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Temperature of water using the change of resistance of a metal

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Introduction

Temperature of water using the change of resistance of a metal

Brief

Undertake an individual instrumentation project, the project may involve one of:

  • Building and testing a sensor
  • Exploring the characteristics of a sensor
  • Designing and assembling a measurement system, and using the sensor to make a measurement

I have chosen to design and assemble a measurement system then use the sensor to make a measurement.

The task is to use a sensing device to put together a system to make a measurement.  The report of results should evaluate the measurement, giving evidence of how reliable it is likely to be.

Introduction

I have decided to make a sensor to measure the temperature of water by measuring the change in resistance of a metal.  I will look at how the resistance changes in a wire passed through a beaker of water at different temperatures.  I can measure the resistance in two ways, one using the built in resistance calculator on the multimeter, or I can run a current through the wire and measure the potential difference across the wire and the current running through it to calculate the resistance using the formula:

                          Resistance (Ω) = Potential Difference (V)

                                                                   Current (I)

I would then calculate the resistivity at the temperature it was at using the formula:

R=ρL           Resistance = resistivity x length/ area

      A

I could then calculate the temperature using the formula: ρT2 =ρT1 (1+βt1 t2 (T2-T1))

Where:

T1 and T2 are the initial and final temperatures in °C

ρT1

...read more.

Middle

Basic Theory

The theory of resistance can be very complex to calculate on a molecular scale and it is mainly done by observation.  However I can explain in simple terms the theory of how temperature changes the resistance of a material.  

Electrons colliding with impurities within the vibrating lattice arrangement of metals cause resistance. The energy lost is given out in the form of heat.  See diagram bellow.

image00.jpg

The amount of collisions is dependant on the amount the atoms in the lattice are vibrating.  The faster the atoms vibrate the higher the probability of a collision.  The temperature of the metal is what affects the vibrating atoms.  More heat energy means the atoms vibrate more thus creating higher resistance.  

Prediction

Due to the theory above I predict that the resistance will increase as the temperature of the water rises.  From the formulas mentioned earlier and the information from the table I obtained from the Internet, I can predict more accurately what my results may look like.

Temperature

Resistivity of Copper at 20

Temperature coefficient

Resistivity at Temperature

Length of Wire

Area

Expected resistance

(Degrees C)

Degrees C  (ohm m)

β

(Ohm m)

(m)

(Cm2)

(Ohms)

0

1.68E-08

0.0042819

1.536E-08

5

3.463E-0

2.2175

10

1.68E-08

0.0041061

1.611E-08

5

3.463E-0

2.3256

20

1.68E-08

0.0039441

1.680E-08

5

3.463E-0

2.4252

30

1.68E-08

0.0037944

1.743E-08

5

3.463E-0

2.5172

40

1.68E-08

0.0036557

1.802E-08

5

3.461E-0

2.6025

50

1.68E-08

0.0035268

1.857E-08

5

3.463E-0

2.6818

60

1.68E-08

0.0034066

1.908E-08

5

3.463E-0

2.7556

70

1.68E-08

0.0032944

1.956E-08

5

3.463E-0

2.8247

80

1.68E-08

0.0031893

2.001E-08

5

3.463E-0

2.8893

90

1.68E-08

0.0030908

2.043E-08

5

3.463E-0

2.9499

100

1.68E-08

0.0029981

2.082E-08

5

3.463E-0

3.0069

image01.png

Plan

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Conclusion

When I took my reading at 100°C I had to keep the Bunsen burner underneath to keep it at this temperature.  This could have heated up the wire more than the water did, thus causing a greater resistance as shown in the graph.

Evaluation

My experiment was successful as I managed to construct and use a system to take measurements.  The results were reasonably accurate and allowed me to confirm my prediction.  I was pleased that my results were strongly correlated.  

There are several things I could have done to improve my experiment:

  • Use of more accurate multimeters would have increased the accuracy
  • I could have used data logging equipment with a thermometer connected to a computer to monitor my results more carefully and accurately
  • I could have done the entire experiment several times and taken different readings each time, instead of taking all three readings immediately after one another
  • I could have used different lengths and thickness of wires to further prove my hypothesis
  • I could have tried a greater range of temperatures of water by raising it’s boiling point by adding a salt or similar, I could also have lowered the temperature using ice and a salt
  • I could have also used a lower current or put in a resistor before the wire to reduce the heating effect
  • I could have used solder connections and thicker wires to reduce any error in faulty connections

If I were to repeat the experiment I would consider the above and in general take more readings to remove any anonymous results.

...read more.

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