Temperature Measurement
Keywords – Thermistor, Thermocouple, RTD, Resistance thermometer, Pt100,
Wheatstone bridge design + balancing + non linearity, Instrumentation
Amplifier, Gain of Amp, Accuracy, Non linearity
1. Compare the RTD, the thermistor and the thermocouple as temperature
transducers. What are the output, range,
sensitivity, advantages and disadvantages of each one?
What is meant by the term Pt100?
How does resistance change with temperature? What other metals are suitable for this type
of thermometry?
2. The following empirical data were obtained for the static
characteristics of a platinum resistance thermometer:
Resistance (W) |
99 |
107 |
115 |
121 |
130 |
138 |
Temperature (°C) |
0 |
15 |
35 |
60 |
75 |
100 |
(i) Plot the resistance versus temperature
characteristic.
(ii) The characteristic equation for this type of sensor is given as
follows:
R(T) = Ro(1+aT)
Where R(T)
is the resistance at any temperature T,
Ro is the resistance at 0 °C,
a is the
temperature coefficient of resistance.
From the graph, estimate Ro and a.
3. Have a look at the two Wheatstone bridge designs shown below. What is meant by the term ‘a balanced
bridge’? At what temperatures are the
two bridges balanced?
If the supply voltage is 15 V, what is the output from each bridge at a
temperature of 75 ºC?
What are the voltages at points D and B relative to ground for each of
the bridge designs?
Which of the two options below for Wheatstone bridge design would you
recommend and why?
4. Which of the four op amp configurations below are suited for use with the output from the Wheatstone bridge and why?
· Inverting
· Non Inverting
· Differential
· Instrumentation
5. What are the key advantages/good features of the instrumentation amp?
6. You are required to design a Wheatstone bridge containing a Pt100 to
give 0 V at -20 ºC and 300 mV at +30 ºC. The supply is 10 VDC. The equation for the Pt100 is above.
To answer this, you must decide:
(i) a location for the
Pt100
(ii) match the resistor opposite the Pt100 to
balance the bridge at the required temperature
(iii) pick the other two resistors to give 300
mV at 30 ºC.
Off you go!