# Instantaneous value

Under instantaneous value or actual value is in the measurement technology , mechanical and electrical engineering , the value of a time-dependent quantity understood at a given time.

Time-dependent quantities can be periodic , transient or random . They are compared to mean values ( equivalent value , rectified value , effective value , etc.).

The formula symbol for a time-dependent quantity already contains the time-dependency and therefore represents the instantaneous value. If upper and lower case letters are used, the lower case letter indicates the instantaneous value and the capital letter indicates an average value. If it is appropriate to emphasize that it is the instantaneous value, the letter in brackets can be added. The same applies if a change in upper / lower case leads to a change in the meaning of the symbol. For example, indicates ${\ displaystyle t}$

 ${\ displaystyle u}$ the instantaneous value of a time-dependent electrical voltage ${\ displaystyle U, \ U _ {\ mathrm {rms}}}$ their effective value ${\ displaystyle u (t)}$ the instantaneous value of the tension, if the time dependence is to be emphasized explicitly ${\ displaystyle \ Phi (t)}$ the instantaneous value of the time-dependent magnetic flux

The metrological acquisition of instantaneous values ​​requires instantaneous value sampling and a data memory . This is best realized with periodic processes with an oscilloscope . In the digital storage oscilloscope version , one-off processes are also displayed. The direct observation of instantaneous values ​​with a display measuring device is only possible with slow processes. In the case of a periodic process, the display can only follow the instantaneous value of the measured variable and the measured value can be consciously read and evaluated at a frequency in the order of magnitude below 0.1 Hz .

In many cases, instantaneous values ​​are recorded, but are then processed further without intermediate storage. For example, instantaneous values ​​of voltage and current are multiplied using analog or digital technology, and the product is averaged in the active power meter , or the product is integrated in the energy meter .

## Individual evidence

1. IEC 60050, see DKE German Commission for Electrical, Electronic and Information Technologies in DIN and VDE: International Electrotechnical Dictionary - IEV , entry 101-14-10 and 103-05-12
2. a b DIN 5483–2: 1982 Time- dependent quantities - Part 2: Formula symbols
3. a b DIN EN 60027–1: 2007 Symbols for electrical engineering - Part 1: General