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Characterizing Thermal Expansion of the Piston Gauge in the Absolute Self-Calibrating Pressure Recorder

  • Author(s): Cook, Matthew James
  • et al.
Abstract

Vertical seafloor deformation can be measured using pressure, since ambient seawater pressure is a proxy for depth. However, most quartz pressure gauges experience drift that can contaminate expected geodetic signals. An instrument called the Self-Calibrating Pressure Recorder (SCPR) was developed to compensate for the effects of drift by using a piston gauge to supply a stable reference pressure. SCPR deployments are limited to two years, so to collect longer geodetic records over years to decades the Absolute Self-Calibrating Pressure Recorder (ASCPR) was developed. The ASCPR uses the same calibration system as the SCPR, but instead makes campaign-style measurements of the true, absolute value of pressure. In order for measurements to be accurate, the calibration system components must be known to a few parts per million (ppm). However, during the deployment and the measurement periods, the ASCPR is susceptible to changes in pressure on the order of 100 ppm, due to temperature changes. In the lab, we exposed the ASCPR to temperature changes while measuring the reference pressure to characterize the effects of thermal expansion on the piston gauge. The corrected pressures were stable at a 95% confidence level of ±0.23 kPa, or 2.3 cm equivalent, corresponding to a 13 ppm effect at a simulated pressure of 19,000 kPa. We also determined the thermal expansion effects had a time constant of 5.9 hours, which allows us to determine the changes over time, thereby reducing the amount of time required for the instrument to stabilize. Finally, the coefficient of thermal expansion that minimized the uncertainty in the pressure was less than the nominal value stated by the manufacturer. The uncertainty remained at the 13 ppm level, larger than our desired uncertainty of less than 10 ppm. At this level, the ASCPR can be used to detect larger geodetic signals, but requires further improvements to monitor slow deformation and sea level. My thesis is focused on evaluating the accuracy of the ASCPR and how it depends on the temperature variations during deployment

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