JNTUH B.Tech III ELECTRONIC MEASUREMENTS AND INSTRUMENTATION Previous Question Paper (October - 2020 - R13)

A basic D.C. current meter (ammeter) is constructed by connecting a low-value resistance, known as a shunt resistor (Rsh), in parallel with a PMMC (Permanent Magnet Moving Coil) meter movement. The PMMC movement has an internal resistance (Rm) and a full-scale deflection current (Im).

The working principle relies on current division. Since the meter movement can only handle a small current, the majority of the current to be measured (I) is bypassed through the shunt resistor. The value of the shunt resistance is calculated using the formula:

where m = I/Im is the multiplying power of the shunt. By selecting an appropriate Rsh, the range of the ammeter can be extended to measure currents much larger than the basic movement's full-scale capacity.

Errors in measurement are categorized into three primary types:

1. Gross Errors: These are primarily caused by human mistakes in reading, recording, or calculating data. They include incorrect adjustment of instruments, misreading scales, or improper application of equipment. These can be avoided by taking multiple readings and exercising care.
2. Systematic Errors: These are predictable errors inherent in the measurement system. They are classified into:
   • Instrumental Errors: Due to inherent shortcomings of the device, 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: Errors like parallax while reading a pointer on a scale.
3. Random Errors: These are unpredictable errors that remain after gross and systematic errors have been corrected. They are caused by small, unknown variations in parameters and are treated using statistical methods, such as calculating the mean and standard deviation of a set of readings.