Part I: Time Constant
Abstract
In this experiment, a water bath is heated with a cooker until it boils and along the
process, its temperature is measured with a few different measuring devices.
After reaching boiling point, the temperature versus time data is plotted into a
graph and their response time is obtained and compared. At the same time, the
significance of the data obtained is analyzed and discussed in the discussion
section.
Objectives
To compare the time constant of different types of temperature measuring
devices with reference to mercury filled thermometer.
To investigate the relationship between resistance and temperature.
Theory
Temperature is a measure of hotness of a body with mass. It indicates the total
thermodynamic energy a body contains. Temperature can be measured in three
major different scales which are Centigrade, Fahrenheit and Kelvin and these
three scales can be interconverted by using respective formulas.
There are many different type of instrument that can be used to measure
temperature such as liquid filled thermometer, bi-metallic thermometer,
thermocouple, vapor pressure manometer, thermistor, and resistance
thermometer. In this different type of measuring instruments, each instrument has
its own response time, which is the time taken to reach 63.2% of a step change.
The lower the response time, the more sensitive the instrument is towards
changes of the surrounding. As per theory, mercury filled thermometer should
have the lowest response time.
Liquid Filled Thermometer
A liquid filled thermometer functions by reading the liquid level contained in the
bore of the thermometer. The working principal is based on linear thermal
expansion of fluid with respect to temperature. As the temperature rises, the
volume of the containing liquid increases linearly, which will produce an increase
in height of the liquid column. As the liquid column height stabilizes, the height
will be read in terms of temperature measurement, usually in centigrade. The
most common type is mercury filled thermometer, but it can also be replaced with
several liquid such as alcohol.
Bi-Metallic Thermometer
Bi-metallic thermometer measures temperature using the metal thermal expansion
principle. If only one metal piece is used, the expansion is far too insignificant
per unit change of temperature to be used to measure the temperature change.
Hence, to amplify the effect of expansion, a bimetal strip with two different
expansion coefficient is used to achieve the objective. Using bimetal with this
property, during unequal expansion, the bimetal strip will bend significantly
which can be detected easily to obtain temperature reading.
Vapor Pressure Manometer
Vapor pressure manometer measures temperature using the relationship between
pressure and temperature of a fluid. As the temperature increase, the pressure will
also increase significantly and this pressure can be converted into temperature
change. Usually, a vapor pressure manometer is connected to a Bordon Gauge
that directly indicates the temperature of the surrounding by measuring the
pressure of fluid in a metal container. This device isnt suitable for high ranges of
temperature as the higher the temperature is, the quicker the vapor pressure will
change. Hence, vapor pressure manometer is only suitable for lower range of
temperature change.
Resistance thermometer
A resistance thermometer functions by measuring the resistance of a material
piece in the thermometer with respect to temperature. The common materials
used are platinum, nickel and copper. Platinum and nickel are more widely used
for high accuracy and for a higher range of temperature while copper is only
suitable for low temperature range as it may oxidize at high temperature. For
small temperature range, we may assume linear relationship between resistance
and temperature but for large measurement ranges, the same assumption cannot
be applied. Example of Resistance thermometer is PT-100 Resistance
thermometer which demotes that the material used is platinum and 100 ohm in
resistance at 0 degree Celsius.
Thermistor
Thermistors consist of semi-conducting polycrystalline material. In the
production of temperature sensors, copper dioxide (CuO2) is preferred. It
demonstrates a severe (non-linear) drop in resistance for an increase in
temperature. It possesses a negative temperature coefficient, which is the reason
why these sensors are called NTC resistors.
Equipment
a) Water Cooker
b) Mercury Filled Thermometer
c) Liquid Filled Thermometer
d) Thermistor
e) PT-100 Resistance Thermometer
f) Type-K Thermocouple
g) Bi-Metallic Thermometer
h) Vapor Pressure Manometer
i) Stopwatch
Procedure
1. The water pot of the cooker is filled half-full with tap water and placed in the
cooker.
2. The lid of the cooker with an opening is closed.
3. Thermistor, bimetallic thermometer, PT-100 resistance thermometer, and
vapor pressure manometer are inserted into respective mountings.
