650:349 – Mechanical Engineering Measurements
Laboratory #6
Fall 2006
Temperature Measurements
I. PURPOSE
The purpose of this laboratory exercise is to demonstrate the use of four different temperature
sensors: Mercury in Glass Thermometer, Resistance Temperature Detector (RTD),
Thermocouple, and Thermistor.
II. BRIEF DESCRIPTION OF TEST APPARATUS AND SENSOR OPERATION.
a. Mercury in Glass Thermometers. The mercury in glass
thermometer is a familiar device which operates on the principle
of thermal expansion. The coefficient of thermal expansion of
the mercury is much greater than that of the glass encasement.
When the thermometer is subjected to changes in temperature,
the mercury expands or contracts depending upon the
temperature increases or decreases. The fluid expansion is quite
small, so a very narrow tube is needed in order to visually
observe the change in volume. The glass encasement is
calibrated so that a change in mercury volume (measured as a
change in length of the mercury column) corresponds to a known
change in temperature.
Figure 1. Typical Mercury
Thermometer
b. Resistance Temperature Sensor (RTD). The electrical
resistance of materials is temperature dependent. For
most materials the resistance increases with temperature.
This property can be utilized in measuring the
temperature. A common material for RTD sensors is
platinum because of its high temperature coefficient of
electrical resistance. A platinum wire of suitable length
Figure 1. Typical RTD Sensor
and diameter is wound around a ceramic cylinder and
encased in a steel protective tube. The resistance change of the sensor can be measured using an
electrical resistance bridge circuit, and the results displayed using a calibrated indicator
registering the resistance of the sensor in Ohms or using a Wheatstone bridge to detect small
changes in voltage associated with small changes in resistance.
c. Thermocouple. When two dissimilar metals are placed in contact, to form a junction a
potential difference is created across the junction. The magnitude of the open circuit potential
difference across the junction is temperature dependent and is called the Seebeck effect. If the
junction is between wires of dissimilar metals which are part of an electrical circuit through
which current flows, then the potential difference measured across the junction is also a function
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of the current flow through the junction
(Peltier effect), as well as the temperature
gradients that may exist along the wire
(Thompson effect). By measuring the open
circuit voltage across the junction (no current
flow) and also by minimizing the effect of
temperature gradients, the measured voltage
across a junction of dissimilar wires can be
used to measure the temperature of the
junction. The voltage measurement must be
carried out at no current flow conditions.
Figure 3. Example Thermocouple Tips
Figure 4. Typical Thermistors
d.
Thermistor. A thermistor operates in the same fashion as an
RTD, but the sensing element is a semiconductor while the
RTD sensor is a metallic element. Thermistor resistance,
however, responds negatively and non-linearly to temperature.
In addition, the thermistor possesses a higher sensitivity with
respect to temperature. Measurement of the resistance can be
accomplished using resistance bridge circuitry (variants of the
Wheatstone bridge).
III. EQUIPMENT
a. A Uniform temperature circulating water bath. The temperature of the circulating water bath
can be adjusted to any value up to the boiling point of water, but should be set not to exceed 80
°C. Some time should be spent familiarizing with the method of operation and the controls. The
cover of the water bath is provided with a set of taps for mounting the sensors at different
positions within the bath.
Important. Do not turn off the circulating pump of the water bath if the
temperature is above 50 °C. Instead, lower the temperature setting and
allow the water to circulate until the temperature drops below 50°C.
Then the unit can be turned off.
b. An Ice/Water bath container. Should be filled with crushed ice to provide a uniform medium
at 0°C.
c. Panel meters.
-A direct measuring temperature indicator for type K thermocouples.
-A mV indicator (or multimeter) with suitable connections for a type K thermocouple
with cold junction compensation at 0°C.
-A resistance measurement panel meter (or multimeter) using resistance bridge circuitry.
d. A Digital Multimeter. For measuring sensor resistances as needed. Some time should be
spent familiarizing with the different function controls.
