Chapter 6 - Temperature, Salinity, and Density
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:
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.
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
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 100–1000 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.
Very accurate measurements of pressure can also be made by measuring
the natural frequency of a quartz crystal cut for minimum temperature
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.