ME3122-1
TEMPERATURE MEASUREMENTS
2012/2013 Formal Lab Report Name: Faizan Abid Matric Number: U096070W Group 3B2 (Makeup session)
Department of Mechanical Engineering National University of Singapore
Aims & Objectives The aim of this experiment is to allow students to appreciate the calibration of different types of temperature sensors. After calibrating the temperature sensors, thetemperature sensors are used to observe and measure the temperature distributionalong a rod and surface temperature. Through these means, students are then able to better appreciate how to measure temperature in heat transfer analysis°
Experimental Results Table 1 Calibration Data Temp Vout(RTD) Vout(Thermistor) Thermocouple Thermocouple w/o ice-pt (C)
Ch 8 (mV)
22.50 41.00 50.50 59.50 70.00 79.50
22.81 41.23 50.51 59.88 70.17 79.47
Ch 9 (mV) 0.04 219.71 348.00 479.60 617.40 730.50
Ch 10 (mV) -0.04 0.71 1.11 1.50 1.95 2.36
with ice-pt Ch 11 (mV) 0.75 1.51 1.90 2.30 2.77 3.17
Calibration Curves (The hand-drawn charts are attached in the end)
Table 2 Transient Readings for Temperature Along Perspex Rod: Clock Time
0 min mV
C
15 min mV
C
30 min mV
2.72
69.00
2.87
72.50
2.90
74.00
1.96
51.50
2.29
59.50
2.40
62.00
1.41
38.50
1.77
47.00
1.92
51.00
1.12
32.00
1.43
39.00
1.58
43.00
1.00
29.00
1.23
34.50
1.35
37.00
0.97
28.50
1.12
31.50
1.21
34.00
0.96
28.00
1.09
31.50
1.16
32.50
28.59
27.50
31.33
31.00
33.68
33.50
o
o
o
C
Channel 1 at 0 mm apart from the hot end
Channel 2 at 10 mm apart from the hot end
Channel 3 at 20 mm apart from the hot end
Channel 4 at 30 mm apart from the hot end
Channel 5 at 40 mm apart from the hot end
Channel 6 at 50 mm apart from the hot end (Embedded thermocouple wire)
Channel 7 for surface thermocouple wire
Channel 8 for surface RTD
Channel 9 for surface thermistor 47.10
26.50
68.84
28.00
96.11
30.50
Calculations and Discussion Sensitivity From the graphs above, the sensitivity of different measuring systems can be determined. Note that sensitivity is given by, S
dV dT
That is, the gradient of the calibration curve obtained. The values are given in Table 3 Table 3 Sensitivity of different temperature measuring systems RTD
Thermistor
Thermocouple w/o
Thermocouple with
(mV/°C)
(mV/°C)
Ice Point
Ice Point
(mV/°C)
(mV/°C)
0.0422
0.0426
0.9959
13.019
The results show that the Thermistors are the most sensitive with a sensitivity of 13 mV/°C, while the thermocouples are the least sensitive with a sensitivity of 0.042 mV/°C. In the experiment, the heater tries to maintain the temperature of the water at 80 °C, but small fluctuations are still expected. Therefore, thermistors are more likely to give a more accurate response of changing temperature since they are the most sensitive.
Temperature Coefficients Temp. coefficient for thermistor,
1 dRt Rt dT
( Rt R3 ) 2 V E o Rt R3 T (
)
(
)
= (
= - 0.05382 /°C
)
( Rt R3 ) 2 S E o Rt R3
Temp. coefficient for resistance thermometer,
1 dRo Ro dT
1 dV iRo dT
S iRo
=
/°C
=
Comparing the above results for the temperature coefficients, one can see that the temperature coefficient of the thermistor is almost 11 times higher than the resistance thermometer (in magnitude). The reason why thermistors have a negative temperature coefficient is that the resistance of the thermometer decreases as the temperature increases.
