Types of Error in Instrumentation

Systematic Errors

Errors which occur due to changes in environment conditions, instrumental reasons or wrong observations. These errors are of three types

(1) Instrumental Errors

(2) Environmental Errors

(3) Observational Errors

Instrumental Errors

These errors occur due to shortcomings in the instruments, improper use of instruments or loading effect of the instrument. Sometimes improper construction, calibration or operation of an instrument might result in some inherent errors.

    For example, weak spring in a Permanent Magnet Instrument might result in too high readings. These errors can be easily detected or reduced by applying correction factors, careful planning of measurement procedure or re-calibrating the instrument.

    Examples include the inability to adjust the zero (reference) point, improper initial settings, using extremely high resistance leads and so on. 

Environmental Errors

These errors occur due to external ambient conditions of the instrument. These conditions include changes in temperature, humidity, availability of dust, vibrations or effects of external magnetic or electrostatic fields. The resultant errors can be minimized by following the following corrective measures:

  

Observational Errors

These errors occur due to a mismatch between a line of vision of the observer and the pointer above the instrument scale. This is also termed as Parallax error which occurs when the observer is unable to have a vision aligned with the pointer. These errors can be minimized by using highly accurate meters (having the pointer and scale on the same plane). Since they occur on Analog instruments, using digital display can eliminate these errors.

Random Errors

These errors occur due to a group of small factors which fluctuate from one measurement to another.  The situations or disturbances which cause these errors are unknown, hence they are termed as Random errors. Sources of these errors are not obvious and not easily figured.

Limiting Errors

For any instrument, the manufacturer defines or guarantees a certain accuracy, which depends upon the type of material and the effort required to manufacture the instrument.  The accuracy is defined within a certain percentage of full-scale reading. In other words, the manufacturer specifies certain deviations from the nominal value. The limits of these deviations are known as Limiting or Guarantee Errors. The error is guaranteed within the limits.

The ratio of error to the specified nominal value is termed as Relative Limiting Error.

Computing limiting error for a combination of two or more quantities, each having a limiting error, is found by considering the relative increment of the function if the result is an algebraic equation.

Gross Errors

Manual errors in reading instruments or recording and calculating measurement results are known as Gross errors. Generally, these errors occur during the experiments, where the experimenter might read or record a value different from the actual one, probably due to poor sight. With human involvement, these errors are inevitable, though they can be anticipated and rectified.

These errors can be prevented by the taking the below-given couple of measures:

• Precautious reading and recording of data.
• Taking multiple readings, by different persons. A close agreement between different readings ensures removal of any gross error.