Laboratory Exercise: Temperature Measurement
Introduction
In order to measure parameters of interest we use different types of sensors
including electrical, chemical, optical and acoustic sensors. Electrical sensors
include voltage generators, current generators, resistance based, capacitance
based and inductance based sensors. Depending upon type of the application,
one should be able to select an appropriate sensor. With respect to using any
type of sensor technology, the most important aspect is calibration of sensor
output with real time measurements and careful understanding is required about
the sensor response in order to obtain a prediction model. The models include
linear and non-linear equations relating the sensor output and the measurement.
In this lab, we will use two types of sensors for temperature measurement:
thermocouples and thermistors. Thermocouples are formed by joining two
dissimilar metals and various types of standard materials are available. Two
metals joined together at a junction generate a very small voltage (millivolts)
when exposed to different temperatures between two junctions. The voltage
generated is a function of temperature. Voltage goes up as temperature goes up.
The voltage is nonlinear with respect to temperature however over small regions
the voltage is approximately linear.
Thermistors (thermally sensitive resistors) are electrical resistors whose
resistance changes with temperature. Thermistors are manufactured from metal
oxide semiconductors which are encapsulated in a glass or epoxy bead. They
have a very high sensitivity, making them extremely responsive to changes in
temperature. Thermistors tend to be low mass which improves response time
and are generally only good over a reduced range of temperature versus other
temperature sensors. The three term Steinhart-Hart equation is the most popular
model used for characterizing thermistors: 1/T = C1+C2*ln(R)+C3*ln(R)3.
Thermistors are resistive devices whose resistance changes proportionally with
temperature. If we know resistance, we can calculate the temperature. We can
estimate temperature from them in two modes: using a voltage divider or using a
constant current source. Current sources are preferred for higher accuracy
measurements as very small currents can be used to minimize self heating.
Objectives
ƒ Understand how to measure temperature with thermocouples and
thermistors.
ƒ Understand basic use of a Fluke Multimeter.
ƒ Understand how to transfer data into MATLAB and create visual outputs.
ƒ Successfully (1) create a 3-point calibration curve (2) find time constants.
ƒ Learn how to create a proper lab write-up.
Required for this lab
ƒ Students must have both MATLAB R2009a and FlukeView Forms already
installed on their computers. See Lauren for assistance ahead of lab time.
ƒ Fluke 289 multimeter, type K thermocouple, thermometer
ƒ Breadboard, 2 resistors (100 ohm, 10k ohm), 9V battery, connector wire,
thermistor
Procedure
ƒ Get into groups of two for this lab.
Part 1 – Thermocouple introduction, 3 point calibration curve
1. Turn the Fluke 289 multimeter on. Turn the dial onto the mV reading with the
temperature symbol next to it. Plug in the K-type thermocouple.
2. Record the temperature of the ice water with the given thermometer. Place
the thermocouple into the ice water and record the mV reading. Repeat this for
the room temperature water and the hot water. This data will be used in post-lab
calculations.
3. (**Can be done outside of class) Using the NIST Polynomial Coefficients
tables, calculate the conversion to temperature for each of the mV readings. A
reference temperature of 22.8°C can be used in the calculations. These
calculations will be needed in your appendix.
4. Create a scatter plot of Voltage (mV) vs. Temperature (°C) in MATLAB (create
two simple arrays and simply use the plot command). On the same graph, add a
scatter plot of Voltage (mV) vs. Thermometer readings (°C). The ‘hold on’
command will allow both plots to appear on the same graph. Label the graph
appropriately.
5. Find the percent error between the thermocouple readings and thermometer
readings. Simply create two arrays in MATLAB and apply the appropriate
equation. Create a table that includes thermocouple readings, thermometer
readings, and percent errors. All coding performed in MATLAB must be included
in the Appendix.
Part 2 – Thermocouple Data Acquisition
1. Set the Fluke dial to the mV reading. Press F2 for the ‘Save’ Function. Scroll
to ‘Record’, and select it by pressing F1.
