Chapter 6 - Temperature, Salinity, and Density

Chapter 6 Contents

6.8 Measurement of Pressure

Pressure is routinely measured by many different types of instruments. The SI unit of pressure is the pascal (Pa), but oceanographers normally report pressure in decibars (dbar), where:

1 dbar = 104 Pa

because the pressure in decibars is almost exactly equal to the depth in meters. Thus 1000 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 1%.

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 0.1%, or better when temperature controlled. Precision is 1001000 times better than accuracy. The instrument is used to detect small changes in pressure at great depths. Snodgrass (1968) 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 0.015%; and precision is 0.001% of full-scale values.

Quartz Bourdon Gage
Accuracy and stability comparable to quartz crystals. It too is used for long-term measurements of pressure in the deep sea.

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