BME 310 Lab 9 Temperature Measurement, John G. Webster
3/27/00
Introduction
The thermistor is the most used sensor for body temperature measurement. The thermistor
is based on the temperature dependence of a semiconductor's resistance, which is due to the
variation of the number of available charge carriers. Thermistors are divided into negative
temperature coefficient (NTC) and positive temperature coefficient (PTC) types, depending on
the purity and doping of the semiconductor. Most thermistors have a negative temperature
coefficient of about –4%/K. A typical circuit puts a constant current through the thermistor and
measures the variation in voltage resulting from temperature changes. An alternative puts the
thermistor in a Wheatstone bridge. The thermistor equation is Rt = R0 exp[(T0 – T)/TT0]
The thermocouple is based on the electromotive force (emf), which exists across the
junction of two dissimilar metals. This emf is dependent on the junction temperature. So there is
an emf difference or voltage between a cold junction and a hot junction at different temperatures.
The voltage is about 40 µV/K so requires high amplification to obtain a usable output. Circuits
are available to maintain a fixed cold junction temperature. Then temperature can be measured at
the hot junction. The equation is E = aT + (1/2)bT2 + . . .
Before the lab: Read material about temperature measurement from J. G. Webster (ed.),
Bioinstrumentation, Section 10.2 at the coursepage
http://www.engr.wisc.edu/cgi/courses/list/bme/310/webster/
More detailed information is in Fraden, J., Handbook of modern sensors: physics, designs
and applications. 2nd ed. Woodbury NY: American Institute of Physics, 1997.
More detailed information is in Fraden J., Noncontact temperature sensors in medicine, in
D. L. Wise, (ed.), Bioinstrumentation and biosensors, New York: Dekker, 1991, pp. 511–550.
Laboratory Equipment
1.
2.
3.
4.
5.
Digital multimeter
Thermocouples, thermistors, and amplifiers
Mercury glass thermometer
Rival hot pot
Infrared thermometer
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BME 310 Lab 9 Temperature Measurement, John G. Webster
3/27/00
Procedure
A. Thermistor
1. Construct the circuit shown below.
1k
5k
5k
Vo
+5V
1k
Thermistor
2. Adjust the power supply to 15 V for the op-amp and use the +5 V on the power supply for
the input to the voltage divider containing the thermistor.
3. Connect the output to a dc coupled oscilloscope and set SEC/DIV to some large value (e.g.
200 ms/div). You would like to be able to have the complete time response on one screen.
4. Place some water in the beaker provided and in the hot pot.
5. Put the thermistor in the beaker and observe the time response on the oscilloscope. Use the
STOP button to capture it and measure the time constant (when the change in voltage is 63%
of the way to the top).
6. Press RUN again and remove the thermistor. Use the STOP button to capture the time
response going from water to air. Measure the time constant.
7. Bundle the thermistor with the thermometer metal tip. Place beaker in hot pot. Heat up the
water to boiling in the Rival hot pot. Unplug the hot pot after the water boils.
8. While the water cools down, take measurements of temperature (from mercury thermometer)
and voltage from the thermistor circuit at 5 C intervals. Be sure to keep the thermistor’s
leads out of the water. Take measurements from 90 C to 30 C. You may find it helpful to
remove the beaker from the hot pot to facilitate cooling.
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BME 310 Lab 9 Temperature Measurement, John G. Webster
3/27/00
B. Thermocouple
1. Construct the circuit shown below.
1k
255 k
V+
V+
K
Vo
V-
GND
R-
2. Adjust the power supply to 15 V.
3. Repeat steps A3 – A8 for the thermocouple.
C. Infrared thermometer
1. When the single infrared thermometer is not in use by another group, measure the ear
tympanic temperature of each member of your group three times.
Results
1. Give the response times from steps A5 and A6. Why are they different?
2. Calculate the output of the thermistor circuit at room temperature. (Assume the thermistor is
1 k at room temperature).
3. Explain the purpose of the cold junction in the thermocouple circuit.
4. Plot your data points for the thermistor and thermocouple in xy-scattered form. Note any
distinct features. Add trend lines for each curve and show the equations.
5. In B5 and B6, give the response times for the thermocouple. Are they different? If so, why?
Compared with the results in A5, which has a faster response time, thermistor or
thermocouple?
6. The resistance of the thermistor changes exponentially with respect to temperature. Is there
any method to approximately linearize the resistance of thermistor in a certain range
(consider a parallel resistor).
7. For the infrared thermometer measurements, calculate the mean and standard deviation.
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