1. Instrument calibration
Due to frequent use of a measuring instrument and also of aging, the instrument may go out of calibration. The measurements made with such an instrument will be in error; this type of error usually is regular and may be called systematic. A comparison with a
standard instrument enables such uncertainty to be accounted for. An instrument should therefore be sent for calibration at frequent intervals.
2. Instrument reproducibility
Even if an instrument has been calibrated under a set of conditions, the measurements may still be in error if the instrument is not being used under conditions identical to those prevailing during calibration. Mechanical defects like sleekness and friction in metal bearings, backlash in micrometer screws etc. lead to errors in measurement. These errors could be either accidental or systematic.
3. Measuring arrangement
The measuring arrangement employed may sometimes influence the measurements. This is particularly so when the comparator law of Abbe is not strictly followed while measuring length. According to this law, errors of first order are avoided when the measuring instrument and scale axes are collinear. Strict adherence to this law for designing and usage of an opto-mechanical instrument is essential. Similar example can be found in other areas.
4. Work piece
The nature of work piece (hardness, roughness etc.) may lead to errors in measurement. Most mechanical and opto-mechanical instruments contact the work piece under constant pressure condition. Therefore hard and soft work pieces would respond differently and this may lead to errors in measurement.
5. Environmental conditions
Environmental conditions such as humidity, pressure, temperature, electrical or magnetic field also influence the measurements if the instrument is not used under conditions prevailing during calibration. A very well-known example is in the area of length measurement where a work piece of nominal length L is measured with an instrument calibrated at 20°C. The error introduced when the measurement is done at temperature t will be = (a2 – a1)L(t - 20),
where a1 and a2 are the thermal expansion coefficients of work piece and of scale of the measuring instrument respectively.
6. Observer’s skills
Human element, despite of automation, often enters into a measurement scheme and plays a decisive role. Therefore observer's skill is to be considered in the act of measurement. It is an established fact that the measurement data of a physical quantity varies from one observer to another, and even for the same observer it may vary with his physical and mental states. Such errors may be of both systematic and random nature depending on the conditions of the experiment.
