Specification of a measured value
The measured value is determined for a quantitative statement about a measured variable, that is the physical variable to which the measurement applies ( DIN 1319 ). The special value of the measurand is expressed by the product of numerical value and unit (also DIN 1313).
Units, often called units of measurement to differentiate them , are internationally agreed, nationally legally established values of physical quantities that are included in the standardization in DIN 1301 with the purpose that all other values of this quantity are to be specified as a multiple of the unit.
According to DIN 1301, numerical values should be between 0.1 and 1000. Instead of larger or smaller values, prefixes for units of measurement should be used, which are also listed in the standard.
|Instead of||should be written|
|20,000 m or 2 · 10 4 m||20 km|
|0.002 m or 2 · 10 −3 m||2 mm|
|2.2 · 10 −10 F||0.22 nF or 220 pF|
The most significant digit that is affected by the error limit or measurement uncertainty (see below) should be the ones digit or a decimal place ( DIN 1333 ); this means that only significant digits are given.
|Instead of||should be written|
|(220 ± 40) pF||(0.22 ± 0.04) nF|
|(220 ± 4) V||(no change made)|
Obtaining a measured value
The measured value is delivered with direct output from the measuring device or the measuring device
- in the case of analog measuring methods, preferably by means of a scale display ; A numerical value is read from a continuum of (real) values which, together with the unit to be indicated on the scale, provides the measured value,
- in the case of digital measuring methods, preferably by means of a numeric display ; A number (e.g. counter reading) is used from a range of values of whole numbers which, together with the smallest increment, provides the measured value. For example, when measuring time, the following can apply: 1 digit increment 0.1 ms. Then a counter reading 1234 becomes the measured value 123.4 ms.
With indirect output, the measured value is indicated by a
delivered, preferably as
- electrical voltage or electrical current , but also as a period of time , number of voltage pulses, etc.
for further processing in a measuring chain , control , regulation or data processing system. Then the measured value is not communicated to the person at all, or it is only brought into readable form after conversion or conversion.
Quality of a measured value
Measured values are always subject to measurement errors. DIN 1319 differentiates between random and systematic measurement errors.
- First and foremost, it will be a measuring device deviation, i.e. a deviation that is caused solely by the measuring device. It is predominantly systematic in nature. If the error limit is known (e.g. due to a class symbol ), this will be stated together with the measured value.
- The influence of random deviations that are contained in the test setup and the measuring equipment (for example noise in amplifiers) can be reduced by repeated measurements and error calculations . The arithmetic mean is used instead of a single measured value . Its uncertainty can also be calculated and must be stated.
- Systematic deviations that are contained in the test setup and the measuring equipment must be calculated out of the measured value if they can be stated - albeit with difficulty. Such deviations are, for example, incorrect adjustment (if not included in the error limit), changes in the measured value due to the reaction of the measuring device on the test setup ( voltage measuring device with insufficient internal resistance of the measuring device in relation to the internal resistance of the voltage source ) or an unsuitable test setup. Since these measurement deviations are to be subtracted, they do not appear as a quality feature.
- Systematic measurement errors that are not known and can only be estimated lead to a further component of the measurement uncertainty that must be specified.
For example, a measurement that determines the acceleration due to gravity to be (9.8 ± 1.0) m / s 2 is quite imprecise, since the measurement can only be used to conclude that the true value is in the range of 8, 8 m / s 2 to 10.8 m / s 2 . The real value is unknown. The fact that the value 9.8 m / s 2 agrees surprisingly well with the value from the literature cannot be seen from the result given. (If you knew the value from the literature, the measurement would also be superfluous.)
Depending on the circumstances, the measured value includes further information such as the type of measuring device or the environmental conditions, including the position of the measuring point and date / time.