Measurement types

Measurement methods may be direct and indirect.

Measurement is direct if the magnitude of physical quantity is directly determined during an experiment.

Measurement is indirect if the required quantity is calculated through the dependence between this quantity and those determined by direct measurements using corresponding formulas or laws. For example, cylinder body density is determined according to a formula:

,

where m, d and h areobtained as a result of the direct measurements.

Specific resistance of a cylinder conductor is determined according to a formula:

.

where R, l and S are its resistance, length and cross section area respectively.

With development of measuring technique some indirect measurements can be considered as direct measurements. For example, power of electric current can be defined by the indirect measurements, that is by measuring current I and voltage U according to a formula:

,

but it is possible to measure power directly by a wattmeter.

Both direct and indirect measurements, depending on the measurement method, measuring means and features of a research object are performed with one- or many-times observations. Measurement performed with one-time observation is called ordinary measurement; measurement performed with many-times observations is called statistical measurement.

Errors types

Errors are divided into three basic types: systematic, random (casual) and gross errorsdepending on their source, character and nature .

Systematic error is a component of the total error of measurement, being constant or dependently variable during repeated measurements of a quantity. It has sign and produces measurement result deviation from true value measured in one side.

Three components of measurement (measurement method, measuring means and an observer) are the systematic error sources. We distinguish systematic methodical, instrumental and personal errors.

Systematic methodical errors arise due to imperfection of the measurement method, inaccuracy of formulas applied for the unknown quantity description and so on.

Instrumental systematic errors arise due to deficiencies of the applied measuring devices only, for example, by device calibration inaccuracy or its incorrect operating conditions. This component of error cannot be avoided. In most cases this error can be determined if the device accuracy range is known. The instrumental error of simple measuring devices (student's ruler, household thermometer, stop watch etc.) is usually determined by the least scale factor of the device.

The personal systematic errors are ones related to the individual features of an observer.

Random (casual) error is a component of total error varying in a casual way at repeated measurements of the same quantity. To minimize a random error influence on the value determination accuracy, it is necessary to do numerous measurements. This component of errors can be analyzed statistically.

Random error appearance results from measuring device (friction in supports, gaps, initial conditions of pointer motion, and so on), external conditions (fluctuation in environmental temperature, vibration, and so on). If a person takes part in measurements, the casual error can appear due to his/her state of health.

Gross (rough) error is a measurement error, which essentially exceeds the error expected at present conditions. Gross errors arise due to strong external factors, operator’s mistakes and improper application of devices. If it is known that some measurements are grosses, they are not taken into account.

Дата добавления: 2015-10-26