Contribuição para o desenvolvimento de um microCTD para monitoramento oceanográfico em águas rasas

Abstract

Obtaining temperature and conductivity profiles of seawater in coastal regions allows the use of instruments with more flexible characteristics in relation to CTD (conductivity, temperature and depth) intended to obtain such measurements on the high seas and at great depths. The popularization of analog and digital electronics in the last decade has stimulated initiatives to develop "free projects" of alternative probes built with accessible materials and capable of acquiring data of sufficient quality for studies in these locations. Of the three variables measured, temperature and depth have a relevant supply of shelf sensors, the second being actually estimated by measuring the pressure of the water column. On the other hand, conductivity, which in turn is used to estimate the salinity of seawater, has few commercial measuring devices suitable for installation in this type of equipment, since they are more present in laboratory instruments. Another important aspect is the enclosure that keeps the sensors in contact with water while protecting the electronic boards that control them. In this work, is presented a device named μCTD probe, which is made with commercially available temperature and pressure sensors, and a conductivity sensor developed from scratch specially to be integrated to the probe, whose enclosure is formed by a set of PVC parts and pipes regularly used for residential hydraulic installations. The first sensor is a digital thermometer whose transducer is a variation of the diode temperature sensor, and the second one, a piezoresistive absolute pressure sensor with transducer built with MEMS (micro- electromechanical system) technology. The conductivity sensor has as transducer a four- electrode conductance cell operated in direct current and governed by a microcontrolled electronic interface equipped with an analog signal conditioning circuit. In bench measurements, the conductivity sensor demonstrated uncertainty dominated by the uncertainties arising from the AD conversion process related to the current and voltage variables. The sensor also presented a relative error between 3.5% and 6.6% and a maximum absolute error of 4.2%. Field measurements were compromised by water leakage into the instrument, but served to validate the μCTD probe as an alternative to a tide gauge by measuring the water column with its pressure sensor.