Absolute scale

An absolute scale is a system of measurement that begins at a minimum, or zero point, and progresses in only one direction. An absolute scale differs from an arbitrary, or "relative," scale, which begins at some point selected by a person and can progress in both directions. An absolute scale begins at a natural minimum, leaving only one direction in which to progress. This natural minimum must be an intrinsic property of the measured dimension rather than a natural side-effect of its progression (i.e.: Water freezes and boils naturally at certain temperatures, but these are not natural minimums or maximums of temperature.)

An absolute scale can only be applied to measurements in which a true minimum is known to exist. Time, for example, has no known beginning or ending, so it can only be measured on a relative scale, set to some arbitrary zero-point. "BC" (before Christ) is an example of a backward-progressing scale while "AD" (annos domini, or "the year of the lord") is an example of a forward-progressing scale, with the birth of Jesus set as the arbitrary zero-point. In sciences like paleontology, astronomy, geology, or archaeology, where extreme accuracy is typically not achievable, a purely relative-scale is used where the constantly-changing present is set as the zero point, and time is measured in terms of "years ago."

Temperature, on the other hand, has a known minimum and, therefore, can be measured from both an arbitrary and a natural, absolute point. Fahrenheit is an arbitrary scale where zero is based on the temperature of a frigorific brine mixture, and Celsius is an arbitrary scale where zero is based upon the freezing point of water. However, water freezes at different temperatures, depending on pressure, so Celsius is also a relative measurement based on the relationship between temperature and pressure. Celsius and Fahrenheit can be measured on both sides of the zero point, in positive and negative values. Kelvin, on the other hand, is an absolute measurement, graduated very similar to Celsius, but which begins at the absolute minimum-temperature that can exist (when all vibrational motion of atoms ceases). The Kelvin scale can progress from zero in only one direction, from coldest to warmer, in positive values alone. Likewise, the Rankine scale is an absolute scale, except it is graduated similar to Fahrenheit.

Pressure is a force that can be measured absolutely, because the natural minimum of pressure is total vacuum. Pressure is frequently measured with reference to atmospheric pressure rather than on any absolute scale, relative to complete and perfect vacuum; it is technologically simpler and cheaper. It may also be more convenient to use relative scales, because, with things like pneumatics and hydraulics, the amount of energy transferred is reduced by the relative "backpressure" of the atmosphere. (e.g.: 15 psi of air in a tank at sea level will become 30 psi in the vacuum of space.) Therefore, with measurements of things like blood pressure or tire pressure, a measurement relative to air pressure is a better indication of "burst pressure" (damage threshold) than an absolute scale. Absolute scales are typically used in science, deep vacuum measurements (where the fluctuating pressure of the atmosphere becomes a nuisance), aeronautics (where precise measurements of the atmosphere are needed to determine altitude), or lighting construction (where the relative pressure of the atmosphere is inconsequential), and are measured in units of "atmospheres" or torr. Barometers do measure absolute pressure by holding a vacuum at the top of the mercury column or one side of a diaphragm, but that vacuum is awkward to achieve and maintain. Thus, while the general public may be familiar with measurements of absolute pressure from weather forecasts, most pressures such as tire pressures and water pressures are measured relative to atmospheric pressure using cheaper and simpler pressure gauges. For this reason, the pressure relative to atmospheric pressure is called gauge pressure and measurements given in units like pounds per square inch (abbreviated lbf/in² or psi) are often shown as psig (the "g" standing for gauge) or psia ("a" for absolute).

Absolute scales are used when precise values are needed in comparison to a natural, unchanging zero point. Measurements of length, area and volume are inherently absolute, although measurements of distance are often based on an arbitrary starting point. Measurements of weight can be absolute, such as atomic weight, but more often they are measurements of the relationship between two masses, while measurements of speed are relative to an arbitrary reference frame. (Unlike many other measurements without a known, absolute minimum, speed has a known maximum and can be measured from a purely relative scale.) Absolute scales can be used for measuring a variety of things, from the flatness of an optical flat to neuroscientific tests.