Types of Errors

The errors are classified as :-

1) Gross errors

2) Systematic errors

3) Random errors

1. Gross Errors

The gross errors mainly occur due to carelessness or lack of experience of a human being. These cover human mistakes in readings, recordings and calculating results. These errors also occur due to incorrect adjustments of instruments. These errors can not be treated mathematically. These errors are also called personal errors. Some gross errors are easily detected while others are very difficult to detect.

The complete elimination of gross errors is not possible but one can minimise them by the following ways :-

1. Taking great care while taking the reading, recording the reading and calculating the result.

2. Without depending on only one reading. At least three or even more readings must be taken and preferably by different persons. The readings must be taken preferably under the conditions in which the instruments are switched on and off.

2. Systematic Errors

The systematic errors are mainly resulting due to the short comings of the instrument and the characteristics of the material used in the instrument, such as defective or worn parts, ageing effects, environmental effects, etc.

A constant uniform deviation of the operation of an instrument is known as a systematic error. There are three types of systematic errors as :-

1) Instrumental errors

2) Environmental errors

3) Observational errors

2.1. Instrumental Errors

These errors can be mainly due to the following three reasons :-

a) Shortcomings of Instruments : These are because of the mechanical structure of the instruments. For example, friction in the bearings of various moving parts, irregular spring tensions, reduction in tension due to improper handling, hysteresis, gear backlash, stretching of spring, variation in air gap, etc. These errors can be avoided by the following methods :-

1) Selecting a proper instrument and planning the proper procedure for the measurement.

2) Recognizing the effect of such errors and applying the proper correction factors.

3) Calibrating the instrument carefully against a standard.

b) Misuse of Instruments : A good instrument if used in abnormal way gives misleading results. Poor initial adjustments, improper zero setting, using leads of high resistance etc. are the examples of misusing a good instrument. Such things do not cause the permanent damage to the instruments but definitely cause the serious errors.

c) Loading Effects : Loading effect due to improper way of using the instrument cause the serious errors. The best example of such loading effect error is connecting a well calibrated voltmeter across the• two points of high resistance circuit. The same voltmeter connected in a low resistance circuit gives accurate reading. Thus, the errors due to the loading effect can be avoided by using an instrument intelligently and correctly.

2.2. Environmental Errors

These errors are due to the conditions external to the measuring instrument. The various factors resulting these environmental errors are temperature changes, pressure changes, thermal e.m.f., stray capacitance, cross capacitance, effect of external fields, ageing of equipment and frequency sensitivity of an instrument. The various methods which can be used to reduce these errors are :-

1. Using the proper correction factors and using the information supplied by the manufacturer of the instrument.

2. Using the arrangements which will keep the surrounding conditions constant. This includes the use of air conditioning, temperature control enclosures etc.

3. Reducing the effect of dust, humidity on the components by hermetically sealing the components in the instruments.

4. The effects of external fields can be minimised by using the magnetic or ' electrostatic shields or screens.

5. Using the equipment which is immune to such environmental effects. For example, in the environment having lot of temperature variations, use of an instrument in which resistance material having a very low resistance temperature coefficient is appropriate.

2.3 Observational Errors

These are the errors introduced by the observer. There are many sources of observational errors such as parallax error while reading a meter, wrong scale selection, the habits of individual observers etc. To eliminate such observational errors, one should use the instruments with mirrors, knife edged pointers, etc. Now a days, the instruments with digital display are available which can largely eliminate such observational errors.

The systematic errors can be subdivided as static and dynamic errors. The static errors are caused by the limitations of the measuring device while the dynamic errors are caused by the instrument not responding fast enough to follow the changes in the variable to be measured.

3. Random Errors

Some errors still result, though the systematic and instrumental errors are reduced or atleast accounted for. The causes of such errors are unknown and hence, the errors are called random errors. These errors can not be determined in the ordinary process of taking the measurements. These errors are generally due to the accumulation of large number of the small effects. These errors are generally small. Hence, these errors are of real concern only when the high degree of accuracy is required.

The random errors follow the laws of probability and hence, these errors can be analyzed statically and treated mathematically. These errors can not be corrected by any method of calibration or other known method of control as the causes of such errors are unknown. The only way to reduce these errors is by increasing the number of observations and using the statistical methods to obtain the best approximation of the reading.