Sealed electrical Instruments, protection

UMEs used in our laboratory were constructed by sealing of carbon fibre into low viscosity epoxy resin (see Fig. 32.4) [118]. This method is simple, rapid and no specialised instrumentation is required. Firstly, the fibres are cleaned with this aim. They are immersed in dilute nitric acid (10%), rinsed with distilled water, soaked in acetone, rinsed again with distilled water and dried in an oven at 70°C. A single fibre is then inserted into a 100- iL standard micropipette tip to a distance of 2 cm. A small drop of low-viscosity epoxy resin (A. R. Spurr, California) is carefully applied to the tip of the micropipette. Capillary action pulls the epoxy resin, producing an adequate sealing. The assembly is placed horizontally in a rack and cured at 70°C for 8h to ensure complete polymerization of the resin. After that, the electric contact between the carbon fibre and a metallic wire or rod is made by back-filling the pipette with mercury or conductive epoxy resin. Finally, the micropipette tip is totally filled with epoxy resin to avoid the mobility of the external connection. Then, the carbon fibre UME is ready. An optional protective sheath can be incorporated to prevent electrode damage. 

The ideal situation for a process analyzer is to have the detector/electrical components in an area physically isolated from the sample stream and any liquid components required for calibration or instrument operation (e.g., solvents for liquid chromatography or gel-permeation chromatography). In this arrangement, isolation of the electrical components ensures that a spark source or hot surface (such as a spectroscopic light source) will not cause a problem with flammable components (both sample stream components and solvents). Also, this isolation wiU limit instrument damage in case of a sample system leak or solvent leak. Although the ideal situation is often not realized in a practical application, one should make every effort to protect the delicate sections of the instrument. In one application of the authors, a fiber optic probe having a quartz window sealed on the end of the probe was installed in the reactor of a process that contained... 

At the surface, the ignition wells for use with downhole electric heaters must have a high-pressure lubricator, stuffing box, flttings, also necessary measurement and control instruments (Fig. 62). The equipment includes a special electric cable of correct length, AC current source, and an air supply system. Special care must be taken to protect the stuffing box seal from electric current. 

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