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!