Purpose:

 

Bimetal sensors are a very clever idea that apparently has it's roots in the design of accurate clocks in the 17th century. John Harrison is the person who seems to get credit for this development.The use of bimetal technology to compensate for temperature dependent effects is still an active area of study as evidenced by this recent paper. In this lab you will be exploring how the physical behavior of the bimetal material is related to the material properties of the metals used.

Procedure:

 

It is my goal to have a number of examples of bimetal sensors available for you to examine. You should spend some time identifying the bimetal component (it's not always obvious) and how it's characteristics are used to induce the desired behavior of the device. Pay particular attention to any household thermostats which may be present and undamaged.

 

Measure how much "longer" the coils seem to get in the coil versions and make an estimate of the temperature change that produces this movement. You will also need to estimate the length of the coil WITHOUT straightening it out of damaging it in any way. You will need to show that this is inconsistent with simple linear expansion for one of your lab deliverables. (this will depend on the availability of such coils)

Once you have established your conceptual understanding and appreciation of the technology you will design an experiment that will allow you to determine experimentally the difference in the coefficient's of linear expansion for the two materials (delta alpha- Da). You will use the provided bimetal strips to do this. Please don't put any more kinks or creases in them than they already have -- doesn't invalidate your experiment but it does make it more difficult. Consider your core conceptual understanding of bimetals -- they curve when you heat them and the amount of curvature is related to the difference between the two materials.

 

Conceptually the issue here is that there are a number of possibilities for the different materials. The amount of curvature will depend on the material characterstics, the temperature difference, and the uncertainty in your measurements (which have been intentionally rigged to make it difficult to be precise). In the past I have expected students to use something like Matlab or FreeMat to generate a plot that looks like the one linked here that reflects these characteristics.

 

 

In your measurements of the temperature change and the radius of curvature there will be some range of plausible values. This defines a region of space on this plot which is commonly known as the solution space. The curve for the actual materials used in the bimetal sensor should pass through your solution space. You may find that your data suggests multiple possible solutions or none at all. Think about what your results mean.

 

LAB DELIVERABLES:

I) From your observations of the coiled bimetal sensors show how you made a reasonable mathematical estimate of the length of the metal that forms the coil. Then show by calculation that the amount of movement of the end of the coil is inconsistent with the linear expansion of any commonly available material.

II) Present your plot of the family of curves with your solution space boxed on the plot. Discuss the meaning of your plot why you chose the range of values illustrated. Describe and support the range of measured values which define your solution space. How did you arrive at your upper and lower limits for the radius of curvature and the change in temperature?

III) What is(are) the difference(s) in the linear coefficients of expansion of the two materials in the bimetal strip that is (are) consistent with your data? How does this compare to the most likely actual materials based on your reading? What are the characteristics of the most likely materials that make the most likely?

Thermal Properties II Rubric: