The Info List - Real Versus Nominal Value

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The distinction between REAL VALUE and NOMINAL VALUE occurs in many fields. From a philosophical viewpoint, nominal value represents an accepted condition, which is a goal or an approximation, as opposed to the real value, which is always present. Often a nominal value is de facto rather than an exact, typical, or average measurement.


* 1 Measurement * 2 Engineering
* 3 See also * 4 Notes


See also: Nominal size

In measurement, a nominal value is often a value existing in name only; it is assigned as a convenient designation rather than calculated by data analysis or following usual rounding methods. The use of nominal values can be based on de facto standards or some technical standards .

All real measurements have some variation depending on the accuracy and precision of the test method and the measurement uncertainty . The use of reported values often involves engineering tolerances .

One way to consider this is that the real value often has the characteristics of an irrational number . In real-world measuring situations, improving the measurement technique will eventually begin yielding unpredictable least significant digits. For example, a 1-inch long gauge block will measure to be exactly 1 inch long until the measuring techniques reach a certain degree of precision. As techniques improve beyond this threshold, it will become clear that 1 inch is not the real value of the gauge block length, but some other number approximates it.


In various subfields of engineering , a nominal value is one for which the "name" for the value is close to, but not the same as, the actual value. Some examples:

* For dimensional lumber in North America, a "two by four" is not 2 inches by 4 inches, but rather between 1⅜ and 1⅝ inches thick and between 3⅜ and 3⅝ inches wide, sized to the equivalent rough hewn piece of softwood for nominal structural design purposes. The ⅛-inch tolerances absorb the natural variation. * Mains electricity
Mains electricity
is nominally 230 V in the European Union
European Union
, but is allowed to vary ±10%. In North America, the nominal voltage is 120 V, with variance allowed from 114 V to 126 V (±5%). Voltage is also supplied at 208 V, 240 V and 480 V with similar tolerances. In general, electrical devices are designed to work with one nominal voltage , which represents a band of possible actual voltages, power factor and AC waveform shapes. * Traction power networks routinely operate well above the nominal voltage, but still within the tolerance. For example, a streetcar traction power could be rated 600 ±10% volts nominal, but the actual overhead line voltage would normally be close to 660 volts, only dropping near the nominal value in exceptional conditions. * A machine is designed to operate at some particular condition, often stated on the device's nameplate. For example, a pump is designed to deliver its nominal pressure and flow while operating at its nominal speed and power. Actual operating conditions may vary. * NiMH
and NiCd rechargeable batteries have a nominal voltage of 1.2 V, but will actually supply real voltages ranging from about 1.45 V to 1.0 V during discharge. * A solar panel designed to charge a "12 V lead–acid battery " will often be called a "12-volt panel", even though the actual voltage while charging (of both the panel and the battery) is around 14 V, and the open-circuit voltage of the solar panel is around 17 V.

Other cases involve diameter, speed, and volume.

Sometimes the word "nominal" is misused in engineering contexts as a synonym for "normal" or "expected"; for example, The rotor resistances on all the other operating wheels are nominal.