6.8: Measurement of Pressure

Introduction to Physical Oceanography · LibreTexts

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    Pressure is routinely measured by many different types of instruments. The SI unit of pressure is the pascal \((\text{Pa})\), but oceanographers normally report pressure in decibars \((\text{dbar})\), where: \[1 \ \text{dbar} = 10^{4} \ \text{Pa} \nonumber \]

    because the pressure in decibars is almost exactly equal to the depth in meters. Thus \(1000 \ \text{dbar}\) is the pressure at a depth of about 1000 m.

    Strain Gage

    This is the simplest and cheapest instrument, and it is widely used. Accuracy is about \(\pm 1\%\).

    Vibratron

    Much more accurate measurements of pressure can be made by measuring the natural frequency of a vibrating tungsten wire stretched in a magnetic field between diaphragms closing the ends of a cylinder. Pressure distorts the diaphragm, which changes the tension on the wire and its frequency. The frequency can be measured from the changing voltage induced as the wire vibrates in the magnetic field. Accuracy is about \(\pm 0.1\%\), or better when temperature controlled. Precision is 100–1000 times better than accuracy. The instrument is used to detect small changes in pressure at great depths. Snodgrass (1964) obtained a precision equivalent to a change in depth of ±0.8 mm at a depth of 3 km.

    Quartz crystal

    Very accurate measurements of pressure can also be made by measuring the natural frequency of a quartz crystal cut for minimum temperature dependence. The best accuracy is obtained when the temperature of the crystal is held constant. The accuracy is \(\pm 0.015\%\), and precision is \(\pm 0.001\%\) of full-scale values.

    Quartz Bourdon Gage

    This has accuracy and stability comparable to quartz crystals. It too is used for long-term measurements of pressure in the deep sea.

    This page titled 6.8: Measurement of Pressure was last modified on Mon, 11 Nov 2024 23:43:50 GMT and is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Robert H. Stewart via source content that was edited to the style and standards of the LibreTexts platform.