Self Calibrating Pressure Recorder
The vertical component of deformation in Earth’s crust can be observed on the seafloor by recording ambient seawater pressure.
As the altitude at a particular site changes with respect to sea level, the hydrostatic pressure on the seafloor changes slightly.
While quartz pressure gauges have excellent short-term resolution, gauge drift adds additional and significant uncertainty in estimates
of long period deformation; drift rates equivalent to 20-30 cm/year have been observed, of the same order as expected deformation signals
atop subsea volcanos, for example. The drift can be estimated with differential pressure measurements to distant and presumably stable
seafloor reference sites. Differential pressure measurements must presume the stability of the reference site; such measurements require
a small work-class remotely operated vehicle and can only be made infrequently due to the expense. Acoustic methods are limited in vertical
resolution by the sound velocity profile.
A piston gauge calibration system uses a calibrated mass loaded onto a hydraulic piston to generate a pressure; the pressure can be accurately known by determining the mass values, piston dimensions and the local acceleration of gravity. Such a system can be transformed into a piston gauge pressure reference, a single-valued and constant pressure source. By in-situ comparing a sea-floor pressure gauge with a pressure reference, the gauge drift can be unambiguously determined and removed from the time series. The pressure reference need only be stable and highly reproducible, a significantly simpler task. A reference based on commercially available instruments could achieve drift control with uncertainties of only a few parts per million, or 1 cm/year at depths of 2000 meters.
We have constructed and tested a laboratory prototype of the piston gauge pressure reference. The instrument was operated at a simulated seafloor pressure (139.3 bar) for over 50 days. The observed repeatability of the corrected reference pressures was 0.52 mbar, equivalent to a depth repeatability of 0.5 cm or 3.7 ppm. We are continuing to refine the instrument and are currently engineering it to operate on the seafloor.
For more information contact: