MAIN CAUSES OF ERRORS DURING MEASUREMENT

1. Ambient Temperature

Since 1932, all countries have adopted 20°C as the standard temperature for taking measurements. In reality, however, it is difficult to maintain the temperature at precisely 20°C (factors include altitude, range, and the machining and operating environment). The effect of thermal expansion will cause errors in the measuring process.

Material		Coefficient of thermal expansion (x0.000001/°C)
Common cast iron		9.2-11.8
Common carbon steel		10-13
Chrome steel		11-13
Molydenum chrome steel		13-15
Iron		12-12.5
Copper		18.5
Bronze		17.5
Brass		18.5
Alloy		23.8
Gold		14.2

According to the above formula and the table of coefficients of thermal expansion, materials with a larger coefficient of linear thermal expansion will have bigger measurement errors. To gather more accurate data, the most straightforward method is to take the parts to be measured into a temperature-controlled room. The parts will then cool down or heat up to the controlled temperature. However, the time needed for the parts to reach the required temperature will depend on their volume, thickness variation, thermal capacity and thermal conductivity.


2. Distortion

Distortion may occur due to:

a. The measuring forces (it is mostly thin parts and soft materials that will be affected).

b. The rigidity of parts (there will be distortion due to gravity if parts are too thin or too long).

c. The rigidity of jigs and fixtures (there will be distortion of jigs and fixtures due to the weight of parts to be
    measured).

d. Inconsistency in the measuring positions. In this case, the measured surface will be distorted by the variation of
    the measuring forces.

3. Offsetting of the Measurement Axis

This occurs when:

a. The part surface is not on the same measuring axes, resulting in a tilting angle (cosine error).

b. The surfaces of the measuring tools are not parallel.

4. Parallax

Readings taken from different angles are different. To get an accurate reading, it is necessary to observe with one eye in the direction normal to the graduation scale. In a simple experiment, 50 inexperienced persons were chosen to take readings with a calliper. The results indicated a 0.04mm deviation.

5. Errors in the measuring tools

This is also a source of measuring errors. All measuring tools must be checked and calibrated before taking actual measurements.

6. Errors occurring during the measuring process

Such errors include:

a. Errors due to the negligence of the operator.

b. Taking wrong graduation scale and figures.

c. Recording errors.

d. Verbal reporting errors.

e. Bad habits of the measuring operator.

f. Calculation errors.