# Measurement

A measurement is the execution of planned activities to a quantitative statement about a measured variable by comparison with a unit . The measured variable is the physical variable to which the measurement applies. The designations for measuring technology are defined for Germany in the DIN standard DIN 1319 .

The term “measurement”, which has grown in physics and engineering , is transferred to other areas, although it is given a different meaning. Because "a transfer of this measurement concept to the social sciences fails because that units in this sense in the social sciences are still lacking." To the term still used to, is formulated in the same source: Measure as "an assignment of numbers to objects or events, as long as this assignment is a homomorphic mapping of an empirical relative into a numerical relative. " Basis for trading on the market in Meppen: The units of length feet and cubits, which are dependent on the individual height , are uniformly defined here.

## Measurement in terms of measurement technology

The aim of a measurement is to obtain a measurement result as a reliable statement about an unknown size of an object. "The activities involved in measuring are predominantly of a practical (experimental) nature, but include theoretical considerations and calculations."

In the first step , the result of the measurement is a measured value , which however contains a measurement deviation and deviates from its true value . Known systematic deviations are to be calculated from the measured value. A complete measurement result is an estimated value obtained from measurements for the true value of the measured variable with quantitative statements on the accuracy of the measurement. "The evaluation of measured values ​​of the measured variable up to the desired result is part of the measurement task and is included in the measurement of the measured variable." The further use of the measured value or measurement result is not part of the measurement, e.g. B.

• the check is met if a condition,
• the rules so that the measured variable approaches a reference variable.

The quantity to be measured can be almost any physical quantity. Most physical quantities cannot be measured directly, but must be calculated from other measured data using physical models and formulas derived from them. One example is the measurement of the speed of an object by measuring its position at two different points in time and calculating the quotient of the distance covered and the required duration.

A measured value or measurement result is expressed as a product of the numerical value and the (measurement) unit (also according to DIN 1313 ). Beginning with the international meter convention of 1875, an international system of units (the SI system, from Système International d'Unités ) was created under the leadership of the General Conference on Weights and Measures . It comprises seven base units: meter , kilogram , second , ampere , kelvin , mole , candela , as well as derived SI units , e.g. B. Volt . There are also generally applicable units outside of the SI, e.g. B. Hour . The SI units are internationally agreed, nationally legally established values ​​of physical quantities that are included in the standardization with the purpose that all other values ​​of this quantity are to be specified as a multiple of the unit. (Established in Germany in the Unit and Time Act and in DIN 1301-1 .)

## Steps to measure

For fairs include:

1. Clear definition of the measurement task (measurement problem) and the measured variable :
The task, the measurement object and the physical measured variable must be specified.
2. Definition of the unit of measurement for the result:
The unit and its symbol are usually defined in accordance with the SI; prefixes for powers of ten can be selected (also in accordance with DIN 1301-1).
Example for the length: mm, cm, m, km.
Example for the speed: m / s or outside the SI km / h or for special areas of application knots (DIN 1301-2).
There are also quantities of the dimension number , e.g. B. refractive index , number , angle , the values ​​of which are given without a unit or with an auxiliary unit of measurement .
3. Compilation of the boundary conditions:
The boundary conditions are e.g. B. Properties of the measuring object (material, surface quality) and the environment (temperature, vibrations) must be observed.
4. Choice of a measuring device or a measuring device :
Based on the measuring principle and the measuring method , a measuring process is developed that is implemented in a measuring device . In many cases, a finished measuring device is already available for the measuring task. (For definitions of the terms see below)
5. Calibration of measuring device / measuring device:
DIN EN ISO 9001 requires all measurements to be traceable to national standards . This is ensured by the process of measuring equipment monitoring. For this purpose, a measuring device should be calibrated at regular intervals . In doing so, the measured value (output variable) is determined when the value of the measured variable (input variable) is considered to be correct. If the measured value does not match the value of the measured variable within the specified error limits , the device must be readjusted (set) or the values ​​determined must be corrected arithmetically afterwards.
6. Definition of the measuring process:
Temporal or spatial sequence of measurements: z. B. Sequence of individual measurements , repetitions, series of measurements under changed conditions; spatial distribution of the measuring points ( measuring points ), measuring profiles , regular grid , etc.
7. Carrying out the measurement and determining the measurement result:
One measurement or several measurements of the same size obtained under the same conditions (comparison / repeat measurements) can be carried out. Then calculate the mean and standard deviation .
Furthermore, measurements of various sizes may be required, from which the measured value of the desired size can be calculated according to defined mathematical relationships.
8. Consideration of the effects of influencing factors :
Correction of systematic measurement errors.
Depending on the circumstances, this also includes a reduction , i. H. a correction to uniform conditions.
9. Determining the complete measurement result:
A complete measurement result consists of the measurement value (if necessary, the mean value from one or more measurement series or the calculated value based on other measurements), supplemented by quantitative statements on the measurement uncertainty .

