Technology 25 San Jose State University LabNotes

5.0 Principles of Thermocouple Operation

A critical parameter in this experiment is the specimen temperature. The temperature is sensed

by a bimetallic device called a thermocouple. When a temperature gradient is present along a

thermocouple, an EMF is developed which is proportional to the temperature difference. Tables

relating the voltage generated for specific thermocouple types, with the external junction fixed at

some reference temperature, are available. Before setting up the test samples, calibrate the

thermocouple measurement system with a reference temperature such as that of boiling H20.

Assure that the thermocouple is properly positioned with respect to the specimen prior to

beginning the experiment.

time

Tem

pera

ture

liquid cools

solidification takes place

solid coolssupercooling

Tm

Figure 1. Near equilibrium cooling curve for a pure metal or a eutectic alloy. Solidification takes place at a single temperature. Supercooling may occur if nucleation is difficult. Tm is the

solidification (or melting) temperature

Pb/Sn Phase Diagram LN 12-1

Technology 25 San Jose State University LabNotes

Tem

pera

ture

time

Tb

Tc

liquid cools

solid cools

solidification occurs

Figure 2. The cooling curve characteristic of a binary alloy where solidification takes place over a range of temperature: Tb is the temperature at which solidification begins and Tc is the temperature at which solidification is complete.

Pb/Sn Phase Diagram LN 12-2

Technology 25 San Jose State University LabNotes

Figure 3. Pb-Sn phase equilibrium diagram.

6.0 Experiment

6.

1. the cooling curve on the appropriate recorder until it indicates that solidification is

complete. Record the temperature (or temperatures) of phase change. Three samples will

be recorded in the cooling blocks at a time.

2. Repeat step 2-4 for all the remaining samples. The thermocouple may be moved

from one test tube to another without removing the glass sheath.

6.4.4 Experiment

Equipment and MaterialsPb/Sn alloy samples, mounted and prepared for metallographic examination.Optical microscopes

Procedures

Take two of the prepared Pb/Sn samples and position them in the optical microscope for viewing.

One of the samples should be chosen by your instructor. You are free to choose the other

sample. Observe the microstructure, and label and sketch it in the area on the data sheet.

7.0 Report

3. The Background section of your report should include brief explanations of the following concepts: phase diagrams and microstructural development, cooling curve measurements, and thermocouple measurements.

4. Procedure: Explain what you did in your own words. Include sketches. Be very careful not to plagiarize the lab notes!

5. The Results section should include a Table containing the phase change temperatures for all the alloys; a Pb/Sn PED with the data from the table superimposed; and the sketches you made of the microstructures.

6. In your Discussion section, suggest possible sources of error and non-reproducibility in the experiment and provide theoretical or practical reasons for the occurrence of such errors. Does your phase diagram agree or disagree with the published data?

7. Summary/ conclusions- Summarize what you did and your results. Comment on the importance/ relevance of the experiment.

8. References

See the grading criteria on the next page.

Pb/Sn Phase Diagram LN 12-3

Technology 25 San Jose State University LabNotes

Grading Guidelines for Laboratory Report

Phase Diagrams

Student Name

Score (6)

Weak

(7) (8) (9) (10)

Effective

Writing Style & Structure Total: /50

Sentence structure

Spelling &Neatness

Paragraph structure, logical flow

Clarity of writing

(Gives ideas directly,

does not complicate ideas)

Voice (Creativity, Originality)

(uses original voice, text not copied from outside source)

Technical Content & Structure Total: /120

Abstract

Introductory (Background) Section [Overall]

Phases diagrams & cooling curves

Thermocouple operation

Results [Overall]:

Cooling curve

Table of phase changes

Phase diagram plot

Discussion [Overall]

Makes some attempt to determine whether results are correct or sensible, links results to theory presented earlier

Conclusion

References

Total Report Score = Sum of above/170

Pb/Sn Phase Diagram LN 12-4

Technology 25 San Jose State University LabNotes

Lab Worksheet: Data Sheet for Phase Diagram Experiment

Date Lab Section

Group Leader

Materials Manager

Safety Expert

Recorder

Other Group Members

Observed Phase Change Temperatures

Alloy

Composition

Chart Recorder

No.

Solidus (oC) Liquidus (oC) SupercoolingO

bserved?

Pb/Sn Phase Diagram LN 12-5

Technology 25 San Jose State University LabNotes

Lab Worksheet: Data Sheet for Pb/Sn Microstructure

Date Lab Section

Group Leader

Materials Manager

Safety Expert

Recorder

Other Group Members

Label and notes:

____________________________

Label and notes:

____________________________

____________________________

____________________________

____________________________

____________________________

Pb/Sn Phase Diagram LN 12-6