with respect to atmospheric pressure. In the absence of differential pressure both levels would be equal, and this would be used as the zero point. The Industrial Revolution saw the adoption of "indirect" pressure measuring devices, which were more practical than the manometer. An example is in high pressure (up to 50 psi) steam engines, where mercury was used to reduce the scale length to about 60 inches, but such a manometer was expensive and prone to damage. This stimulated the development of indirect reading instruments, of which the Bourdon tube invented by Eugène Bourdon is a notable example. In the front and back views of a Bourdon gauge on the right, applied pressure at the bottom fitting reduces the curl on the flattened pipe proportionally to pressure. This moves the free end of the tube which is linked to the pointer. The instrument would be calibrated against a manometer, which would be the calibration standard. For measurement of indirect quantities of pressure per unit area, the calibration uncertainty would be dependent on the density of the manometer fluid, and the means of measuring the height difference. From this other units such as pounds per square inch could be inferred and marked on the scale. See also Calibration curve Calibrated geometry Calibration (statistics) Color calibration – used to calibrate a computer monitor or display. Deadweight tester EURAMET Association of European NMIs Measurement Microphone Calibration Measurement uncertainty Musical tuning – tuning, in music, means calibrating musical instruments into playing the right pitch. Precision measurement equipment laboratory Scale test car – a device used to calibrate weighing scales that weigh railroad cars. Systems of measurement References Sources Crouch, Stanley & Skoog, Douglas A. (2007). Principles of Instrumental Analysis. Pacific Grove: Brooks Cole. . Accuracy and precision Standards Measurement Metrology