Datalogging in Science

Calibration of a thermocouple

Overview
The chosen thermometer is a thermocouple and it is calibrated using a datalogging thermometer as standard. The ice point and steam points are recorded but many intervening points are recorded to confirm the linear nature of the thermometric property. Current or voltage may be taken as the thermometric property of a thermocouple. This document has described the procedure in terms of current.

Apparatus

List: Thermocouple, two beakers, crushed ice, heat source, Current Sensor (100 mA), temperature sensor, datalogger.


Theory
A thermocouple consists of two wires of different materials whose ends are joined. If one junction (called the reference junction) is maintained at a steady temperature and the temperature of the other junction (called the test junction) is allowed to change, then an emf is induced. The greater the temperature difference is, the greater the induced emf (and so the greater the current that will flow). This is known as the Seebeck effect.


Procedure
1. Set up the apparatus as shown.

2. Connect the current sensor and temperature sensor to the datalogger.

3. Connect the datalogger to the computer.

4. Open the Graph program and set it to record
current and temperature in real time.

5. Allow the junctions to come to thermal equilibrium with their environment before taking measurements.

6. Start with crushed ice in both beakers.

7. Begin recording and heat one of the beakers while maintaining the other at zero Celsius.

8. The datalogger will record current and corresponding temperature continuously.

9. Observe the graph of current versus temperature develop on the computer screen as the water heats.

Precautions
Make sure that the tip of the temperature sensor is at the same depth in the liquid as the test junction.

A long slow heating time gives the best results, if the heating is too quick the conduction patterns in the water will give local hotspots which can interfere with the results.


Results
A graph will develop on the computer screen that illustrates the
relationship between current and temperature for a thermocouple.
It serves to emphasise that current is a thermometric property.
It also serves as a calibration graph, which enables the thermocouple
to function as a thermometer.
Usually it is necessary to rescale the graph and also smooth it.
(Smoothing is needed because of the fluctuating temperature
due to convection currents in the water. The rescaling is the
consequence of the difficulty in predicting suitable values for the
“Sensor axis limits”).

Questions
1. Does the graph suggest a linear relationship between the induced current and the temperature?
. What does your answer to question one suggest about the range of measurement possible with this thermocouple.

3. What is meant by a calibration curve?

4. What are the types of situation where thermocouples are commonly used?
5. Why are thermocouples suitable for the situations you have indicated?

Extensions
Use the thermocouple (with current sensor attached) together with your calibration graph to measure temperature of various objects in the laboratory, starting with your own body temperature. To do this, proceed as follows

1. Detach the datalogger from the computer for mobility purposes.

2. Detach the temperature sensor from the datalogger (because calibration is complete)

3. Press the Meter button on the datalogger to operate it in measuring mode (rather than recording mode).

4. Keep the reference junction in crushed ice and place the test junction in contact with the object whose temperature you wish to measure. When thermal equilibrium has been reached note the value of current in the visual display of the datalogger.

5 (a) If using a printed calibration graph, seek the relevant value of current on it and identify the corresponding value of temperature.
or
5 (b) If using the graph on the computer screen one

Footnote:
Thermocouples are rugged, accurate and sensitive. They are usually small enough not to interfere with the object whose temperature is being measured. Output can be fed directly to computers, which can be remote from any hot object under investigation.

Data Harvest users

1. Connect the sensor to the datalogger and the logger to the computer
2. Then click the Set Up icon to the right of this message.
3. When the software opens, click the Play button.