# Calibration, History

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#### Summary

Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device.

#### Details

The words "calibrate" and "calibration" entered the English language during the American Civil War, in descriptions of artillery. 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 AD 1800.

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 pressures. Eugene Bourdon fulfilled the need for high pressure measurement with his Bourdon tube pressure gage.

In the direct reading hydrostatic manometer design on the left, an unknown applied pressure Pa pushes the liquid down the right side of the manometer U-tube, while a length scale next to the tube measures the pressure, referenced to the other, open end of the manometer on the left side of the U-tube (P0). 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.

This direct measurement of pressure as a height difference depends on the density of the manometer fluid, and a calibrated means of measuring the height difference.

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.

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. A hemispheric viewing window allowed consumers to 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.

#### Related Topics

• WikipediaMaster Calibrators Association - Industry Experts on Electrical Calibration, Adjustment and MetrologyElectrical Calibration VideoVideo which explains the relationship between Calibration - Traceability - Accreditation with flow measuring devices

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