Real versus nominal value
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 always is actually present. Often a "nominal" value is a de facto standard rather than a typical or average measurement.
When measurement is involved, 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 gage block length, but some other number that is always out of reach.
Only counting can yield an accurate and reproducible real value; see metrology for an explanation.
[edit] Engineering
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 and cannot be reduced without risk.
- Mains electricity is nominally 230 V in the 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.
- 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.
- 56 kbit/s modems cannot exceed 53.3 kbit/second.[citation needed]
Other cases involve diameter, speed, and volume.
Sometimes the word "nominal" is also used in engineering contexts as a synonym for "normal" or "expected"; for example, The rotor resistances on all the other operating wheels are nominal.[1]
[edit] See also
[edit] Notes
- ^ "Rear Wheel Trouble Continues". JPL. 2009-12-10. http://jpl.nasa.gov/news/news.cfm?release=2009-191&icid='NewsFeaturesHome'. Retrieved 2009-12-10.