4. Spirit-filled and mercury-filled thermometer is inserted into the opening of
the cooker lid.
5. All measuring devices are switched on and the initial reading of all devices
are recorded.
6. The cooker is switched on and the stop watch is pressed to start
simultaneously.
7. Reading of all the devices are recorded for every 2 minutes.
8. This recording procedure is repeated until one of the devices show the
temperature of 100C which is the boiling temperature of the water.
9. The cooker is switched off and all measuring devices are switched off.
10. The lid of the cooker is opened to allow the water to cool before pouring it
away.
Results
Table 1: Temperature reading on various measuring instruments with respect to time
PT-100
Bi-Metallic Vapour Pressure Spirit Filled Mercury Filled Type K
Resistance
Time (sec) Thermometer Manometer Thermistor (C) Thermometer Thermometer Thermocouple
Thermometer
(C) (C) (C) (C) (C)
(C)
0 24.0 28.2 23.7 19.0 26.0 26.2 24.5
120 24.5 30.0 28.8 24.0 30.0 33.1 35.3
240 27.9 34.0 34.1 31.0 39.0 39.1 39.3
360 32.0 38.0 40.7 39.0 44.0 44.7 48.0
480 37.0 46.0 46.9 44.0 52.0 52.0 56.3
600 43.0 52.0 53.5 52.0 59.0 58.0 60.6
720 50.0 58.0 62.4 61.0 67.0 65.1 66.0
840 57.0 66.0 70.2 66.0 74.0 72.8 72.9
960 64.0 73.0 75.6 73.0 80.0 78.0 79.8
1080 73.0 81.0 84.2 83.0 88.0 86.9 87.3
1200 78.0 87.0 88.4 88.0 92.0 91.3 91.5
1320 86.0 94.0 92.5 95.0 98.0 98.3 95.8
1440 96.0 99.6 94.5 99.0 100.0 100.4 96.7
Graph 1: Graph of temperature vs time
120
100
Bi-Metallic Thermometer
80 Vapor Pressure Manometer
Temperature
Thermistor
60
Spirit filled thermometer
40 Mercury filled thermometer
PT100 Resistance Thermometer
20 Type K thermocouple
Step response (63.2 C)
0
0 500 1000 1500 2000
Time(s)
Zoomed view of Graph 1 with pink line as the step response value and intervals
representing elapsed time
Measuring Device Time Constant (seconds)
Bi-Metallic Thermometer 960
Vapour Pressure Manometer 800
Thermistor 720
Mercury Filled Thermometer 660
Spirit Filled Thermometer 760
PT-100 Resistance Thermometer 680
Type-K Thermocouple 660
Part II: Type K Thermocouple
2.1 Abstract
In this experiment, we investigate the working principle of a Type-K Thermocouple.
A water bath is boiled with a cooker and its temperature is measured using the Type-
K thermocouple and at the same time, the potential difference across the coupling
rods is measured using a voltmeter. The relationship between voltage and temperature
is established and at the same time, the sensitivity of the thermocouple is calculated
and compared with the theoretical value.
2.2 Objectives
1. To investigate the working principle of Type-K Thermocouple and the
sensitivity of this thermocouple.
2. To find a relation between voltage output and temperature.
2.3 Theory
A thermocouple is an electrical device consisting of two different conductors forming
electrical junctions at differing temperatures. It can produce a temperature dependant
voltage as a result of thermoelectric effect, which can be used to measure temperature.
Thermocouple are widely used in many fields serving industrial purposes, commercial
purposes as sensors and safety devices. There are many types of thermocouples such
as Nickel alloy thermocouples, Platinum/Rhodium alloy thermocouples and
Tungsten/Rhenium alloy thermocouples. In this experiment, the targeted
thermocouple is Type-K Thermocouple.
Type-K Thermocouple consist of metal couples made of chromel and alumel, is the
most common general purpose thermocouple. The sensitivity of the Type-K is
approximately 41V/C, which means for every change of one centigrade, the voltage
difference produced will be 41 micro Volt. They operate well in oxidizing
atmospheres, but may have insensitivity if a reducing atmosphere occurs.
Theoretically, the voltage output increases proportionally to the increase in
temperature.
2.4 Equipment
a) Type-K Thermocouple
b) Water Cooker
c) Voltmeter
2.5 Procedures
a) The water container from the cooker is filled half full with tap water and
placed in the cooker.
b) The lid of the cooker is closed tightly.
c) The two conductors of the Type-K thermocouple is inserted into mountings on
the cookers exterior.
d) Each of the conductor is connected to a probe, which connects to the voltmeter.
e) The initial reading on both the thermocouple and the voltmeter is recorded.
f) The cooker is switched on and the readings on both thermocouple and
voltmeter is taken every 2 minutes until the water boils. The readings are
recorded and the cooker is switched off.
g) The cookers lid is left open to allow the water to cool before pouring away.
2.6 Data and Results
Table 1: Voltmeter and Type-K Thermocouple Temperature Reading Data
Temperature Voltmeter
Time (minutes)
(C) (mV)
0 24.5 0.79
2 35.3 0.85
4 39.3 0.92
6 48.0 1.07
8 56.3 1.18
10 60.6 1.21
12 66.0 1.51
14 72.9 1.85
16 79.8 2.14
18 87.3 2.17
20 91.5 2.20
22 95.8 2.25
24 96.7 2.33
Part III: Humidity
3.1 Abstract
In this experiment, the working principle of a whirling psychrometer is being
investigated and the air humidity of the room is obtained by using both wet and dry
bulb thermometer readings from the psychrometer along with a psychrometric chart.
The humidity obtained is compared with the data obtained from a wall hygrometer
gauge and the difference between readings obtained is investigated and analyzed.
3.2 Objectives
This experiment aims to understand the concept of whirling psychrometer
(hygrometer), at the same time investigating the wet and dry bulb thermometer
involved in a hygrometer and how to utilize the readings obtained to determine room
humidity.
3.3 Theory
Humidity is defined as the amount of water vapor in the air, which indicates the
likelihood of precipitation, dew or fog. A few types of humidity are such as relative
humidity which is the ratio of vapor pressure to saturated vapor pressure at a given
temperature, or specific humidity, which is the ratio of water vapor mass to total air
mass. Humidity can be measured with many devices.
A hygrometer can be used to measure humidity of air, while a psychrometer serves to
measure the humidity. A psychrometer consist of two thermometer which is the wet
bulb thermometer which is kept in a wet condition, and the dry bulb thermometer.
Generally, wet bulb thermometer has a lower temperature reading as compared to the
dry bulb thermometer due to the temperature lowering effect as a result of evaporation
of water at the wet bulb area. Using readings from both thermometer, the relative
humidity of surrounding air can be estimated using a psychrometric chart by locating
the point of intersection between lines representing temperatures from both
thermometers.
In this experiment, the psychrometer used is a whirling psychrometer which may
obtain the relative humidity of ambience air by spinning the psychrometer in air for a
few minutes, then obtaining the wet and dry bulb temperature which is then used in a
psychrometric chart.
Figure 1: Psychrometric Chart
3.4 Equipment
a) whirling hygrometer b) wall hygrometer
Figure 2: Wall Hygrometer Gauge
Figure 3: Whirling Hygrometer
3.5 Procedure
a) A whirling hygrometer is obtained with wet bulb and dry bulb thermometer.
b) The water chamber connecting to the dry bulb thermometer is filled with water.
c) The temperature on both wet bulb and dry bulb thermometer is recorded.
d) The whirling hygrometer is spun in the air for 5 minutes continuously to allow
partial evaporation of water at the area of wet bulb.
e) After 5 minutes, the readings of both thermometers are recorded.
f) Using the psychrometric chart, the relative humidity of air in the laboratory is
obtained.
g) The reading on the wall hygrometer is also recorded.
3.6 Data and Results
Table 1: Humidity Data obtained from different instruments
Initial After Spinning
Wet Dry Wet Dry
Bulb Bulb Bulb Bulb
(C) (C) (C) (C)
Bulb Reading 22 25 21 25
Humidity (wall hygrometer) (%) 76
Humidity (psychrometric Chart) (%) 70
Sample Calculation
1) Percentage Error = (Theoretical Experimental)/Theoretical * 100%
|70 76|
= 100%
70
= 8.57%
