Theory of Measurement

Notes

Static Performance Parameters

Measurement is the process of comparison between the unknown physical quantity to its predefined standard so as to find the value of the unknown physical quantity.The performance characteristics of an instrument can be divided into two distinct categories as Static characteristics and dynamic characteristics. Accuracy,Precision,Sensitivity, Linearity, Reproducibility,Repeatability, Resolution, Threshold,Drift,Stability,Tolerance etc are the static characteristics. Speed of response,Measuring lag,

Dynamic Performance Parameters

The set of criteria defined for the instruments, which changes rapidly with time, is called ‘dynamic characteristics’. The term dynamic characteristics are used to study the behavior of the system between the time that the output value changes and time ,that value has settled to its final steady-state value.

Errors in Measurement and its Statistical Analysis

The deviation of measured value(M.V) from the true value(T.V) of the quantity of measurement. is termed as Error in measurement. Basically,Gross errors, Systematic errors, Random errors are the main types of error in measurement. Statistical approaches are normally preferred to minimize the random errors.

Measurement of Voltage and Current

Ammeters and voltmeters, except electrostatic voltmeters, operate on the same principle. In the case of analog or pointer indicating ammeter and voltmeter, there are two classes of instruments depending upon the operations viz. Moving coil Instrument(voltmeter/ammeter) and Moving iron Instrument(voltmeter/ammeter). Moving-iron instruments are generally used to measure alternating voltages and currents while the permanent magnet moving coil Instrument are used to measure dc voltages and currents.

Measurement of Low, Medium and High Resistance

]Resistances are classified as Low, medium and high resistances. Low resistance is the range of 0.1 ohm to 1 ohm ,Medium resistance is the range of 1 ohm to low mega ohm and High resistance is 0.1 ohm to a higher range.Various methods are used to measure low,medium and high resistances.

Ac Bridges and Measurement of Inductance and Capacitance:

A bridge is a special class of instrument consisting of four arms, where three arms have known values and an unknown value of the remaining arm is calculated.Normally, bridges are used for the measurement of passive circuit component such as Resistance, inductance, capacitance etc. Generally, Bridges are classified on the basis of input base as Dc bridge,Ac bridge .