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.
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. There are basically three types of systematic errors, namely –
i. Instrumental – They are inherent in measuring instruments, because of their mechanical structure (e.g.: irregular spring tensions or stretching of a spring).
ii. 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.
iii. 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.
iv. In general, these are also subdivided into two – static and dynamic.
II. 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.
Methods of minimization –
Gross errors cannot be completely eliminated, but can be minimized by taking proper care in reading and recording of the measurement parameter. One should, therefore, not be completely dependent on a single reading
Instrumental Systematic errors can be avoided by
a. selecting a suitable instrument for the particular measurement applications
b. applying correction factors after determining the amount of instrumental error
c. calibrating the instrument against a standard
Environmental Systematic errors can be avoided by air conditioning, hermetically sealing certain components in the instruments, and using magnetic shields
Observational Systematic errors can be avoided by concentrating on one particular measurement process at a time. Clearing out the area where the instrument is placed will also help the observer focus
Random errors can be treated mathematically using laws of probability. The idea is to repeat the measurement to gain high precision.