BME 353 – BIOMEDICAL MEASUREMENTS AND INSTRUMENTATION
EXPERIMENTS ON TEMPERATURE SENSORS
A: INVESTIGATION OF THERMISTOR CHARACTERISTICS
The purpose of this exercise is:
1.
2.
To familiarize with temperature sensors.
To study the resistance versus temperature characteristics of a thermistor and develop a
mathematical model for it.
Introduction
Temperature is used as an indicator of the physical and physiological well-being of patients and it is the
most commonly measured variable. In other fields of engineering, many engineering applications
require either direct or indirect measurement of the temperature. Medical applications generally covers
a narrow temperature range from 32°C to 42°C (sometimes from 29°C to 45°C) but require accuracy of ±
0.1°C. Also, medical thermometers tract the maximum temperature reached and hold it until the user
resets (initialize) them.
The thermistor is the most used sensor for body temperature measurement. It is based on the
temperature dependence of a semiconductor's resistance, which is due to the variation of the number
of available charge carriers. Most thermistors have a negative temperature coefficient of about –4%/K.
The thermistor has a non-linear resistance versus temperature characteristics. Several mathematical
models have been developed to represent the characteristic of the thermistor. The commonly used
equation in medical applications is 𝑅𝑇 = 𝑅0 𝑒
𝑇 −𝑇
𝛽( 0 )
𝑇∗𝑇0
A typical circuit puts a constant current through the thermistor and measures the variation in
voltage resulting from temperature changes. The thermistor has a slow response to step changes in
temperature due to its relatively long thermal time constant as verified in a previous experiment. This
doesn’t pose any problem in temperature measurements since the change in temperature takes place
very slowly.
Clinical thermometers work in a very narrow temperature range, but require a sensitivity of 0.1 °C.
They must be robust so that it can withstand the harsh use in medical environment, provide accurate
and reliable reading, easy to handle and use, electrically and hygienically safe to use.
Experimental Procedure
1.
2.
Measure the temperature of the coldest water and hottest water that you obtain in the lab
using a thermometer.
Obtain the resistance versus temperature characteristic of the bead type thermistor supplied to
you in the lab. Use cold water/ tap water/ hot water combinations to obtain a range of
measurement between 15C and 60C by taking a reading at approximately every 5C as you
3.
4.
5.
increase the temperature of the water. Wait long enough before you read the resistance so that
the tip of the thermistor reaches thermal stability.
Repeat step-2 for decreasing temperature.
Make a graph of the resistance versus temperature behavior you have obtained.
Determine the thermistor model. Calculate its  and sensitivity.
B: I.C. TEMPERATURE SENSOR-BASED THERMOMETRY
An innovation in thermometry is the integrated circuit temperature
sensors. These are available in both voltage and current-output
configurations. Both supply an output that is linearly proportional to
absolute temperature. Typical values are 1 µA/K and 10 mV/K.
Experimental Procedure
1.
2.
3.
4.
Construct the circuit shown. Adjust the power supply to for
the op-amp 15 V and apply +5 V to the sensor. Measure the
output voltage using a DVM.
Repeat steps 2 and 3 in Part-A for the I.C. sensor.
Make a graph of measured voltage against temperature.
Determine the model for the I.C. sensor.
Results and Discussions
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6.
Observe the plots carefully and comment on the linearity responses for both sensors.
Comment on the hysteresis in the responses.
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 or series resistor)?
Comment on the accuracy and precision of your measurements.
If you need to make a thermometer for the following application, which sensor you would
choose and why?
a. Measuring the body temperature form a patient;
b. Measuring the ambient temperature outdoor.
Make general comments on the experiment and suggest ways to improve it, if possible.