JNTU B.Tech III ELECTRONIC MEASUREMENT AND INSTRUMENTATION Previous Question Paper (October - 2020 - R16)

A D.C. current meter, or ammeter, is constructed by connecting a low-resistance shunt resistor in parallel with a PMMC (Permanent Magnet Moving Coil) galvanometer. The shunt resistor allows the majority of the current to bypass the sensitive meter movement, enabling the measurement of currents significantly higher than the meter's full-scale deflection current.

The working principle relies on the magnetic effect of electric current. When current flows through the meter, the magnetic field produced by the coil interacts with the permanent magnet's field, creating a torque that deflects the pointer. The shunt resistance value is calculated using the formula:

Where R_s is the shunt resistance, R_m is the internal resistance of the meter, I is the total current to be measured, and I_m is the full-scale deflection current of the meter. By selecting an appropriate shunt, the range of the ammeter can be extended to any desired value.

Errors in measurement are broadly classified into three categories:

• Gross Errors: These are primarily caused by human mistakes in reading, recording, or calculating results. They include incorrect adjustment of instruments or misinterpretation of data. These can be minimized by taking multiple readings and using high-quality equipment.
• Systematic Errors: These are inherent to the measuring system and occur consistently. They are divided into:
  • Instrumental Errors: Due to inherent shortcomings in the instrument, such as friction in bearings, improper calibration, or loading effects.
  • Environmental Errors: Caused by external conditions like temperature, pressure, humidity, or magnetic fields affecting the instrument.
  • Observational Errors: Caused by parallax errors or incorrect alignment of the observer's line of sight.
• Random Errors: These are unpredictable errors that remain after gross and systematic errors have been accounted for. They are caused by unknown, small fluctuations in experimental conditions and are typically treated using statistical methods, such as calculating the mean and standard deviation of a series of observations.