Accuracy testing using the Test Rig

In order to understand the effects of the polymer resin and yarn fibres on the accuracy of the ETS yarn a test rig had to be designed. The main focus during the design of the test rig was to develop an equipment capable of gradually increasing the temperature of the sample over a prolonged period of time. It is assumed that body/wound temperature would change gradually over time rather than having step changes. The samples stated in section 4.2.1 were tested with the custom made test Rig.

4.3.1 Developing the Test Rig

The test rig was constructed using textile-based heaters produced by the Advanced Textiles Research Group at Nottingham Trent University. Silicone was used to make the base for the test rig. Silicone was chosen because of its good thermally insulating property, to prevent the heat produced by the textile heaters escaping through thermal conduction. A mixture of silicone, silicon carbide and carbon (SiO, SIC, C) was laid on top of the silicone base. The mixture of SiO, SiC, C was chosen because of its thermally conductive and electrically insulating property. This provided an uniform temperature surface for the samples to be tested. Three commercially available k type thermocouples from Pico Technology were placed at equidistance’s in-between the two layers of silicone and silicon carbide. The thermocouples were bent slightly to ensure the measuring heads were on the same level as the surface of silicon carbide. This structure was regarded as the test bed and it is shown in figure 4.4.

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Figure 4-4: Fabric heaters positioned on top of the test bed

To hold the samples being tested and two other commercially available thermocouples a flexible sensor holder (FSH) had to be designed. A potion off the edge of the test bed was cut for this purpose. This was done to ensure that the test bed and the sensor holder had the same composition. Three slits were cut on the sensor holder at equal distances (4 mm apart) to the thermocouples on the test bed. The sample being tested was placed on the middle slit and the two thermocouples were placed on the other two.

Thereafter the sensor holder was placed directly on top of the thermocouples in the test bed. This ensures that the sample and the two thermocouples would be positioned right on top of the three thermocouples in the test bed during the experiment. A rectangular metal plate was placed on top of the sensor holder, this was done to ensure that there was an evenly distributed weight holding the sensor holder in place. The two thermocouples on the FSH were used to observe if there would be any changes in the temperature recorded by them, when compared to the temperature captured by the thermocouples on the test bed (which were positioned right underneath them).

Four two course fabric heaters were placed on the lower end of the thermally conductive SiO, SIC, C layer. Fabric heaters were made by knitting FabRoc (EXO2, USA) and interconnecting it using knitted copper filaments (Mbise et al., 2015). The heaters were knitted with a band on both sides and a Velcro strip was sewn onto its edges to ensure the heaters were held in place.

In order to eliminate the effects of room temperature on the readings the test Rig was placed inside an insulation box. The insulation box was constructed with Floor mate 300A (Dow building solutions, MI, USA) which has a very low thermal conductivity of 0.033 Wm-1_K-1 _{(data sheet given in appendix}

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kg metal weight. When used in a wound dressing it is assumed that the bandage would provide the insulation against room temperature and create a micro-climate. The entire test rig is given in figure 4.5.

Figure 4-5: Test-Rig

The fabric heaters were powered using a laboratory power supply with 5 V, 2 A. The fabric heater elements heated the silicon carbide gradually increasing its temperature. The program was developed in LabVIEW to read the temperature off the five thermocouples and the sensing yarn (the program given above in section 4.2.3.1). The LabVIEW program was adjusted (using a time delay express VI) to record temperature measurements every minute. The results were saved onto a spread sheet.

The thermocouples were connected to a PICO-TC08 unit (Pico Technology, UK). The resistance of the thermistor was determined with a potential divider circuit by using a NI USB 6008 DAQ unit. The PIC0-TC08 and the NI 6008 DAQ both were interfaced to a computer. The values of the resistors used in the potential divider circuit was determined with an Agilent 34410A 6 ½ Digital multi-meter which has a precision of 0.01 %.

4.3.2 Results

Preliminary tests were conducted to evaluate the functionality of the test rig, whereby a thermocouple was used instead of sensing yarn. As the results were encouraging the samples mentioned in section 4.2.1 were tested using the custom made test rig.

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Figure 4-6: Results from the test rig. Where thermistor measurements are shown in green and the thermocouple measurements are shown using the rest of the colours. Where a) gives the results when a thermistor is tested, b) when a thermistor is encapsulated using 9-20801 resin to form a cylindrical micro-pod with a diameter of 5mm, c)When it is encapsulated using 9001-E-V-3.5 resin to form a cylindrical micro-pod with a diameter of 5mm, d) when polyester fibres are inserted within the 0.87mm diameter encapsulation created using 9-20801 resin, e) when zylon fibres are inserted within the 0.87mm diameter encapsulation created using 9-20801 resin and f)when the final ETS yarn is tested.

The thermocouples used for these experiments were of class one tolerance which means they have an accuracy of +/-1.5 °C within a temperature range of -40 to 375 °C. Therefore the accuracy of the test rig is +/-1.5 °C. The Tolerance of the Murata NTC 0402 thermistors is ±1 % according to the manufacturer. The results from the first experiment (figure 4.6a) shows that the temperature measurements obtained from the thermistor fall on top of the temperature measurements captured by the thermocouples. This suggests that there is no difference between the temperature measurements captured by the thermistor and the thermocouples.

The results from the second set of experiments (figure 4.6b) demonstrated that the thermistor readings were not significantly influenced by the quantity of multi-cure 9-20801 resin used to form the micro-pod. Even the 5 mm diameter micro-pod did not have any effect on the temperature measured by the thermistor. Therefore one could conclude that the amount of resin used for encapsulation has minor effect on the sensing ability of the thermistor.

One could conclude from the results from the third set of experiments (figure 4.6c) that the thermally non-conductive resin 9001-E-V-3.5 also has minor effect on the temperature measuring

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ability of the thermistor. This may be due to the minute difference in between the thermal conductivities of the resins.

The results from the fourth (figure 4.6d) and fifth set (figure 4.6e) of experiments show that the inclusion of additional yarn fibres (PE and Zylon) within the micro-pod to enhance the mechanical strength of ETS yarns do not affect the thermistor readings significantly.

From the final results (figure 4.6f) one could conclude that the inclusion of polyester fibre sheath around the thermistor, interconnects and micro-pod through the knit-braiding process would not affect the performance of the thermistor significantly. It can be also observed that the ETS yarn can be used to measure temperature over a prolonged period of time. All the experiments were carried out for an excess of 250 minutes.