This article reports the performance of an improved, newly developed, compact, low power, lifetime-based optical sensor (optode) for measuring partial pressure of dissolved CO 2 gas (pCO 2 ) in natural waters. The results suggest that after preconditioning, these sensors are stable in water for time periods longer than 7 months. The wide dynamic range of about 0-50000 μatm opens possibilities for numerous applications of which some are presented. In normal marine environments with pCO 2 levels of 200-1000 μatm, the best-obtained precision was about ±2 μatm, and the absolute accuracy was between 2-75 μatm, depending on the deployment and the quality of the collected reference water samples. One limitation is that these sensors will become irreversibly poisoned by H 2 S and should thus not be deployed in sulphidic environments.
The candela, the SI (syste`me internationale) unit for optical radiation, has been one of the base units since the inception of the system. The latest definition was in 1979, when it was linked to the derived unit, the watt. Advances in optical technology and the needs of the communication sector suggest that it is timely that consideration be given to redefining the candela in terms of fundamental quantum optical entities, i.e. photons. Validation of this approach will require comparison against the most accurate conventional technique, cryogenic radiometry. A definition in terms of photon number and the requirements for demonstrating equivalence with existing techniques is discussed, together with new possibilities which would result from further improvements in accuracy. Work being carried out at the National Physical Laboratory (NPL) towards these goals is described, drawing on developments of photon-counting calibration techniques and low temperature measurements, and research into single photon sources and detectors.
Chemical sensing is of great importance in many application fields, such as medicine, environmental monitoring, and industrial process control. Distributed fibre-optic sensing received significant attention because of its unique feature to make spatially resolved measurements along the entire fibre. Distributed chemical sensing (DCS) is the combination of these two techniques and offers potential solutions to real-world applications that require spatially dense chemical measurements covering large length scales. This paper presents a review of the working principles, current status, and the emerging trends within DCS.
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