Lab: Thermometric Properties

Introduction:

Temperature is the measure of the hotness or coldness of an object or substance. This can be accomplished (though not always recommended) to a degree by touching the object. However, as one can notice on a cold day, the feel of two objects outside in cold weather can be quite different when they should be the same temperature. For example, touching a plastic tool handle on a cold day can be done with bare hands without too much discomfort, however a metal bar setting next to the tool handle will be quite uncomfortable. If one wishes to communicate the coldness of the objects, it is difficult to do unless the exact same situation is available to the other person to touch.

In many situations it is useful to have a way of communicating the hotness or coldness of an object. For this reason we look for materials that change an observable property as heat is added to or removed from the material. This property is called a thermometric property. A record is then made of this property's characteristics, as heat is added or removed. This record then becomes a standard against which the hotness or coldness of other objects can be compared. A variety of substances have these thermometric properties. We will be looking at one type of thermometric property, changing electrical resistance.

We will create this record (scale), calibrate it with standard records (scales), and use our newly constructed thermometer to measure temperature. Heat added to a material causes such changes as melting, boiling, expansion, etc. When these changes are compared with a scale (any scale), we use the word "temperature." This is just a convenient way of communicating the hotness or coldness of an object. Heat and Temperature are very different things.

Digital Temperature Probe

Construction

A thermistor is a resistor that changes resistance with temperature. When connected to a resistor as shown below and the Voltage, Vout measured with a meter, we have a simple circuit for an electronic thermometer, sometimes called a temperature probe. If a digital multimeter is used to measure the voltage as shown, then you have the makings for a digital thermometer. As with all thermometer scales, you will need to calibrate this thermometer by using a standard thermometer and creating some type of reference chart.

This type of temperature probe is extremely useful in situations where the probe needs to be physically separated from the scale (meter) that is read, remote sensing. For example a temperature probe might be located outside an airplane in flight, inside a piece of electronic equipment, or perhaps at the bottom of a river. In each case it would not be practical to take a thermometer to that location and read it.

The diagrams below show how the temperature probe was constructed. You may not know how to read a circuit diagram, but you will in a few weeks so don’t lose this paper. The voltmeter will have to be connected to the terminals on the circuit board. If you have never done this before, check with your instructor before turning anything on.

Qualitative Observation of a Thermistor

Note the physical appearance of a thermistor before and after warming it with your hand. You should have difficulty observing a change. A thermistor is a device that changes its electrical property, resistance when heat is added or removed. To observe this, connect each end of the thermistor to one terminal of a multimeter. (The multimeter should be set to measure resistance.) How does the resistance change as the thermistor is warmed with your hand?

"How does the object change as heat is added?"

or

"How does the object change as heat is removed?"

Your answer to these questions is a description of the relationship between the amount of heat added or removed and the thermometric property of the object. At this time your description is qualitative. To better describe the changes we will need to answer the question how much does it change. This is a quantitative description. With a quantitative description, we can compare how "hot" one item is to another, we have a thermometer.

## Experimental Design

It is often the case that qualitative observations precede the decision to do an experiment. One might notice some effect and as a result, ask the formal question (purpose) about the relationship between the two variables that were noticed to have had some kind of on each other. You have just made such an observation. Stop now and write the purpose for an experiment. When your group agrees on a purpose, you may check it with the instructor.

More Observations

To collect quantitative information about how the temperature probe characteristic referred to above changes when heat is added or subtracted, we will need to place it in contact with an object and take a reading. Water baths at various temperatures are easy to set up. However, the thermistor will need to be isolated with an epoxy coating or some other watertight material that will conduct heat efficiently.

Measuring the amount of heat added will not be a thing that we can directly measure. Instead, we will have to measure the temperature of the water. A standard thermometer, either Celsius or Fahrenheit can then be used to find the temperature in standard units. For this lab, we will use Celsius.

Immerse both the thermometer and the digital probe in the water-bath, being careful that the temperature probe is no deeper that the watertight coating will allow. Neither the probe nor the standard thermometer should touch the container. Let them set in the water long enough for the readings to stabilize.

The information about how the voltage readings on the probe change when the temperature of the object changes is sometimes called a calibration. To calibrate your thermistor you must measure the Voltage (Vout from your circuit) versus Temperature (measured with an accurate standard thermometer). Record these values. Change the temperature of the object and repeat several times. Make sure that you choose temperatures with as wide a range as possible.

Some things to consider in the analysis:

Your V vs T information is really a calibration report. Though we generally do graphs and write equations, this is not the only way to convey the calibration information. All of the following are different formats for expressing the calibration information. Make sure that you have included each format in your report.

Verbal (Written) Calibration Report: Write a short statement explaining how the Voltage readings changed as the Thermometer readings changed.

Calibration Table: Organize your readings into a neat table, which can be used to compare V and T readings.

Calibration Graph: Plot your Voltage vs. Temperature values on a graph. V should be on the vertical axis and T on the horizontal axis. Draw a smooth line (curve) through your points.

Equation: Choose linear sections of your line (2 or 3 sections). Write an equation that represents the relationship shown by each straight-line section of your graph. When you state the equation be sure to give the Temperature range within which each equation is valid. Be careful to read the correct “b” values from your graph for each segment. You will have to extend each one with a light mark to see where it crosses the vertical axis.

Report:

This lab was not a standard experiment like those you have seen in physics so far. In many ways, it is more like what an engineer might be asked to do. Even so, the report should contain all the basic elements usually found in a lab report. Sometimes these elements are modified to fit this situation and given a new title.

Read the document found at: http://evan.lemley.org/courses/engr_report_format_spring_2005.pdf

You will provide a report that might be useful to someone wanting to use your set-up as a thermometer. Include as many different representational forms as might be useful to someone using the device as a thermometer.

For this lab, you will write a short engineering report that includes the following sections: