Mumbai University > Electronics and Telecommunication > Sem3 > Electronic Instruments and Measurements

Marks: 5M/10M

Year: Dec 13, June 14

[Please note that the answer to this question is an extended and edited version of that of Q2 of this chapter. It is recommended that you observe the changes, and read the question properly before answering, during the exam.]

The static error of a measuring instrument is the numerical difference between the true value of a quantity and its value as obtained by measurement. This causes the repeated measurement of the same quantity to give different indications, and thus, precision is an important characteristic in electronic instruments.

Following are the three main types of static errors:

I. Gross Errors – These errors are mainly due to human mistakes in reading or in using instruments or errors in recording observations.

• Errors may occur also due to incorrect adjustment of instruments and computational mistakes.

• One of the basic gross errors that occurs frequently is the improper use of an instrument.

• These errors cannot be treated mathematically.

II. Systematic Errors – These errors are due to shortcomings of the instrument, such as defective or worn parts, ageing, or effects of the environment on the instrument.

• The errors are sometimes referred to as bias, and they influence all measurements of a quantity alike.

• In other words, a constant uniform deviation of the operation of an instrument is known as a systematic error.

• There are basically three types of systematic errors, namely –

  a. Instrumental – They are inherent in measuring instruments, because of their mechanical structure (e.g.: irregular spring tensions or stretching of a spring). They can be avoided by appropriate instrument selection and calibration.

  b. Environmental – They 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. They can be avoided by air conditioning, etc..

  c. Observational – They are introduced by the observer. The most common error is the parallax error (when the pointer fluctuates between different readings, and the reading is taken arbitrarily) 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.

  a. In general, these are also subdivided into two – static and dynamic.

  b. Static errors are caused by imitations of the measuring device or the physical laws governing its behaviour, while dynamic errors are caused by the instrument not responding fast enough to follow the changes in a measured variable.

III. Random Errors – These are the errors that remain after gross and systematic errors have substantially been reduced or at least accounted for.

• They are generally an accumulation of a large number of small effects and may be of real concern only in measurements requiring high degree of accuracy.

• They are due to unknown causes, not determinable in the ordinary process of making measurements.

• Such errors are normally small, and follow the laws of probability. Thus, they can be treated mathematically.

• For example, suppose a voltage is being monitored by a voltmeter which is read at 15 m minute intervals. Although the instrument operates under ideal environmental conditions and is accurately calibrated before measurement, it still gives readings that vary slightly over the period of observation. This variation cannot be corrected by any method of calibration or any other known method of control.