Abstract
Salinity measurement in water is typically performed with conductivity sensors. However, for long-term marine deployments, loss of precision is observed, mainly due to electrode drift (oxidation and degradation occurs in the presence of water, salts and bio-fouling), which results in inaccuracy of measurements. A cost-effective, low-power, four-probe salinity sensor is presented, to accurately measure long-term deployments in oceans, rivers and lakes. The four-probe methodology overcomes many of the drift problems, and the use of low-cost stainless-steel electrodes (avoiding platinum or titanium materials) can still achieve good long-term stability, in the practical salinity scale range from 2 to 42 PSU. Low-power electronics (200 µA in sleep-mode and 1 mA in active-mode) based on a ratiometric ADC conversion, and a low-power microcontroller with non-volatile memory, complements the proposed sensor, to achieve an autonomous salinity sensor for long-term marine deployments, with autonomy above 1 year with a 1 min-1 sample rate, using a common 2400 mA x 3.7 V lithium battery.

Abstract
A cost-effective (less than 20€) and low-power device is present for in situ continuous monitoring of suspended sediments (SPM) concentration in estuarine and coastal areas. The sensor uses nephelometric technique for SPM values less than 20g/L and backscatter technique for higher ones. A transmitted infrared (IR) and ultraviolet (UV) channels are used to perform the distinguish of inorganic from organic matter in the suspended particles. It is explained the design and built of the sensor as its calibration and preparation for in field tests. The sensor was deployed for one week in a small dock in the estuarine zone of Cavado river (Esposende, Portugal) where is analyzed the SPM and organic/inorganic matter change with the tidal cycles.

Abstract
A cost-effective optical sensor for continuous in-situ monitoring of turbidity and suspended particulate matter concentration (SPM), with a production cost in raw materials less than 20 (sic), is presented for marine or fluvial applications. The sensor uses an infrared LED and three photodetectors with three different positions related to the light source-135 degrees, 90 degrees and 0 degrees-resulting in three different types of light detection: backscattering, nephelometry and transmitted light, respectively. This design allows monitoring in any type of environment, offering a wide dynamic range and accuracy for low and high turbidity or SPM values. An ultraviolet emitter-receiver pair is also used to differentiate organic and inorganic matter through the differences in absorption at different wavelengths. The optical transducers are built in a watertight structure with a radial configuration where a printed circuit board with the electronic signal coupling is assembled. An in-lab calibration of the sensor was made to establish a relation between suspended particulate matter (SPM) or the turbidity (NTU) to the photodetectors' electrical output value in Volts. Two di fferent sizes of seashore sand were used (180 mu m and 350 mu m) to evaluate the particle size susceptibility. The sensor was tested in a fluvial environment to evaluate SPM change during sediment transport caused by rain, and a real test of 22 days continuous in-situ monitoring was realized to evaluate its performance in a tidal area. The monitoring results were analysed, showing the SPM change during tidal cycles as well as the influence of the external light and biofouling problems.

Abstract
The advances in wireless communications are still very limited when intended to be used on Underwater Communication Systems mainly due to the adverse proprieties of the submarine channel to the acoustic and radio frequency (RF) waves propagation. This work describes the development and characterization of a polyvinylidene difluoride ultrasound transducer to be used as an emitter in underwater wireless communications. The transducer has a beam up to 10 degrees x 70 degrees degrees and a usable frequency band up to 1 MHz. The transducer was designed using Finite Elements Methods and compared with real measurements. Pool trials show a transmitting voltage response (TVR) of approximately 150 dB re mu Pa/V@1 m from 750 kHz to 1 MHz. Sea trials were carried in Ria Formosa, Faro (Portugal) over a 15 m source-receiver communication link. All the signals were successfully detected by cross-correlation using 10 chirp signals between 10 to 900 kHz.

Abstract

Abstract
Pressurised networks are widely used to transport gas through extensive distances. To secure the gas transport at safety levels and also economic viability, the networks are thoroughly monitored. Paramount to network control and analysis is the modelling of the gas dynamics in the pipelines and its consequent simulation. In this work, the pipeline is represented by a quasi-hyperbolic PDE, whose exact solution is not easy to withdraw, and in alternative we opt for an approximation. The construction of the initial function, very important to obtain a good approximation, is done using a separation of variables. Special relevance is given to issues as consistency, stability and convergence in order to evaluate a class of FD methods for the solution of gas network models, in particular the quasi-hyperbolic equation. Horizontal pipelines are considered as well as some particular centred schema for an inclined pipeline.