Epoxies temperature sensors

This technique employs a single thermistor serving as both a temperature sensor and a heater. Typically in this technique, either a thermistor is inserted through the lumen of a hypodermic needle, which is in turn inserted into the tissue, or the thermistor is embedded in a glass-fiber-reinforced epoxy shaft. Figure 2.4 shows the structure of a thermistor bead probe embedded in an epoxy shaft. Each probe can consist of one or two small thermistor beads situated at the end or near the middle of the epoxy shaft. The diameter of the finished probe is typically 0.3 mm, and the length can vary as desired. Because the end can be sharpened to a point, it is capable of piercing most tissues with very minimal trauma. 

In order to make a FPI chemical sensor, the FP cavity needs to be made accessible by the analyte molecules. One way to achieve this is to use a holey sleeve to host the cavity. Xiao et al.7 reported such a fiber FPI gas sensor formed by bonding two endface-polished fibers in a holey sleeve using epoxy. The holey sleeve allows gas to freely enter and leave the cavity. A resolution of 10 5 was estimated in monitoring the changes in the refractive index caused by varying the gas composition. However, the sensor assembly was complicated and required the use of epoxy. In addition, the various components used in sensor construction were made of different materials. As a result, the device had a strong dependence on temperature. 

A compact sensor of greatly reduced dimensions (outer diameter x length 36 x 46 mm) has been constructed and is shown in Fig. 2. In order to conveniently accommodate enzyme columns and to ensure isolation from ambient temperature fluctuations, a cylindrical copper heat sink was included. An outer Delrin jacket further improved the insulation. The enzyme column (inner diameter x length 3x4 mm), constructed of Delrin, was held tightly against the inner terminals of the copper core. Short pieces of well-insulated gold capillaries (outer diameter/inner diameter 0.3/0.2 mm) were placed next to the enzyme column as temperature-sensitive elements. Microbead thermistors were mounted on the capillaries with a heat-conducting epoxy. Two types of mini system has been constructed as discussed below. 

Various sensors for temperatures, air flow, etc., all require exceptional dimensional stability, property retention at elevated temperatures, fluid resistance and creep resistance. Ignition components require many similar properties but have the additional need for good electrical properties, i.e., high dielectric strength, and good adhesion to epoxy potting compounds. Materials that see these applications include PBT/PC and PPE/HIPS. 

Figure 24 Plots of ionic conductivity against time during the cure cycle of a glass fibre reinforced epoxy resin panel using an interdigitated sensor. Temperature profile measured by a resistance thermometer in the sensor...

Mattana et al. (2013) reported using stainless steel yams for this purpose. The yams were aligned with the strips and conductive epoxy was applied to bind them together. The observations showed a very linear behaviour of the sensors, which was needed in order to apply Eq. [23.2] and calculate the resulting temperature. However, it was noted that the printed sensors had a temperature coefficient of resistance quite a bit lower than the sensors made with more conventional techniques such as photolithography. 

Fibre Bragg-grating (FBG) sensors can be used for quasi-distributed measurement of strain. The FBG sensors excel in precise point-wise strain measurements up to 0.8% with a resolution of 3 ps, or temperature determinations with a resolution of 1 K. The FBG fibres exhibit low breakdown strain of only 1 %. It has been already seen that POF sensors can be processed directly by textile machineries, whereas FBGs can be embedded in an apposite tube inserted at the production stage into the textile matrix (although other approaches are under study) or directly in the epoxy resin of composites. 

Applications of the fibre optics transmittance or ATR probe are in quality control, reaction monitoring, skin analysis, goods-in checking, analysis at high and low temperature, radioactive or sterile conditions, and hazardous environments. Applications of the reflectance probe are for turbid liquids, powders, surface coatings, textiles, etc. By using an on-line remote spectrophotometer, real-time information is gathered about a chemical process stream (liquids, films, polymer melts, etc.), as often as necessary and without the need to collect samples. This determines more reliable process control. Remote spectroscopy costs less to maintain and operate than traditional techniques. Fernando et al. [48] have compared different types of optical fibre sensors to monitor the cure of an epoxy resin system. 

---