Errors may come from different sources and are usually classified under three main headings.
– Gross errors
– Systematic errors
– Random errors
Gross errors are essentially human mistakes that are the result of carelessness
– Misreading of an instrument
– Incorrect recording of readings
– Wrong computation
– Improper use of instrument (loading effect of voltmeter)
Systematic errors are divided into two categories
– Instrumental Errors: Due to shortcomings of the instrument
– Environmental Errors: Due to external conditions affecting the measurement
– Friction in bearings of various moving components
– Zero setting not adjusted properly
– Calibration error due to aging
– Faulty display circuit in digital instruments
– Ambient parameters such as temperature, pressure, humidity, magnetic and electric fields, dust and other such external parameters can affect the performance of the instrument.
– Though gross errors and systematic errors can be avoided by taking proper care, some errors of unexplainable origin can also occur in measurements.
– These errors are classified as random errors.
– Random errors can be minimized by taking many readings and determining the mean value.
Absolute errors and relative errors
– If a resistor is known to have a resistance of 500 Ω with a possible error of ±50 Ω, the ±50 Ω is an absolute error. This is because 50 Ω is stated as an absolute quantity, not as a percentage or fraction of the 500 Ω resistance
– When the error is expressed as a percentage or as a fraction of the total resistance, it becomes a relative error. Thus the ±50 Ω is ±10 % relative to 500 Ω or ±1/10 of 500 Ω. So the resistance can be specified as R = 500 Ω ± 10 %.
– Another method of expressing an error is to refer to it in ppm relative to the total quantity.
– For example, the temperature coefficient of 1 MΩ resistor might be stated as hundred ppm/Co, which means 100 parts per million / degree Celsius. One millionth of 1 MegaΩ is 1 Ω. Consequently, 100 ppm of 1 MΩ is 100 Ω. Therefore, a 1 Co change in temperature may cause the 1 MΩ resistance to increase or decrease by 100 Ω.