Process optical sensors

This type of isolation device (also called a chemical barrier) is similar to deflagration suppression systems used on process equipment. This barrier system consists of an optical sensor, installed in the pipeline or duct between two items of equipment, that detects an oncoming deflagration... 

With regard to the development of infrared sensors during the last decade, some major fields of application can be identified, covering biological, biochemical or medical applications, environmental monitoring and process monitoring, with the latter being considered as the area closest to a widespread application of IR optical sensor systems. 

Sensitivity impacts upon the limit of detection and resolution of the device, making it a key performance parameter. Recently, several strategies have been developed in order to provide sensitivity enhancements for optical sensor platforms based on both optical absorption and fluorescence phenomena. These strategies are the result of rigorous theoretical analyses of the relevant systems and, combined with polymer processing technology and planar fabrication protocols, provide a viable route for the development of low-cost, efficient optical sensor platforms. 

As a conclusion it has been demonstrated that optical sensors have a wide field of applications in the process control, surveillance and biosciences. They supply the chance to monitor processes an-line and with short time resolution. Interferometric sensors are rather sensitive and applicable in... 

The simplest way to form arrays of integrated optical sensors is to put them all in parallel while all are fed from one input channel the input power being divided over all sensors by using e.g. a star coupler. Every sensor has its own output, detector and electronic processing unit and all sensors can be monitored simultaneously. 

The optical sensors are composed of ion-selective carriers (ionophores), pH indicator dyes (chromoionophores), and lipophilic ionic additives dissolved in thin layers of plasticized PVC. Ionophores extract the analyte from the sample solution into the polymer membrane. The extraction process is combined with co-extraction or exchange of a proton in order to maintain electroneutrality within the unpolar polymer membrane. This is optically transduced by a pH indicator dye (chromoionophore)10. 

Optical sensors for oxygen are among the few sensors, which have found practical application for process-monitoring and clinical diagnostics. They are generally based on compounds such as platinum porphyrins or ruthenium phenanthroline derivatives (Table 17) which show a decrease in luminescence upon exposure to molecular oxygen15. 

The use of sensors for surveillance of industrial combustion processes is state of the art. Especially optical sensors detect the existence of a flame. With decreasing furnace performance the amount of installed sensors declines. In residential appliances there are nearly no sensors installed because the costs for both, sensors and actuators, have to be balanced with the technical profit. The efforts for setting up combustion controls are very ambitious but in many cases not successful. First a distinction has to be made between sealed boilers and those that are open towards the room in which they are installed. The resulting controls cannot be interchanged between these two groups of appliances. 

Energy transfer [3,12] between molecules has also been used in the design of optical sensors. Here, an excited molecule (donor) can transfer its electronic energy to another species (acceptor). This process occurs without the appearance of a photon and results from dipole-dipole interaction between the donor-acceptor molecules. The rate depends on the fluorescence quantum yield of the donor, the overlap of the emission spectra of the donor with the absorption of the spectrum of the acceptor, and their relative orientation and distance. It is the overlap of... 

Newer techniques for measuring the refractive index allow for instantaneous, real-time measurement in process streams, or alternatively, a special continuous-flow sample well can be installed on bench top instruments. Small, pocket-sized refractometers also make held measurement very simple and reliable. Fiber optic sensors find uses in biomedical applications. 

A. L. Harmer, Fibre optic sensors for offshore process control instrumentation, Proc. Optical Fibre Sensors Conference (OFS 86), Informal Workshop at Tusukuba Science City, 1986, VII. Pub Institute of Electronics and Communication Engineers of Japan, Tokyo, 1986. 

In reviews on the use of in situ sensors" or optical sensor systems" for bioprocesses, UV-vis does not play a major role. An example for the application of at-line UV-vis spectroscopy was presented by Noui et al. The selective flocculation processes of Saccharomyces cerevisiae homogenates were followed with a newly developed direct UV spectrophotometer. The results from a PLS regression model were in good agreement with those from off-line chemical assays. 

As noted earlier, on-line process monitoring is among the application areas that can benefit most from the use of sensors provided they can be incorporated into portable systems. Electrochemical sensors are generally more suitable for these applications, even though optical sensors can also be readily adapted for this purpose (e.g. by using LEDs or photodetectors instead of conventional photometers) at the expense of somewhat degraded spectral resolution and sensitivity. 

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