7. Experiment # 7: Using the Temperature Measurement and
Calibration Unit
7.0
Objectives
The objectives of this experiment are to
1. Familiarize students with different ways of temperature measurements, fixed points and
transient response.
2. Indicating differences between measuring devices and showing calibration procedure for
some probes (Thermistor)
3. Verifying the Seebeck effect experimentally
7.1 Unit Description
Please refer to the Schematic Diagrams in Figures 1 and 2.
7.1.1 The Control Console:
The control console contains a number of instruments, round Din connectors and 2mm connector
sockets. These are used together with the sensors supplied and a set of 2mm stackable connecting
leads to investigate the various electronic methods of temperature measurement. In addition,
inside the console is a voltage amplifier and a constant current source that are also used as part of
the investigations.
For operator safety, on the rear panel is an earth leakage circuit breaker (RCCB) (21). This
will isolate the unit from the mains in the event that the incoming and outgoing currents do not
balance by more than 3 mA as in a leakage to earth situation.
7.1.2 The Heater Plate
A heater plate(25) and stainless steel beaker are also supplied to provide students with a
variable temperature source that can be used to provide a test environment for the various
sensors. The hot plate power is supplied from either of the two auxiliary power outlets (22,23)
on the rear panel.
The stainless steel beaker is supplied with a number of temperature indicating self adhesive strips
attached, These demonstrate to students an additional method of temperature indication and the
use of color change crystals that are sensitive to a particular range of temperatures.
On the front panel, the Main switch (1) is a combined double pole miniature overload cut out
and switch. The components on the front panel are arranged in logical groups according to their
function. The main components are as follows:
7.1.3 Platinum Resistance Thermometer, PRT
The direct reading platinum resistance thermometer (12) is connected to the socket PRT
thermometer input (11). Connecting the PRT probe supplied to this socket will allow the
Page 1 of 15
temperature to be displayed on the digital display. Note that the PRT probe supplied has a plug
that will only fit the PRT associated sockets on the panel.
A second socket PRT probe resistance (10) allows the resistance of the probe to be directly
measured using the digital hand held meter supplied. To investigate the operation of a PRT
probe, an internal constant current source (9) is available. Adjacent to this are sockets connected
to internal resistors that can be used to simulate connecting wire resistances (8).
Finally an additional 4 wire probe input output (7) socket allows students to investigate the
wire connections of the PRT probe that is supplied
7.1.4 Thermistor
The direct reading thermistor thermometer (15) is connected to the socket thermistor
thermometer input (14). Connecting the Thermistor probe supplied to this socket will allow the
temperature to be displayed on the digital display. Note that the Thermistor probe supplied has a
plug that will only fit the Thermistor associated sockets on the panel. A second Thermistor
Resistance socket (13) allows the resistance of the probe to be directly measured using the digital
hand held meter supplied.
7.1.5 Thermocouples
The direct reading thermocouple thermometer (16) is connected to the thermocouple sensor
sockets (17) and the associated switches. Any of the type K thermocouples supplied may be
connected to the sockets and when the switch is pressed Down, the thermocouple is connected
directly to the thermocouple thermometer(16).
In order to investigate the fundamental principles of thermocouples, the connectors grouped as
amplifier input (3) connect any of the three types of thermocouples supplied (and any locally
available thermocouple) to an internal voltage amplifier. The resulting amplifier output is
available at the amplifier output(4) terminals. The amplifier has a fixed gain to allow
calculations to be undertaken. In order to display the resulting voltage a panel mounted millivolt
meter(6) is supplied.
To allow investigation of the effect of lead resistances and the input impedance of measuring
instruments three internal resistors(19) off different ‘values are located at the bottom of the
panel. There are also sockets (18) that are not connected and spring terminals(2) (for bare
wires) that allow thermocouples to be linked in various ways.
7.2 Accessories
A number of accessories are supplied with the unit and are normally contained in a carrying case.
7.2.1. Red Spirit in Glass Thermometers
A number of red spirit in glass thermometers are provided. For new students these are probably
the most familiar type of temperature measuring device.
