Different types of static errors in a measuring system

Different types of static errors in a measuring system

The numerical difference between the true value of a quantity and its value as obtained by measurement, i.e. repeated measurement of the same quantity give different indications called the static error of a measuring instrument.
Static errors are categorized as gross errors or human errors, systematic errors and Random errors.

Gross Errors:-

This error is mainly due to human mistakes in reading or in using instruments or errors in recording observations. Errors may also occur due to incorrect adjustments of instruments and computational mistakes. These errors cannot be treated mathematically. The complete elimination of gross errors is not possible, but one can minimize them . Some errors are easily detected while others may be elusive. One of the basic gross errors that occur frequently is the improper use of an Instrument the error can be minimized by taking proper care in reading and recording the measurement parameter. 
In general, indicating instruments change ambient conditions to some extent when connected into a complete circuit.

Systematic Errors:-

These errors occur due to shortcomings of, the instrument, such as defective or worn parts, or ageing or effects of the environment on the instrument.
These errors are sometimes referred to as bias, and they influence all measurements of a quantity alike. A constant uniform deviation of the operation of an instrument is known as a systematic error. There are basically three types of systematic errors:

1. Instrumental, 
2. Environmental,
3. Observational

Instrumental Errors:-

Instrumental errors are inherent in measuring instruments, because of their mechanical structure. For example, in the D'Arsonval movement friction in the bearings of various moving components, irregular spring tensions, stretching of the spring or reduction in tension due to improper handling or over loading of the instrument. Instrumental errors can be avoided by

1. Selecting a suitable instrument for the particular measurement applications.
2. Applying correction factors after determining the amount of instrumental error.
3. Calibrating the instrument against a standard.

Environmental Errors:-

Environmental errors are due to conditions external to the measuring device, including conditions in the area surrounding the instrument, such as the effects of change in temperature, humidity, barometric pressure or of magnetic or electrostatic fields. These errors can also be avoided by 

1. Air conditioning, 
2. hermetically sealing certain components in the instruments, and 
   
3. using magnetic shields.

Observational Errors:-

Observational errors are errors introduced by the observer. The most common error is the parallax error introduced in reading a meter scale, and the error of estimation when obtaining a reading from a meter scale. These errors are caused by the habits of individual observers.  For example, an observer may always introduce an error by consistently holding his head too far to the left while reading a needle and scale reading.

In general, systematic errors can also be subdivided into static and dynamic Errors. 
Static errors are caused by limitations of the measuring device or the physical laws governing its behavior.

Dynamic errors are caused by the instrument not responding fast enough to follow the changes in a   measured variable.