THEORY
Answer each of the following question with one or two sentences (phrases): (Yes/NO
answer require justification)
a) Mention what three laws govern the thermocouples.
b) Why the reference junction is important in the measurement with thermocouples?
c) What is the reason of using a Wheatstone bridge in the calibration of a thermistor?
d) What is the main difference between an RTD and a thermocouple when they are
measuring temperature?
e) Explain the difference between Seebeck, Peltier and Thomson effect.
f) The measurement system of an oven uses a thermocouple type K (max 1372oC) as
a sensor which had a irreparable damage. Here is only a thermistor in the
warehouse. Is it possible to replace the thermocouple with the last one? Justify
your answer.
g) What is the physical meaning of RC for a capacitive circuit? Explain.
Problem 1
Determine the value of resistance for the different coded colors:
Problem 2
For the Wheatstone bridge circuit shown in the Figure 1 calculate the following:
a) The Rx value.
b) Determine the current I2.
Resistor
R1
R2
R3
Ei
Value
1500
3500
1500
70V
R3
R2
R1
Eo
Rm
Rx
I2
Voltmeter
E1
Figure 1.
Problem 3
The Type J thermocouple circuit shown in Figure 2 is used to measure the temperature
T1. The thermocouple junction labeled 2 is at a temperature of 25 oC, maintained by an
ice-point bath. The voltage output is measured with a potentiometer and found to be
8.132mV. What is T1?
Iron
T1
Potentiometer
emf=8.132mV
Constantan
Ice bath
Iron
2
Reference
Joint 25oC
Figure 2.
Problem 4
An RTD forms one arm of an equal-arm Wheatstone bridge, as shown in Figure 3. The
fixed resistances, R2 and R3 are equal to 25. The RTD has resistance of 25 at a
temperature of 0oC and is used to measure a temperature that is steady in time. The
resistance of the RTD over a small temperature range may be expressed as:
RRTD  R0 1   T  T0  
Suppose the coefficient of resistance for this RTD is 0.003925oC-1. A temperature
measurement is made by placing the RTD in the measuring environment and balancing
the bridge by adjusting R1. The values of R1 required to balance the bridge is 37.36 .
a) Determine the temperature of the RTD
b) If the required uncertainly in the measured temperature is  0.5oC, would a 1%
total uncertainly in each resistor that make up the bridge be acceptable? Neglect
the effects of lead wire resistances.
R3
R2
G
R1
RTD
E1
Figure 3.
Problem 5
The material constant  is to be determined for a particular thermistor by using the circuit
shown in Figure 4. The thermistor has a resistance of 60k at 25oC. The reference
resistor in the circuit, R1, has a resistance of 130.5 k. The dissipation constant, , is
0.09mW/ oC. The voltage source used for the measurement is constant at 1.546V. The
thermistor is to be used at temperatures ranging from 100oC to 150oC. Determine the
value of .
Temperature [oC]
100
125
150
E1 Voltage [V]
1.498
1.521
1.543
Ei
R1
RT
T
Figure 4
E1
Problem 6
An RC circuit is used to build a temperature measurement system by means of the time it
takes a 100μF capacitor to charge up to 2.5 V. Assume the output of the system is
measured at 35°C and eo=5 V, R1=800 Ω, C=100μF.
t=0
R1
RRTD
eo
C
A RTD is used to sense the experimental procedures. The measured resistance of the
RTD for 0°C and 100°C were 4000 Ω and 16000 Ω, respectively.
1. Determine the RTD’s calibration equation.
2. What is the value of the RTD resistance?
3. Determine the time constant τ.
4. How much time did it take the resistor to charge to 2.5 V.