calculated using the formula:

The diagram consists of a PMMC movement symbol in series with a resistor labeled R_s, with the input voltage V applied across the entire series combination. As the voltage increases, the current through the coil increases proportionally, causing the pointer to deflect across a calibrated scale marked in volts. By switching between different values of R_s, a multi-range DC voltmeter can be constructed to measure various voltage levels.