rapor5 - In engineering applications and modern...

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In engineering applications and modern experimental studies temperature measurement is an important topic because in many cases accuracy of these temperature measurements determine the total error of the experiments.Most of the time we look for the way to measure temperature that yields: Accuracy and speed of response required by the application Acceptable initial investment Low maintenence cost Long life and stability of calibration All methods of temperature measurement can be divided into two groups: 1)Indirect methods(contactless) for instance optical methods.(I mainly talk about it in the questions part). 2)Direct methods (contact methods).Here are some possibilities for this method: Thermometers.It can be alcohol(-40 ° C,+80 ° C) or mercury(-5 ° C,+350 ° C) Resistance wire termometers Temperature sensors Thermocouples
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In our experiment we mainly deal with thermocouples so we have to define it.A thermocouple is a system composed of two homogenous metal wires dissimilar in chemical composition that are soldered ,fused or welded together at one end forming a closed circuit when the free ends are connected to an indicating instrument which is mostly a potentiometer. Thermocouples are available in different combinations of metals or calibrations. The most common calibrations are J, K, T and E. There are also high temperature calibrations R, S, C and GB. Here are the properties of mostly used ones(of coarse there are other kinds of thermocouples for specific purposes.): For our particular aim we need to draw temperature vs time graph.To do this correctly of course we have to make necessary corrections considering the room temperature as we learn in our experiment.We take our sample at 250° C and cool it to nearly 211° C ;by the way we write the data seen in computer in every 30 seconds.After making our graph(temperature vs time) we have found the melting point of Sn by measuring the thermo- emf between the thermocouple ends. We have found the melting point of Sn is about 226 ° C but the melting point of pure Sn is 232 ° C. This shows that our sample is not pure.In our particular case we do not observe undercooling but it can be undercooling in some cases as we are learned in our experiment so at this point of my report I want to give information about the undercooling. Undercooling is a very useful process in many cases. If done right, the temperature of a liquid can be lowered below the normal freezing point while its remains unfrozen, or unsolidified, and still in the liquid state. This is undercooling. The undercooling phenomenon occurs in pure, undisturbed substances that are slowly cooled. So long as no molecules join to form a solid nucleus (called "nucleation"), the sample remains liquid. Once a solid does form, it spreads rapidly through the sample, and another contradiction occurs. The temperature rises rapidly as the latent heat of fusion is released in an effect called "recalescence" -- and a flash of light often is seen. Then the sample cools as it completes the change from liquid to solid
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rapor5 - In engineering applications and modern...

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