Errors
Error is the difference between the measured value and the true value of a measurement. It is the deviation from the expected perfection. Any measurement, however carefully it is conducted, is subject to measurement errors. These errors make it difficult to
ascertain the true value of the measured quantity. The nature of the error may be ascertained by repeating the measurement a number of times and looking at the spread of the values. If the spread in the data is small, the measurement is repeatable and may be termed as being good. If we compare the measured quantity obtained by the use of any instrument and compare it with that obtained by a standardized instrument the two may show different performance as far as the repeatability is concerned.
The ideal or perfect instruments would have perfect sensitivity, reliability and repeatability without any spread of values and would be within the applicable standards. However, in many measurements there will be imprecise and inaccuracy due to many internal and external factors.
Types of errors
When determining the performance of an instrument, it is essential to evaluate the errors in measurement. There may be many sources of errors. In instrumentation systems the errors can be broadly classified as follows:
1. Systematic or Bias Error
It occurs due to imperfect calibration of measurement instruments, imperfect methods of observation and interference of the environment with the measurement process. They can be either constant or proportional to the actual value of the measured quantity or proportional to the value of a different quantity such as humidity and surrounding temperatures. Incorrect zeroing of an instrument leading to a zero error is an example of systematic error in instrumentation. Thus, systematic errors always affect the results of an experiment in a predictable direction. The systematic error may be of three types.
• Instrument error: It occurs due to poor design of the instrument like errors in graduation of Mile, inequality of balance arm, etc. It can be avoided by calibrating the instrument against a suitable standard or applying a suitable correction after determining its amount.
• Environmental errors: It occurs due to variation of conditions external to the measuring device like changes in surrounding temperature, humidity, pressure, etc.
• Loading errors: It occurs due to carelessness of the observer like parallax error. It can be avoided by using the instruments carefully and intelligently.
2. Accidental or Random Error
The random error may vary from observation to observation. It is closely related to the precision. The higher the precision of a measuring instrument, the smaller is the variability or the fluctuation in its readings. In other words, the random error is caused by unpredictable fluctuations in the readings of the measuring instrument or in the observer's interpretation of the instrument's reading. The random error is due to factors which we are unable to control. It may be too expensive or we may be too ignorant of these factors to control them. It may so happen that whatever we are trying to measure is changing in time. Random error often occurs when instruments are pushed to their limits. For example, it is common for digital balances to exhibit random error in their least significant digit. The random error may be of three types:
• Inconsistencies Associated with Accurate Measurement of Small Quantities: It occurs when very accurate measurements are to be made.
• Presence of Certain System Defects: It occurs due to presence of certain defects in the system. Backlash error occurs due large tolerances in mating parts and slackness occurs due to presence of friction.
• Effects of Unrestrained and Randomly Varying Parameters: It occurs due to certain uncontrolled disturbances such as line voltage fluctuation, vibrations of the instrument supports, etc.
3. Gross or Miscellaneous Error
There are certain other errors that cannot be classified either as systematic or as random errors. Such errors are included as gross errors. The gross error may be of three types:
• Personal or human error: It occurs due to the limitations in the human senses. The observer may read a measurement high or low at one time and slightly different at the other time.
• Error due to faulty components or their adjustments: It occurs due to misalignment of moving pans, electrical leakage, etc.
• Error due to improper application of the instrument: It occurs due to the use of instrument in undesired operating conditions. In such casts it becomes necessary to stop the experiment until the disturbing element is eliminated.
