This experiment is an alternate to the four performed in MET 3140 for the Fall Semester, 2006. It is to be performed over two lab sessions. The first session will concern the construction of the thermocouples (as shown in figure 13-9 below), and their bonding to metal test specimens. The second session will concentrate on taking transient and steady-state measurements from the specimens while maintaining the reference junctions in ice baths. Each student will make two thermocouples as needed for two measurement locations on the test specimen. Specimens will be steel objects of different simple shapes, and they will be heated at one end or side using an oxyacetylene tourch.

The objectives of this experiment include (hands-on for each student):

• Construction of two iron-constantan thermocouples having soldered measurement and reference junctions according to figure 13-9.
• Maintaining a suitable reference junction temperature.
• Measurement of the Seebeck voltage at two locations on a precision (calibrated) voltmeter resulting from the heating of the test specimen at one end.
• Recording of observations during the transient and steady-state phases of heat conduction in the speciment noting that cooling of it will occur by convection and radiation as the heat source is removed and the specimen cools.
• Discussion of the resulting temperature profiles based on knowledge gained in MET 3120 up to now.
• Discussion of the thermocouple’s time constant and measurement error as it may apply to this test as covered in Chapter 13.
• Documentation of the experiment with a digital camera and webcam.

This lab experiment involves the construction of thermocouple measurement sets from Omega Iron and Constantan thermocouple wire (Wires A and B as shown in figure 13-9) joined to copper lead wires to form a reference junction and which connect to the voltmeter. Junctions will be twisted together and soldered. The two reference junctions will be immersed in ice water.

The measurement iron-constantan junction will be mechanically attached to the steel test specimens. Test specimens will be selected pieces of steel scrap available from the metal shop. These may include: rounds of steel plate, sections of box beams, bars and angles.

A steel test object will be distributed to each student. Students and their test object will be located around work benches in the metal shop. Spools of iron and constantan thermocouple wire, and copper lead wire will be available on each work bench to permit each student to make two thermocouples as shown above. Junctions are to be formed by twisting the wires together then soldering the twisted length with a light coating of solder as shown below. Solder, and a soldering iron will be available (to be shared) at the workbench.

After the two thermocouples have been made up and soldered by each student, the student’s thermocouples are to be attached by screw mounting to two locations on their test specimen. Noting that one end of the specimen (pointed to on the diagram) is to be heated with the tourch in session 2 of this lab, thermocouple locations spaced at about 3 and 6 inches from this edge (the X’s) should be chosen as shown below. This spacing will permit observation of both the transient and steady-state heating of the specimen.

Each student should prepare two water-ice baths in a styroform cup using cups and ice to be provided. The reference junctions of their thermocouples are to be soaked in these baths when taking voltage readings.

Two voltmeters will be used for each student’s thermocouple test. The individual thermocouple’s plus and minus leads are to be connected as shown in Figure 13-9.

The lab coordinator will prepare the shop’s oxyacetylene tourch for use and will light it and adjust the flame. Either the lab coordinator, or the lab instructor will heat each student’s test specimen for up to 5 minutes as needed to warm both thermocouple locations sufficiently to produce a measureable Seebeck voltage in both thermocouples as a function of time. Be sure to insulate the test specimen in a shop vise with wood spacers for heating otherwise the vise will act as a heat sink distorting the results.

A total of four voltmeters will be available to permit heating one student’s specimen while the previously heated specimen of another student is cooling and measurements are being recorded. In this way the tourch and voltmeters will be shared. Each student’s measurement time will be about 10 minutes.

Conversion of the time-tagged Seebeck voltages to temperature is done according to Example 13-2 from the text which is shown below. The polynomial coefficients used are from Table 13-1 shown below.

Thermocouple polynomial coefficients for use in Eq. (13-5)

Type J, Temperature Range 0 – 760 C

Your report should document this experiment according to good practice as discussed in your lab manual. Draw diagrams of the experiment and include a digital picture if possible.

Plot your observations in Excel, and comment on them using what you have learned about one and two dimensional heat transfer and diffusion.

How did this lab experiment go? What problems occurred? What would you do differently next time? Etc.