The performance characteristics of a measuring instrument are categorized as Static and dynamic characteristics. A static error is one of the static characteristics.
We know that the static error is the difference between the true value of the measured variable to the value indicated by the instrument not changing with time.
The static error may arise due to a number of reasons. They are classified as
Gross errors occur due to the lack of experience of human beings. These cover human mistakes in noting down the readings, recording the process variable, and calculating results. These readings occur due to the incorrect adjustments of the instruments.
These errors can not be treated mathematically. These errors are called personal errors. Some gross errors are easily detected while others are difficult to detect.
Complete elimination of gross errors is not possible but can be minimized in the following ways.
- Great care should be taken while taking readings while recording and calculating.
- Without depending on only one reading. One or more persons to be involved in taking at least three or even more readings.
Constant and uniform deviation of the error is called Systematic error. Systematic errors occur due to incorrect or poor calibration of the instrument. One more reason is that the instruments used with substandard spare parts, due to worn, environmental effects, aging.
1. Instrumental errors:
They are classified again as
a) Shortcomings of instruments: friction in bearings of various moving parts, irregular spring tension,
b). Misuse of instruments: A good instrument if not properly used leads to giving abnormal readings.
c). Loading of instruments:
As shown in the above figure, a pressure gauge is used to measure the highest pressure range of 40 bar. If it is subjected to pressure more than 40 bar, spring inside will lose its tension due to the overloading effect. In the next measurement, it will show error while measuring any range. The errors due to the loading can be corrected by using measuring instruments intelligently and correctly.
2. Environmental errors:
These errors are due to the conditions external to the measuring instrument. Ambient parameters such as temperature, pressure, humidity, dust, the effect of external magnetic and electrostatic fields, aging of the instrument, frequency sensitivity of the instrument.
Such errors can be avoided by providing air conditioning, cleaning the instruments, housing the instruments in a proper protecting cover.
Various factors resulting in these environmental errors are due to the
The effects of electromagnetic or effect of the external field can be avoided by providing magnetic or electrostatic shields or screens.
3. Observational errors:
These are the errors introduced by the monitoring person. Parallax error while reading a meter, wrong scale selection, etc.,
Although the systematic and instrumental errors are reduced by taking proper care, these errors are due to the accumulation of small effects in large numbers. These errors are generally small. These errors are of real concern when high instrument accuracy is accounted for.
- These errors can not be corrected by doing any method of correction or calibration as the causes for the error is not known.
- To avoid these errors, the frequency of measurement is to be increased. That means the same parameter is to be measured often.
Author: PSS BAPU RAO