7.2.2. Bi-metalic Thermometer
A bi-metal thermometer is provided using the differential expansion of metals.
7.2.3. Vapour Pressure Thermometer
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A thermometer using the change in vapor pressure of a volatile fluid is supplied.
7.2.4. Thermocouples
Three types of thermocouples are supplied
Type
Thermocouple
Nickle-Chrome + Nickle-Aluminium
Type K
Copper+Copper-Nickle.
Type T
lron+Coppcr-Nickle
Type J
Color Code
+ve Green
+ve Brown
+ve Black
-ve White
-ve White
-ve White
Tables showing the voltage output from type K, T and J Thermocouples are given in appendix A
Of the type K thermocouples, one set are shrouded within a stainless steel tube and the others are
exposed. All of the thermocouples are connected to stackable 2mm plugs that fit fire various
sockets on the panel.
7.2.5. Connecting Leads
In order to interconnect the various components on the panel, a number of color coded 2mm
stackable leads are supplied.
7.2.6. Rubber Disc
A large robber disc with holes is also supplied to be placed on top of the stainless steel beaker.
The holes may be used to support the various temperature sensing devices when undertaking
experiments.
7.2.7. Vacuum Insulated Flask
A vacuum insulated flask is provided to contain either an ice-water mixture (for the zero 0oC
reference) or alternatively heated water. Due to the insulation the ice in the water will remain
solid for an extended period of lime and the heated water temperature will remain constant for
long enough to allow temperature measurement,
It is recommended that the accessories are kept in the carrying case for security.
Page 3 of 15
Page 4 of 15
Page 5 of 15
7.3 PRECAUTIONS AND WARNINGS (SAFETY)
The Temperature Measurement Methods and Calibration unit H981 involves the use of both
electrical equipment and heated water.
The following precautions must be observed.
1. In the event of a water spillage on or near the unit, DO NOT TOUCH THE CONTROL
CONSOLE. Isolate the unit electrically at the local supply point and then remove the power
supply lead from the main power inlet (20) on the rear panel. The unit can then be dried in
the normal manner.
2. The heater plate (25) can (as its name implies) reach temperatures that will cause severe
burns if touched with bare hands.
3. In order to demonstrate the boiling water reference point, the beaker is used together with the
heater plate (25), rubber disc and one or more temperature sensors inserted in the boiling
water. Students should be made aware of the potential for steam to vent through the various
boles in the rubber disc and the severe burns that steam can cause. Obviously the potential
for bums resulting from spillage of the boiling water must also be mentioned to the operators.
It is recommended that when the heater plate (25) and stainless steel beaker are being used,
the operators are provided with industrial gloves, or similar with which to move the
temperature sensing devices and the beaker.
4. In order to ensure the best possible accuracy and hence illustration of the procedures being
demonstrated, it is recommended that pure water distilled is used for both the ice being
generated for use in the vacuum insulated flask and the water for the beaker.
5. It is possible for the internal amplifier, constant current source and panel mounted millivolt
meter (6) to be severely damaged by incorrect connection when setting up the various
experimental circuits.
6. BEFORE turning on the main switch (1), ensure that the connecting leads supplied have been
correctly connected according to the diagrams and illustrations detailed in each of the
relevant experimental procedures.
Page 6 of 15
7.4 OPERATING PROCEDURES
It is assumed that the installation procedures have been carried out and that the heater plate (25)
is connected to power outlet (22, 23) sockets on the rear panel.
Ensure that the Precautions and Warnings detailed on page 6 have been read, understood and
explained to the students/operators.
Control console
Ensure that there are none of the sensors or stackable connecting leads attached to the
instrumentation console. Turn on the main switch (1) and the instruments on the panel should
illuminate. If this is not the case check the earth leakage circuit breaker (21) on the rear panel.
Heater Plate
When power is supplied, the main switch on the heater plate (25) may be turned on. The heater
plate(25) has a temperature control dial, This is purely an indication of approximate temperature
and is similar in accuracy to the scale that may be found on any domestic cooker or electrical
appliance. The control method is also similar in that the control is a simple on-off thermostat.
Water and Ice
For many of the experiments an ice and water mixture is required together with water for heating
in the stainless steel beaker. As detailed in the Precautions and warnings on page 6, the water
should be pure distilled water or at least de-ionized. If there is an appreciable amount of salts or
other impurities in the water, the melting and boiling temperature can vary considerably from 0
and 100oC.
After using the stainless steel beaker and vacuum flask it is recommended that these are dried
and cleaned in order to prevent the build up of any residual salts on the inside surfaces.
Obviously it is recommended that a store of ice is available in advance of the experimental
period.
For the ice reference it is necessary to have a mixture of solid ice and liquid water in the vacuum
flask. The 0oC reference will not be obtained until there is a mixture of BOTH solid and liquid
water in the flask.
Care of the Equipment
As the unit consists of a number of loose items it is recommended that at the end of each
experimental period, the components are checked off against the packing list supplied with the
unit and the loose items are stored in the carrying case supplied.
USEFUL DATA
Internal Amplifier Gain:
Voltage Out = Voltage In × 40
Internal Constant Current Source:
Current Output = 0.5 mA = 0.5 × 10-3 Amps
Page 7 of 15
7.5 TEMPERATURE MEASUREMENT OF FIXED SCALE POINTS.
7.5.1 Temperature
Temperature is a difficult concept to understand and describing it as the degree of “hotness or
coldness” of a body is not sufficient where measurement is concerned. Technically, it is defined
as an indication of internal energy.
7.5.2 Temperature Scales and Fixed Points
Temperature differs from properties such as mass length and time and cannot be measured by
comparison to basic standards. In devising a scale for measurement of temperature, an easily
reproducible physical state of a material has to be utilized. The temperature at which this state
occurs is called a “fixed point” and a scale may be created by reference to such points.
For example, the centigrade scale is based on the points at which (pure water) ice melts and pure
water boils (at standard atmospheric pressure). The ice point is given the value 0 Centigrade and
the boiling point 100 Centigrade. The range between these two fixed points is divided into 100
equal parts.
Modern developments in thermometry have indicated that the ice point be substituted by the
triple point of water on the grounds of reproducibility. The triple point of water is an easily
reproducible state of pure water existing as a mixture of ice, liquid and vapor in equilibrium. The
temperature of the triple point is given the value of 0.01 Celsius.
The Celsius scale is the same as and replaced the centigrade scale in modem thermometry. The
zero having been shifted to give:
0.010 Celsius (0.0l oC) at the triple point
Alternative scales have been derived to define temperature. One example attributed to Lord
Kelvin is the Thermodynamic Scale. This scale is independent of material properties and relates
to a thermodynamically reversible heat engine. The unit of measurement is the Kelvin and the
triple point of water is given the value 273.16 Kelvin (273. l6 K)
A similar scale is the Absolute scale which is defined by the relationship PV = RT for a perfect
or ideal gas. In both cases, the unit of measurement corresponds to the Celsius unit and absolute
measurement of temperature may be stated by adding 273.16 to the Celsius value. e.g. 1000C =
373.16K.
7.5.3 International Temperature Scale (ITS9O)
The international temperature scale was devised to permit rapid calibration of scientific and
industrial instruments. The scale agrees with the Celsius scale at defined fixed points below and
above the 0 – 100 oC range. Typical fixed points are listed below:
Page 8 of 15
•
Between 0.65K and 5.0K temperature is defined by the vapor pressure-temperature
relationship of (liquid) Helium.
•
Between 3.0K and the triple point of neon( at 24.5561 K), temperature is defined by
means of a helium gas thermometer calibrated at three experimentally realizable
temperatures.
•
Between the triple point of hydrogen (at 13.8033K) and the freezing point of pure silver
(at 96l.780C) is defined by means of platinum resistance thermometers calibrated at
specified sets of points using specified interpolation methods.
•
The variation in electrical resistance of platinum wire with temperature can be used to
measure temperature.
