ME3122 – ME3122 – 1 Temperature Measurements
Informal Lab Report
Done by:
Tang Sin Hong (A0072446E)
Group:
3D2
Date of Experiment:
3/9/2012
Experiment Set No:
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OBJECTIVES 1. Calibration of different types of temperature sensors. 2. Measurement of temperature distribution along a rod 3. Measurement of surface temperature.
CALCULATIONS AND DISCUSSION Tables and Graphs are attached behind this report.
a) Sensitivity of the sensors From Graph 1, Thermocouple with ice-point, S = dv/dt Gradient
= (3.52 – 0) / (85 – 1.5) = 0.04216 mV/ ⁰C
Thermocouple without ice-point, S = dv/dt Gradient
= (2.54 – 0) / (83.5 – 22) = 0.04130 mV/ ⁰C
From Graph 2, RTD, S = dv/dt Gradient
= (79.57 – 0) / (79.6 – 0) = 0.9996 mV/ ⁰C
From Graph 3, Thermistor, S = dv/dt Gradient
= (805 – 0) / (85 – 22.5) = 12.88 mV/ ⁰C
From the sensitivity values obtained, we can see that thermistor has the highest sensitivity among all 4 temperature sensors and thermocouple has the lowest sensitivity value. This means that for thermistor sensor, there will be a bigger jump in voltage for every degree Celsius increment. Having a larger sensitivity would enable one to detect small changes in temperature more accurately.
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Temperature Coefficient of Resistance For Thermistor, β
= -S(Rt + R3)2 / EoRtR3 = -(12.88 x 10 -3)(30000 + 7599) 2 / (1.5 x 30000 x 7599) = -53.25 x 10 -3 K-1
For RTD, β
= S / (i x Ro) = (0.9996 x 10 -3) / (2.1 x 10 -3 x 100) = 4.76 x 10 -3 K-1
From the magnitude, we can see that the thermistor has a larger temperature coefficient of resistance than RTD. This implies that the thermistor experiences a bigger change in resistance per unit change in temperature compared to RTD. This reconfirms the previous conclusion that thermistor is more sensitive than RTD.
b) The temperature profile is expected to be linear along the axial direction of the rod. Since the rod is insulated circumferentially, according to the heat diffusion equation we can assume that the heat transfer is allowed only in the axial direction. Furthermore, there is no heat generation within the rod, and the temperature is held constant with respect to time, hence we can expect a linear temperature profile.
However the temperature profile of the Perspex rod recorded during the experiment, display a non-linear relationship. This could be due to the material property of Perspex which does not conduct heat as readily as other materials. It also shows the material’s resistance to heat conductivity thus resulting in the hyperbolic temperature profile graph obtained experimentally across the length of the rod instead of an ideally linear relationship.
In a steady state situation, the temperature distribution tends towards linearity. This can be seen from the 15 min graph, compared to the one at 0 min. However the time taken for this experiment is still not long enough to achieve full steady state, where temperature across the Perspex rod does not vary with time. In addition, there are also unaccounted heat losses due to radiation, thus explaining the non-linear relationship. 3
c) Relative % error of temperature deviation between surface temperature measurement At 0 minute, Temperature of the surface of the exposed end (embedded thermocouple) = 24.5 ⁰C
Surface Thermocouple wire
= [(25 – 24.5) / 24.5] x 100% = 2.04%
Surface RTD
= [(25 – 24.5) / 24.5] x 100% = 2.04%
Surface Thermistor
= [(25 – 24.5) / 24.5] x 100% = 2.04%
At 15 minute, Temperature of the surface of the exposed end (embedded thermocouple) = 25.5 ⁰C Surface Thermocouple wire
= [(26.5 – 25.5) / 25.5] x 100% = 3.92%
Surface RTD
= [(27 – 25.5) / 25.5] x 100% = 5.88%
Surface Thermistor
= [(26 – 25.5) / 25.5] x 100% = 1.96%
From the relative % error, we can see that the surface thermistor has the least error, which supports our conclusion that the higher the sensitivity the sensor has the more accuracy it has.
Possible sources of errors
Parallax errors are involved when reading off values from the liquid-in-bulb thermometer.
Improper insulation, which cause heat loss to the surrounding in all directions.
The readings were not recorded at the 0 min and 15 min instant, this lag may cause inaccuracy in the readings obtained.
There are fluctuations in the readings as it is not a steady state process.
Internal resistance within the wires may fluctuate with temperature.
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Ways to Improve this Experiment
A digital thermometer could be used to eliminate the parallax error.
A data logger could be used so as to obtain all the reading of all the sensors at the same point in time.
CONCLUSION Sensor
Advantages
Thermocouple
Measure the temperature differential between two junctions of dissimilar metals, provides a relative measure. Able to use at very high temperatures. Good response even though at high temperature response.
Resistance Thermometer (RTD)
Thermistor
Good indication of the absolute temperature. The relationship between temperature and resistance is linear. High contamination resistance. Low drift Relatively high accuracy, due to high sensitivity
Disadvantages Less sensitive as the range of measurement can be very wide. For direct temperature indication, one junction must be accurately maintained at or compensated to a known reference temperature. Accuracy at extremely high and low temperatures is usually limited. Hard to calibrate Measures only temperature differences. Expensive. Has poor temperature response due to large mass of sensors. Expensive. At high temperatures the accuracy decreases.
The importance of calibration of different temperature sensors allows us to be able to read temperatures accurately at locations where we are unable to use a thermometer to measure the. Different sensors have different sensitivity, temperature coefficient, response time, and accuracy. As shown from the experiment, the thermistor has a higher sensitivity than the resistance thermometer due to the higher magnitude in temperature coefficient of resistance. However, for the accuracies of each sensor thermometers, it is seen from the deviation calculations that the thermistor has the highest accuracy among the sensors used. We have studied and understood the characteristics of each sensor and understood the measurement of temperature and surface temperature of a rod.
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