Temperature Profile and Distribution Based on the calibration curves obtained above, the temperature profile is them calculated based on the voltage output given in Table 2. The profile is plotted below:
Temperature Profile 80.00 70.00
Temperature ( °C)
60.00 50.00 40.00
0 min 15 min
30.00
30 min 20.00 10.00 0.00 0
10
20
30
Distance (mm)
40
50
60
From the graph above, it can be seen that the temperature in the Perspex rod decreases along the direction away from the heat source. Also, as the duration increases, so does the temperature at all the test points, which is why the graphs increase vertically for different time durations.
Did you obtain a linear temperature profiles? If not, can you explain why? Would you expect a linear temperature profile? Under steady state, one dimensional conditions with no energy generation the temperature profile will be linear in homogeneous media based on the heat transfer equation:
(
)
Therefore, one would expect a linear profile. However, in this experiment we did not obtain a linear temperature profile. This could be due to the following reasons: a) The heat transfer in Perspex rod had not yet reached steady-state b) The heat transfer in Perspex rod is not an ideal one-dimensional c) The Perspex rod might not be homogeneous, k is not constant.
Relative percentage error Clock Time o
C
Channel 6 at 50 mm apart from the hot end (Embedded thermocouple wire)
0 min % Error
15 min C % Error
o
30 min C % Error
o
28.50
0.00
31.50
0.00
34.00
0.00
28.00
-1.75
31.50
0.00
32.50
-4.41
27.50
-3.51
31.00
-1.59
33.50
-1.47
26.50
-7.02
28.00
-11.11 30.50
-10.29
Channel 7 for surface thermocouple wire
Channel 8 for surface RTD Channel 9 for surface thermistor
The findings from above result:
a) The relative percentage error is quite small for all cases, which means it is feasible to use surface-mounted sensor to measure the body temperature without embedding the sensor into the body if doesn’t require high accuracy.
b) Among 3 sensors, the thermocouple wire has smallest relative percentage error, probably because it the same type of sensor as Channel 6, so the calibration error is minimized, while for RTD and thermistor, since they are calibrated using own calibration curve, which might introduce some error due to linearity and sensitivity difference Possible Sources of Error Possible source of errors are list as follow: A. Human error a) When take the reading from master thermometer, the eye level may not align with the mercury level, which causing parallax error and it will affect the accuracy of readout. b) The mercury level falls between two small divisions was read based on estimation. c) To determine whether the thermal-steady state has been reached, just use “gut feeling” to judge, it may not be accurate. B. Equipment error a) Some of the sensor channel is instable; the reading is oscillating all the time, the readout has been obtained based on estimation. b) When taking the reading from Ch. 1 to Ch. 9 for transient state measurement, need to switch to different channel and record reading, it will not represent the actual temperature at that particular time. c) The resolution of master thermometer is 0.5 C, which will affect the accuracy of readout. d) The temperature controller does not response fast enough due to its PID control algorithm. The temperature of the system fluctuated slightly when On/Off the heater. C. System error a) When calculate the gradient of calibration curve, different fitting method to form a straight line will affect the result.
b) We assume the calibration curve is linear, so the nonlinearity of the sensor has been ignored.
Ways to improve the experiment Base on above observations, here are some suggestions to improve the experiment: a) Use digital thermometer as master thermometer to eliminate read out error. b) Use LabView and Data Acquisition hardware to capture the voltage output from sensors, the sensor output from all channels can be captured concurrently, without any time delay caused by switching of different channels. c) Use a better temperature controller to minimize the fluctuation of system temperature
Conclusion After this experiment, I had learnt characteristics of different types of temperature sensors and how to measure the temperature distribution along a Perspex rod. I also learnt how to calibrate each type of sensor and measure the surface temperature using different sensors. After this experiment, I had better understanding about the temperature distribution along a body.
A comparison of the three sensors is shown below:
Following is a table showing the advantages and disadvantages of the different sensors:
