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Calibration
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.
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.
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.
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.
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