## More terms for measurement

### Measuring principle

"The scientific basis of a measurement process." (VIM: 1994); "Physical basis of the measurement." (DIN 1319-1: 1995),

z. B. the Lorentz force as the basis of a measurement of the electrical current.

### Measurement method

"Special type of measurement procedure independent of the measuring principle" (DIN 1319-1),

z. B. deflection measuring method , zero balancing measuring method, differential measuring method

or - according to another, independent point of view - analog method, digital method, see below or digital measuring technology .

### Measurement method

"Practical application of a measuring principle and a measuring method" (DIN 1319-1),

z. B. Determination of mass with a beam balance and weights using the zero balance measurement method.

### Influencing factor

Quantity that is not the subject of the measurement, but influences the measured quantity or the information provided by the measuring device about the measured value (according to DIN 1319-1), (see also cross-sensitivity ),

z. B. Ambient temperature, electromagnetic field strength.

### Measuring device, measuring device, measuring mechanism

A measuring device is defined as "a device that is intended for the measurement of a measured variable alone or in conjunction with other equipment" (DIN 1319-1). For general characteristics of measuring devices, see measuring equipment .

A measuring device is often part of a measuring device , which is defined as the "totality of all measuring devices and additional devices to achieve a measurement result" (also DIN 1319-1).

The term measuring instrument does not appear in the "Glossary of Metrology", in DIN 1319-1: 1995 the translation of " en: Measuring instrument " is also a measuring device .

The measuring mechanism is the active part in a mechanical measuring device. The moving element with pointer and parts that are important for the mode of operation, e.g. B. permanent magnet , coil .

### Measurement object

“Carriers of the measured variable” - “Objects to be measured can be bodies, processes or states.” (DIN 1319-1), e.g. B.

• the measured variable "volume of a present vessel" is a property of a measured object "vessel"
• the measured variable “flux density of an existing magnetic field” is a property of a measurement object (state) “magnetic field”.

## Measurability

A variable can be measured if there is a measuring principle according to which it can be measured, i.e. if it can be meaningfully defined from a physical point of view and is therefore particularly quantifiable . This also includes all requirements for the reproducibility of the measurement result.

Physical quantities can be measured . Some non-physical quantities can be traced back to physical quantities such as volume on sound pressure , color perception on the distribution in the light spectrum .

The determination of no physical quantities, such as obtained using statistical methods inflation rate, the intelligence quotient or customer satisfaction is also sometimes referred to as measurement. From a physical point of view, this is usually disputed because there is no physically defined unit.

A characteristic that can only be assessed subjectively, such as B. Beauty (such as a color) or cunning is not generally recognized and for that reason alone cannot be quantified.

Values ​​that are too small to be measured with today's methods are sometimes referred to as “unmeasurable”, but are simply “not detectable”.

## Distinctions

### Direct and indirect measurement

Direct measurements are those whose results can be read directly on the measuring device , for example measurements with a ruler, protractor or tape measure .

With indirect measuring methods, the result is only available after a few intermediate stages (see measuring device ), e.g. B. Temperature determination of stars from their electromagnetic spectra .

### Analog and digital measurement

In an analog measurement of the measurement value by a step- free processing of the measurement signal detected in a digital measurement by a step- wise processing (DIN 1319-2).

• In the case of an analog measurement, an intermediate distance or angle is often generated so that the measured value can be read on a scale with an adapted scale division.
• In the case of a digital measurement, an intermediate variable which can be set in steps or can be determined by counting is often generated, so that the measured value can be read on a numeric display based on the step position or the counter reading .

Due to the "use of counting measuring devices, counting is increasingly being used in measuring technology as a special type of measuring" (DIN 1319-1).

For a more detailed comparison of these two measurement methods, see digital measurement technology .

## Limits for measurements

In the Copenhagen interpretation of quantum mechanics , measurement has a crucial place. This is reflected in the fact that in addition to the Schrödinger equation , the time describes development of a quantum state, its own laws on the behavior of the system at a quantum mechanical measurement is. The uncertainty relation also describes a fundamental limit for measurements, regardless of the accuracy of the apparatus.

But even in classical physics there are limits to the accuracy of measurements, since every measurement must be an interaction. A well-known example of the influence of the measurement itself on the measurement object comes from electrical engineering, see feedback deviation . This means that the open circuit voltage of a real voltage source cannot be measured exactly with real measuring devices.

Furthermore, it should be considered that the speed of light has a finite size, so that one will never know what is being seen or perceived at the exact point in time of the present. Because the speed of light is not infinite, the information needs time to get from the object to the subject (observer). So you always see a picture of the past. Even the term “present” does not have to be the same for two subjects, since they cannot exchange ideas about it (when exactly is “now”? When someone sends a signal, it is already the past for another when it is received).