•
No single platinum resistance thermometer can be used to cover the entire scale but a
range of platinum resistance probes are used and calibrated at a range of repeatable fixed
points Mathematical equations are then used to define the variation in resistance between
the fixed points.
The repeatable fixed calibration points are defined between ranges of points as follows.
Between triple point of hydrogen (at 13.8033K) and the triple point of pure water (at
273.16K) the thermometer is calibrated at the triple points of:
•
•
•
•
•
•
Hydrogen (1 3.8033K)
Neon (24.5561K)
Oxygen (54.3584K)
Argon (83.8058K)
Mercury (234.3 156K)
Water (273.l6K)
From 0oC to the freezing point of pure silver (961.78 oC), the thermometer is calibrated
at the following freezing points:
o
o
o
o
Tin(231.928 oC)
Zinc(419.527 oC)
Aluminum (660.3 23 oC)
Silver(961.78 oC)
Above the temperature 961.78 the international Temperature standard refers to the
Planck law of black body radiation which is beyond the scope of this experiment.
Page 9 of 15
7.6 Experiment 7a: The use of different temperature measuring units
to measure fixed scale points
7.6.1 Introduction
Having defined a temperature scale, the first experiment will illustrate two fixed calibration
points using a red spirit in glass thermometer, vapor pressure thermometer and Bi-metallic
expansion thermometer.
The liquid-in-glass thermometer uses the volumetric expansion of a colored spirit inside a very
small bore glass tube together with a scale engraved on the outside of the tube. The only fixed
points on the scale are defined by the melting point of ice(0oC) and the boiling point of
water(100 oC). The 100 (and smaller) divisions between are purely equal divisions of length,
based upon the assumption that over that range the volumetric expansion of the spirit is linear
with respect to temperature.
The vapor pressure thermometer utilizes fixed relationship between pressure and temperature
that exists when a a liquid and its vapor (only, no other gases or air) are contained in a closed
vessel. The P-T relation is well defined. It consists of a metal bulb, partially filled with liquid
and its vapor fills the space on top of the liquid. The fluid is connected to the sensing element of
a Bourdon tube gage. The gage is calibrated directly to units of temperature. The P-T relation is
nonlinear, this is the reason the sensitivity of the unit decreases at low readings.
The Bi-metallic expansion thermometer is made of two thin metal strips, having different
coefficients of expansion, are mechanically fastened together. This results in a strip that bends
significantly when heated. One end is fixed while the other end is attached to a pointer. It is
widely used whenever accuracy is not so important.
The Platinum resistance sensor
The Platinum resistance sensor is one of the most accurate devices available for temperature
measurement and is often calibrated for reference purposes. The sensor uses a small length of
platinum wire trimmed to have a particular resistance at specific temperature. The current device
has a resistance of 100 Ohms at 0oC. The advantage of the PT100 device is that above and below
the reference temperature, the resistance change is essentially linear.
The Thermistor
An alternative device that is used in a similar manner to the PT 100 probe is a thermistor. This is
a small resistor that exhibits similar characteristics to the P17100 sensor in that its resistance
changes with temperature. However in most cases the resistance change is nonlinear but has a
much greater range than the PT100 sensor In addition, the thermistor sensors can have a negative
or positive change of resistance with temperature. Because of their low cost, they are often also
used in electronic thermostats.
The Thermocouples
Peltier Thermo-Electric Effect
A thermocouple consists of two dissimilar metals joined together at one end. When the metallic
junction is heated, a very small voltage is generated known as the Peltier voltage. This voltage is
a function of the metals involved and the temperature of the junction.
Page 10 of 15
The Seebeck Thermo-Electric Effect
If two junctions of a thermocouple were maintained at different temperatures, a voltage will be
generated. It is proportional to the temperature difference. Hence the two junctions can be used
to determine the differences in temperature.
Ice point was used as a reference junction to calculate the temperature using a thermocouple.
However, if a junction temperature is known, it is possible to add or subtract the electronic signal
resulting from this temperature to or from the measuring junction to determine the measuring
temperature. This method is used in thermocouple instruments that display the temperature
directly.
Ensure the operators have read and understood both the Operating Procedure on page 7 and the
Precautions and Warnings on page 6.
7.6.2 Experimental Procedure
1. Partially fill the vacuum flask with ice and water and place one of the glass tube
thermometer, the vapor pressure thermometer and the Bimetallic thermometer in the
mixture. Carefully stir the mixture with the thermometer to ensure that the thermometers
reach the temperature of the mixture.
2. Periodically examine the thermometer in the ice water mix and observe the indicated
temperature. This will be close to 0 oC. Note that it is unlikely that the indicated
temperature will be Exactly 0 oC.
3. Now remove the vapor pressure thermometer and the Bimetallic thermometer from the
mixture in the vacuum flask. Instead, place the PRT100, Thermistor and a K-type
thermocouple in the ice-water mixture. The other ends of the PRT100, thermistor and
thermocouple probes are to be connected to sockets (11), (14) and (17) in the control
console, respectively. Readings are to be recorded from the displays (12), (15), and (16),
respectively.
4. Meanwhile 2/3 fill the stainless steel beaker with pure water and place the rubber disc on
top. Place this on the heater plate (25) and turn on the main switch (1) and the heater
plate (25). Set the heater plate to a temperature of approximately 2000C.
5. As the water is heated, record the readings of the PRT100, Thermistor, liquid-in-glassthermometer and the thermocouples (in Table 1) until water boils.
6. Once the water in the stainless steel beaker has reached boiling point(steam will be
issuing from the holes in the rubber disc) turn the temperature setting down to
approximately 120-1500C. This will ensure the water continues to boil but does not
generate excessive amounts of steam.
7. Remove the probes from the ice-water mixture and place the vapor pressure thermometer
and the Bimetallic thermometer carefully into the boiling water. Allow the thermometers
Page 11 of 15
time to reach a stable temperature and carefully remove sufficiently to observe the scale
measurement. This will be close to 100 oC.
Notes
i.
Note that it is unlikely that the indicated temperature will be exactly 100 oC due to the
general purpose nature of the thermometers used.
ii.
Note that thermometers can be purchased with specified accuracies, specified immersion
lengths for specific purposes and calibrated over various ranges depending upon the fluid
used and the application
iii.
The boiling point of water occurs at 100 oC for standard atmospheric pressure only. i.e.
the pressure must be 760mm Hg. At lower atmospheric pressure. boiling will occur at
lower temperature and, conversely. at higher atmospheric pressures, boding will occur at
higher temperatures. For example, a rise in atmosphere of 27mm Hg. above the standard
pressure will result in a rise of 1oC in the boiling point of water.
iv.
A scale which is used in various parts of the world is the Fahrenheit scale. This scale
gave the values 32 oF to the ice point and 212 oF to the boiling point of water, being
divided into 180 equal divisions. Instrumentation is still used calibrated in degrees
Fahrenheit, but these tend to be dual calibration, incorporating Celsius equivalents.
Where conversion is necessary,
o
C= (oF-32)/1.8
o
F= (oC × 1.8) +32
Table (1) measurement of temperature using different sensors
No
1
2
3
4
5
6
7
8
9
10
11
12
Type
thermometer
of Liquid
in
glass
Ice-water mixture
Vapor
pressure
Bimetallic PRT100
Thermistor KThermocouple
Boiling water
Page 12 of 15
7.7 Experiment 7b: Investigation of the Resistance Change of a
Negative Temperature Coefficient Thermistor Sensor with
Temperature.
7.7.1 INTRODUCTION
One obvious disadvantage of the platinum resistance temperature probe is the fact that it utilizes
a precious (and hence expensive) metal. This makes the probe relatively expensive and, in
addition, it has to be protected. In addition, the electronics required to utilize the probe also tend
to be relatively expensive.
7.7.2 Experimental Procedure for Investigation of the Resistance of a
Thermistor Probe.
•
•
•
•
•
•
•
Select the Thermistor probe and insert the probe in the thermistor resistance
socket(13).
Take the digital multimeter supplied and insert the red probe into the red socket and
the black probe into the black socket as shown above.
Set the multimeter selector switch dial to the resistance (Ohms) scale and a range to
accommodate a resistance of approximately 1000 -30000 Ohms.
Partly fill the vacuum flask with pure ice-water and 2/3 fill the stainless steel beaker
with pure water (replace the existing water with water at room temperature). Place
the flask on the heater plate (25), turn on the main switch(1) and set the heater plate
to approximately 120oC.
Place the Thermistor sensor in the ice-water and observe the resistance. Record the
final resistance when the probe has reached a minimum. It may be necessary to
switch the meter to a higher resistance range if necessary
Connect the Platinum Resistance temperature sensor probe to the PRT Thermometer
input (11) and then place the probe in the stainless steel beaker. The PRT
thermometer (PT100) will be used for this experiment to determine die temperature of
the heating water.
At regular intervals measure the temperature of the water and the resistance of the
thermistor sensor up to the point where the water is boiling.
Page 13 of 15
Table 2: Measurements of PRT100 Temperature and Thermistor resistance
Water temperature oC (PRT100)
Thermistor Resistance, Ohm
Plot the data (resistance versus temperature). As may be seen the resistance change with
temperature is not linear. In addition, as the temperature approaches 100 oC the rate of change of
resistance becomes smaller and therefore the thermistor must be chosen with a range of variation
that suits the application and the expected range of temperatures. It is to be noted that thermistors
are available with a wide range of resistance variation and a wide range of temperatures.
7.8 Experiment 7c: Verifying the Seebeck thermo-electric principle
7.8.1 The following procedure is intended to verify the Seebeck Thermoelectric effect
•
•
•
•
•
•
Select two of the shrouded type K thermocouples (green and white insulation)
Partially fill the vacuum flask with ice and water
Meanwhile 2/3 fill the stainless steel beaker with pure water and place the rubber disc on
top. Place this on the heater plate (25) and turn on the main switch (1) and the heater
plate (25). Set the heater plate to a temperature of approximately 2000C.Connect the two
thermocouples as shown in Figure ensuring that both of the black thermocouple plugs are
inserted in one of the white sockets (18) that are not connected. This forms an electrical
joining of the similar metals of the two thermocouples. The thermocouples are shown in
the figure as “K” and “K”
Connect the two red plugs from the thermocouples into the red and black socket of the
Amplifier Input sockets.
Connect the Amplifier output to the Voltmeter Input using red and black connector cable
(red to red and black to black).
Turn on the main switch (1).
Page 14 of 15
•
•
•
Place both thermocouples in the vacuum flask and gently agitate. The millivoltmeter
display will be close to 0.0 (no temperature difference, then no voltage difference)
Note, the reading of the millivoltmeter is magnified 40 time. Thus it should be divided by
40 to get the reading as millivolts (note that the thermocouple tables in appendix 1
provide a reading in micro volts)
Take one of the thermocouples and place it in hot water, the voltmeter reading will
change and the reading can be converted to temperature difference using the Tables in
Appendix 1 (divide the reading by 40 then multiply it by 1000 to convert it to microvolts,
then obtain the corresponding temperature difference). For example, a reading of 103 mV
is converted to a true value by dividing it by 40 so it becomes 2.575 mV = 2575 V.
The tables indicate that this is equivalent to approximately 63 oC
Therefore, we used two thermocouple junctions at two different temperatures to calculate the
temperature difference.
7.9 Report Requirements
A technical report is expected where it would include an introduction, objectives of the
experiment(s), brief description of what you learned from each of the three procedures. Data
tables, graphs and discussions are also expected. Finally, your discussions are expected to shed
some light on the following issues:
1. What are the sources of errors in a thermocouple reading
2. What are the advantages of the digital temperature sensor over the thermometer
3. What are the disadvantages of the digital temperature indicator
Finally, the report is to have a conclusion (the conclusion may include your suggestions for
improvement) and a list of references (if you used any).
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