2. Set the duration to one minute and the sample interval to one second.
3. Place thermocouple in the given hot water bath. Start recording by pressing
F2.
4. Over the one minute of readings, change the temperature of the sensor by
placing ice in the water, heating the end with your fingers, etc.
5. The Fluke will stop making readings after the duration. Press F2 twice to save
the data.
6. Using the given USB connector, connect the Fluke to FlukeVIEW on your
personal computer. Press OK in FlukeView to “Get Data Now”. Pressing the
“Export Data” button on the right will save the data file. Save the file as
“TemperatureChanges.csv” in MyDocuments\MATLAB.
7. In the MATLAB Command Window, run:
>> [Temperature Time info] = readFlukeFile(‘TemperatureChange.csv’)
This puts the two variables measured by the Fluke into your workspace.
8. Create a scatter plot with Time on the x-axis and Temperature on the y-axis.
Label the graph appropriately. This will be part of your results section.
Part 3 – Thermistor and Thermocouple Time Constants
- When the surrounds of a thermocouple change in temperature, the
thermocouple reading will show a response to this change. The speed of this
response can be quantified in terms of a time constant.
1. Create an ice/water mixture as close to 0 degrees Celsius as possible. Leave
thermometer in the water.
2. Use the Fluke with its current setup to the type-K thermocouple. Press F1 and
change the visual output to Celsius. Begin recording data through the same
procedure in Part 2, except collect data over 2 minutes at 1 second intervals.
When the Fluke is showing a consistent room temperature, place the
thermocouple in the ice bath.
3. Connect the Fluke to FlukeVIEW on your personal computer again. Export
the data from the Fluke as you did in Part 2, with the file saved as
‘TypeKTimeConstant.csv’.
4. In the MATLAB command window, run:
>> [Temperature Time info] = readFlukeFile(‘TypeKTimeConstant.csv’)
The variables uploaded into your workspace will be used to find the
thermocouple’s time constant.
5. Calculating the time constant: Two time/temperature data points are needed
for the time constant calculation. The first point should be right when the
temperature begins to drop from room temperature. The second point should be
right before the temperature begins to level off at freezing point. Calculate your
Type-K thermocouple’s time constant using equation (1).
‫ * ح‬dTTC / dt + TTC = Tenv
↓
TTC(t2) – Tenv
TTC(t1) – Tenv
Where:
t1
t2
Tenv
=
e ^ (-t1 / ‫)ح‬
= initial time point (sec)
= final time point (sec)
= temperature of surrounding environment (°C)
(1)
TTC
‫ح‬
= temperature at time (t) (°C)
= time constant
6. Insert the resistors (R1 and R2) into the breadboard to form connections
between R1 and R2.
- R1: place one end in a negative hole and place the other end in the
same column, row H.
- R2: place one end the same column as R1, in row I. Connect the other
end into any other hole in the same row. This will create an ‘L’ shape.
- The thermistor given to you is already soldered to green connector wire.
Place one end in row I, column 1. Place the other end in the first positive
terminal (near 1A).
- Place the red wire of the 9 volt battery into the positive terminal next to
the thermistor. Place the black wire into the negative terminal next to R1.
This completes the circuit.
7. Make sure the ice bath is as close to 0°C as possible. Repeat Part 3, but
using the thermistor instead of the thermocouple. Use the Fluke to measure the
voltage across the thermistor.
Results Section Should Include:
1. Table of Part 1 raw data and calculated percent error.
2. All resulting graphs from MATLAB.
3. Time constants from Part 3.
Discussion Questions:
1. Discuss the principles of operation of thermocouples and thermistors.
2. Discuss the results of the time constant calculations.
3. What errors may be introduced by measuring temperature with a
thermocouple?
4. If you were to monitor poultry being cooked in a factory (temperature must get
to 165 degrees Fahrenheit), which device would you use to monitor temperature,
and how would you use the thermocouple or thermistor?
Appendix Should Include:
1. Voltage-to-Temperature conversions from Part 1.
2. Time constant calculations.
3. NIST polynomial conversions