Many of the earliest measuring devices were intuitive and easy to conceptually validate. The term "calibration" probably was first associated with the precise division of linear distance and angles using a dividing engine and the measurement of gravitational mass using a weighing scale. These two forms of measurement alone and their direct derivatives supported nearly all commerce and technology development from the earliest civilizations until about 1800AD.
The Industrial Revolution introduced wide scale use of indirect measurement. The measurement of pressure was an early example of how indirect measurement was added to the existing direct measurement of the same phenomena.
Before the Industrial Revolution, the most common pressure measurement device was a hydrostatic manometer, which is not practical for measuring high pressure. Eugene Bourdon filled the need for high pressure measurement with his Bourdon tube pressure gage.
In the direct reading hydrostatic manometer design on the left, unknown pressure pushes the liquid down the left side of the manometer U-tube (or unknown vacuum pulls the liquid up the tube, as shown) where a length scale next to the tube measures the pressure, referenced to the other, open end of the manometer on the right side of the U-tube. The resulting height difference "H" is a direct measurement of the pressure or vacuum with respect to atmospheric pressure. The absence of pressure or vacuum would make H=0. The self-applied calibration would only require the length scale to be set to zero at that same point.
In a Bourdon tube shown in the two views on the right, applied pressure entering from the bottom on the silver barbed pipe tries to straighten a curved tube (or vacuum tries to curl the tube to a greater extent), moving the free end of the tube that is mechanically connected to the pointer. This is indirect measurement that depends on calibration to read pressure or vacuum correctly. No self-calibration is possible, but generally the zero pressure state is correctable by the user.
Even in recent times, direct measurement is used to increase confidence in the validity of the measurements.
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In the early days of US automobile use, people wanted to see the gasoline they were about to buy in a big glass pitcher, a direct measure of volume and quality via appearance. By 1930, rotary flowmeters were accepted as indirect substitutes. Visible in the picture above on the right (above the price digit register) is a hemispheric viewing window where consumers could see the blade of the flowmeter turn as the gasoline was pumped. By 1970, the windows were gone and the measurement was totally indirect.
Indirect measurement always involve linkages or conversions of some kind. It is seldom possible to intuitively monitor the measurement. These facts intensify the need for calibration.
Most measurement techniques used today are indirect.
There is no consistent demarcation between calibration and metrology. Generally, the basic process below would be metrology-centered if it involved new or unfamiliar equipment and processes. For example, a calibration laboratory owned by a successful maker of microphones would have to be proficient in electronic distortion and sound pressure measurement. For them, the calibration of a new frequency spectrum analyzer is a routine matter with extensive precedent. On the other hand, a similar laboratory supporting a coaxial cable manufacturer may not be as familiar with this specific calibration subject. A transplanted calibration process that worked well to support the microphone application may or may not be the best answer or even adequate for the coaxial cable application. A prior understanding the measurement requirements of coaxial cable manufacturing would make the calibration process below more successful.
The calibration process begins with the design of the measuring instrument that needs to be calibrated. The design has to be able to "hold a calibration" through its calibration interval. In other words, the design has to be capable of measurements that are "within engineering tolerance" when used within the stated environmental conditions over some reasonable period of time. Having a design with these characteristics increases the likelihood of the actual measuring instruments performing as expected.
The exact mechanism for assigning tolerance values varies by country and industry type. The measuring equipment manufacturer generally assigns the measurement tolerance, suggests a calibration interval and specifies the environmental range of use and storage. The using organization generally assigns the actual calibration interval, which is dependent on this specific measuring equipment's likely usage level. A very common interval in the United States for 8–12 hours of use 5 days per week is six months. That same instrument in 24/7 usage would generally get a shorter interval. The assignment of calibration intervals can be a formal process based on the results of previous calibrations.
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