| Wikipedia: 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. When measurement is involved, the real value has the characteristics of a non-mathematical transcendental number. In all measuring situations, improving the measurement technique will always begin yielding unpredictable least significant digits. For example, a 1 inch long gage block will measure to be exactly 1 inch long until the measuring techniques reach 1 micro inch accuracy. As techniques improve beyond this threshold, it will become clear that 1 inch is not the real value of the gage block length. It is some other number that is always out of reach, but not precisely 1 inch.
Only counting can yield an accurate and reproducible real value, see metrology for an explanation.
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 1½ in. × 3½ in, sized to the equivalent rough hewn piece of softwood for nominal structural design purposes. The true width and depth of the 2x4 are never known. The lumberyard uses calibration to set their saws so that the wood is within 1/8 in Engineering tolerance of the 1½ in × 3½ in size. By the 21st century, sawmills could easily cut 40 or more 2x4 pieces with a cumulative width and depth variation of less that 1/8 in, so the prevailing actual values are closer to 1 3/8 in. by 3 3/8 in. but are still unknown. Wood is a natural product. The 1/8 in tolerances absorb the natural variation and can't 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 rechargable batteries have a nominal voltage of 1.5 V, but actually supply 1.25 V at most.
- 56 kbit/s modems cannot exceed 53.3 kbit/second.
Other cases involve diameter, speed, and volume.
Sometimes the word "nominal" is also used in engineering contexts as a synonym for "normal."
Economics
In economics, nominal values are the face value of currency over long periods of time (years), whereas real values have been corrected for inflation.
External links
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