Pressure in the ocean is typically measured using either a strain gauge or quartz crystal resonator. Strain gauge sensors are metallic foils on a flexible backing which sense deformation, due to the induced stress of pressure. Quartz crystal resonators measure pressure by detecting the change in frequency of oscillation of the resonator due to induced stress. The output pressure is compensated for temperature by using the signal from temperature-sensitive crystals which are a component of the instrument. Strain gauge sensors typically have 0.1% of full scale accuracy while the resonators are accurate to about 0.01% of full scale and more stable over long periods of time with less hysteresis. Quartz sensors are generally preferred where high-accuracy is needed, for example to measure the surface wave signal at depth. Strain gauge sensors are used where a simple measure of water depth, for example mooring or instrument depth, is needed. Most pressure data in the time-series database have been collected using Paroscientific quartz crystal sensors (http://www.paroscientific.com ) (fig. 21). The strain gauge sensors are generally an integral part of data logging systems and are not tracked as separate instruments. Pressure data will be found as variables in Seacat, Microcat, ADV, PCADP, and MIDAS files.

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Figure 21. Two Paroscientific Digiquartz® pressure sensors.