This happens because the metals react differently to the temperature difference. During a one sided heating the velocity of the electrons in the hot end increase more than that of the cold side. Therefore the electrons on the hot side get the tendency to move towards the cold side.
This creates a charge transfer with electrons concentrating on the cold side and making it more electronegative, while the hot side becomes more electropositive. This uneven charge distribution creates an electric field that produces a force that tries to repel the electrons to the hot end, consequently creating a voltage potential or thermoelectric voltage. To make a thermocouple thermometer and create an uneven charge distribution in response to a change in temperature, two dissimilar metals are required.
The change in voltage created by this temperature difference can be measured by a sensitive voltmeter. A thermocouple always measures a voltage difference due to a temperature difference, therefore two different temperatures are required to obtain meaningful results. So a reference temperature must be set. In this case the reference temperature was set as ice in water with a temperature of 6. Pure nickel wire and nickel chromium wire were used to make the thermocouple, since these both have different thermal conductivities.
If the voltage V T measured by the voltmeter is plotted against the temperature measured by a thermometer, a linear relationship can be obtained for NiCr and Ni for lower temperatures. The gradient or slope of this linear graph gives the value of k.
This relationship can be used to calibrate the thermocouple with the relationship:- 1 is a function of the temperature. Different metal combinations can be used for different measurement ranges, like platinum and platinum alloy combinations can be used for the largest measurement ranges, since as can be seen in Fig. This means the measurement accuracy in noble elements is much smaller for low temperatures. The common temperature range for nickel materials is from K to K whereas for platinum materials it is a much higher range from 10K to K.
Once and are obtained with 2 different readings, and is calculated, the new thermocouple voltage reading is divided by the factor to determine the temperature. These wires were connected as shown in the schematic in Figure. Figure Connection schematic of Ni and Ni-Cr wires. The measurement and the reference ends were made by twisting the two ends of the wires together.
The reference head was dipped in a beaker filled with the ice water at a temperature of 6oC and the measuring head was dipped in boiling water at oC. The other two ends were connected with alligator clips to the voltmeter positive and negative terminals respectively as shown in figure.
The temperature for the boiling water was read from the thermometer inserted in boiling water. The voltage reading was noted from the voltmeter. This was the first set of readings and represented the first point on the linear graph Next the thermocouple was tested in the atmosphere. The room temperature was read off a thermometer kept in an open space in the room.
This was the second set of readings and represented the second point on the linear graph. As T1 increases, so does V. The EMF also depends on the temperature of the open ends T2.
This is called the reference junction. Practical Thermocouple Construction A thermocouple construction consist of two conductors, welded together at the measuring point and insulated from each other long the length.
It will usually have an outer protection sheath. Welding is most common. All three types J, K, and T are available as insulated duplexed pairs from 0. For accuracy, and minimum system disturbance, the smaller the wire the better, but wire smaller than 0.
The Iron wire is magnetic. Junctions can be made by welding or soldering, using commonly available solders and fluxes. Iron-Constantan thermocouples can generate a galvanic EMF between the two wires and should not be used in applications where they might get wet. The Alumel wire is magnetic. Junctions can be made by welding or soldering, but high temperature silver-solders and special fluxes must be used. Why to prefer Type B Thermocouple?
Maintains a high level of accuracy and stability at very high temperatures Highest temperature limit of all of the termocouples Stable voltage generating equipment that possesses the quality of resisting high temperature oxidation.
Wide measuring range High mechanical strength. Further Reading. November October September Two calibration options. Tolerance testing; Characterization of thermocouples; Also included is a useful thermocouple tolerance summary table and thermocouple characterization linear algebra operation steps. Experimental are analyzed and the … sri venkateswara college of engineering and technology autonomus 1 ics lab manual 1. Calibration files for the Thermocouple are provided on Vernier lab interface program disks.
Experiment with the Thermocouple. Place the reference junction in an ice-water bath. Start data collection and check the temperature of a candle flame or some hot water. For thermocouple compare the ITS 90 Scale for calibration. A reference cold junction and thermocouple wires are used, in addition to electrical instrumentation, to calibrate indicators or simulators equipped with a CJC feature.
Suitable attention Thermocouple home experiment This home experiment is designed to teach you about amplifying signals, introduce you to generating data files for later use. This project and report may be done in teams of two. The report is to be in the standard memo format.
This sensor could be used for any of the following experiments: Study how the temperature inside a Bunsen burner flame varies with position. Compare the temperatures of different flames candles vs. Bunsen burner. Check the calibration of ovens calibration procedure for thermocouple datasheet, cross reference, circuit and application notes in pdf format. In this case, the extension or compensating cable is part of the thermocouple and should never be replaced by other wires even of the same type or batch.
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