Documentation of the U.S. Geological Survey Oceanographic Time-Series Measurement Database

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 percent of full scale accuracy while the resonators are accurate to about 0.01 percent 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. 24). 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.

Pressure-Based Wave and Tide Measurements

Sea-Bird Seagauge

Sea-Bird Electronics model SBE 26plus (Seagauge) (fig. 25) are employed to make water level measurements and are fitted with a Paroscientific Digiquartz sensor to obtain the desired level of measurement accuracy (0.01-percent full scale). The instrument may be programmed to sample in a variety of ways, including burst sampling of up to 4 Hz. The housing also has a temperature sensor in the head, and allows deployment at depths up to 600m. Water level is resolved to 0.2mm for tides and 0.4mm for waves. The manufacturer’s software is used to compute wave statistics and spectra from the raw measurements. For more information see http://www.seabird.com/sbe26plus-wave-tide-recorder.

RBR D|wave

“The RBR Virtuoso D|wave [fig. 26 in this report] is a small submersible pressure logger capable of sampling continuously or in burst mode at speeds up to 6 Hz and can store 30 million individual readings. This unit was temperature compensated and had a pressure sensor with a depth rating of 10 m. The manufacturer-specified accuracy was ±0.05 percent of full scale (about 0.005 m water depth) and resolution was 0.001 percent of full scale (about 0.0001 m water depth).” (Dickhudt, 2014)

The manufacturer’s software is used to compute wave statistics and spectra from the raw measurements. For more information, see http://www.rbr-global.com/products/small-tide-a-wave-loggers/rbrsolo-d-wave-logger.

Onset HOBO U_20

“The Onset Hobo U20 water-level logger [fig. 27 of this report] is a small submersible pressure logger capable of recording about 22,000 pressure and temperature samples at rates up to 1 Hz. Two models of Onset Hobo U20s were deployed: model U20–001–04 pressure sensors with ranges of 0 to 15 decibars (about 0–4 m water depth) were used as atmospheric and submersible sensors; and model U20–001–01 pressure sensors with ranges of 0 to 21 decibars (about 0–9 m water depth) were used as submerged sensors. The U20–001–04 sensor has a water-level accuracy of ±0.075 percent of full scale (about 0.003 m water depth) and water-level resolution of 0.001 m water depth. The U20–001–01 used as a submerged sensor has a water-level accuracy of 0.05 percent of full scale (about 0.005-m water depth) and water-level resolution of 0.002 m water depth.” (Dickhudt, 2014).

Click on figures for larger images.

Figure 24. Two Paroscientific Digiquartz® pressure sensors.

Figure 25. Sea-Bird Seagauge mounted on Micropod 1000 prior to deployment south of Fire Island, New York, in 2014.

Figure 26. Photograph of the RBR Virtuoso D|wave sensor used on Dauphin Island, Alabama, in 2013.

Figure 27. One of the Onset Hobo U20 pressure loggers and threaded stainless-steel rod used on Dauphin Island, Alabama, in 2013.