Thermocouples

Introduction

In thermo-fluid sciences, the measurement of temperature plays a key role. During this lab we tested two different types of thermocouples, a T-type and a J-type. Measuring with a thermocouple allows us to measure the temperature difference between two junctions. There are several laws that deal with the thermocouples and reading of temperatures. We use the first law of thermodynamics which states that there is a conservation of energy. Also the law of homogenous metals is in effect. This law basically states that two different metals are needed for the thermocouple to function properly. The law of successive or intermediate temperatures allows a thermocouple that’s calibrated with a reference temperature to be used with a different reference temperature. Because of this, extra wires with the same thermoelectric characteristics are able to be added to the circuit and not change the total emf. The law of intermediate metals states that the addition of different metals to the circuit will not affect the total voltage created.

Discussion
During the calculations a few assumptions were made. For the calculation of the time constant for the T-type, the entire thermocouple is considered copper. The J-type was considered entirely Constantan. After analyzing the time constant graphs for each thermocouple type, several things can be concluded. For the twisted wire thermocouples, the volume was calculated as if it was a cylindrical shape. Also for the soldered thermocouples, the tip was considered to be a spherical shape when finding the volume.
The T-type thermocouple is labeled as TC_T on the data. The T-type seems to be better rated for lower temperatures. This is supported by the increasingly linear relationship as the temperature decreases and as the temperature rises, the relationship becomes more parabolic. The J-type thermocouple, TC_J, is more accurate at higher temperatures. This is also supported by the graph showing a linear relationship when the temperature is higher. Overall the T-type thermocouple is more accurate in a range of 0-80 degrees Celsius.
The adding of solder when fabricating the thermocouples seemed to help the thermal conductivity. It becomes more sensitive to slight temperature changes with the solder as opposed to simply twisting the two wires. When needing accurate temperature measurements, soldering is much better than twisting wires. This assumption about solder being the better choice is supported in the individual graphs. In the soldered graphs, more drastic temperature changes are present showing its sensitivity.
The time constants calculated from the data are within reason. The concept of time constant measures how long it takes for a thermocouple to respond to temperature change. The actual theory is how long it takes for the probe to reach 2/3 of the way to the second temperature. There is another way to go about calculating the time constant. This is done by using things such as the volume, density, surface area, specific heat, and thermo conductivity. This method isn’t very difficult but it can be more difficult when the system becomes transient in nature. The things needed to calculate the time constant are there but because it was already calculated, [ln(θ)], it wasn’t necessary. If this way was used to calculate the time constant, a difference in numerical valued would be noted due to the surface area of the cylinder (twisted thermocouples) and spheres (soldered thermocouples).
When analyzing the thermocouple composite graph there was an error in that we did not allow the thermocouples acclimate to room temperature for enough time. This can be seen between temperature gradients when it does not plateau long enough between trials.
Conclusion
Thermocouples aren’t the most efficient and accurate way to measure temperature, but they are very cost effective which is why they are widely used. There are many different types of thermocouples that are better for varying ranges of temperatures. In the methods of fabricating thermocouples that were done in this experiment, the soldering of the wires resulted in a much better finished product. It increased the sensitivity resulting in a more accurate and responsive thermocouple.