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e. Temperature Sensors. Mercury in glass thermometer, RTD sensor, thermistor sensor, three
type K thermocouples, and a fourth K type thermocouple for cold junction compensation.
h. PC controlled Data Acquisition System. A PC has been fitted with a data acquisition card and
an external eight channel terminal board to acquire, display, and store eight different analogue
signals. For the present experiment, the system will be used with only two of the channels
active. These channels are connected to a thin K-Type thermocouple, and a thick K-Type
thermocouple. The software use is LabVIEW, a package used for monitoring and controlling
experiments in an industrial environment.
IV. PROCEDURE
a.
Using the thermometer, measure the room temperature.
b.
Measure the temperature of the ice bath using a type K thermocouple, using the direct input
or temperature panel meter.
c.
Insert the RTD probe into the ice bath and plug the detector output into the RESISTANCE
THERMOMETER INPUT of the panel meter (or the multimeter). Set the OHM/mV
switch to OHMS and let the sensor reaches thermal equilibrium. After reaching thermal
equilibrium, record the resistance at 0°C. Then re-insert the RTD detector in one of the
water bath top taps.
d.
Insert the thermistor probe into the ice bath and measure its 0°C resistance after it reaches
thermal equilibrium. Then re-insert the thermistor probe into one of the water bath top
taps.
e.
Connect one of the thermocouples to the direct reading meter panel and the other
thermocouple to the HOT JUNCTION panel meter terminals. The cold junction
thermocouple should be placed into the ice bath and its terminals connected to the COLD
JUNCTION panel terminals as in Figure 5. Record the voltage from the hot/cold
thermocouple circuit using the multimeter or panel meter at room temperature. The third
thermocouple should be wired to the inlet board of the PC data acquisition system.
f.
Set the temperature of the water bath to 25 °C using the combination pump/heater. Launch
the LABVIEW program for monitoring the
Yellow Wire
bath temperature. After steady state is reached
(you’ll know by checking the thermocouple
Red Wire
attached to the PC), record the following: the
B
R
resistance of the RTD, the resistance of the
Hot
thermistor, the voltage (mV) of the cold
Red Wire
compensated thermocouple connection, and
B
the temperature of the bath. Check for
temperature uniformity within the water bath
R
Yellow Wire
(i.e. move the thermocouples to different
insertion points in the bath to see if the
Cold
temperature is the same).
Figure 5. Cold junction compensated
thermocouple connection
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g.
h.
i.
Yellow Wire
Increase the bath temperature to 55 °C in
10°C steps and record the same sensor
readings (temperature, voltage, or
resistance) as in (f) at each step.
Periodically check the temperature
uniformity within the bath.
With the bath at a steady temperature of
55
°C
disconnect
the
hot/cold
thermocouple circuit and connect three Ktype thermocouples as shown in Figures 6
(a thermopile circuit). Record the emf
reading (voltage) from the thermopile
circuit using the multimeter or panel
meter. Also using a glass thermometer,
record the temperature in the vicinity of
the panel meter thermocouple terminal
inputs.
Red Wire
Hot
Room
Temp
Yellow Wire
R
B
Red Wire
Hot
Red Wire
B
R
Yellow Wire
Cold
Figure 6. Series hot junction thermocouple
connection (thermopile).
Measure transient responses of the two thermocouples connected to the PC data acquisition
system and LABVIEW.
1)
Measure the diameter of the thin and thick thermocouple shields.
2)
Start the LABVIEW data acquisition system to record the temperatures of both
thermocouples. Use a fast data rate. QUICKLY insert the thin and thick tube
thermocouples into the 55 °C water bath. Let them reach thermal equilibrium
(about 2-3 second). Then remove them (quickly) from the bath and let them cool
to ambient temperature. Try to hold the thermocouples as still as possible after
removing them from the heat bath.
V. CALCULATIONS AND RESULTS
In the absence of a temperature standard for measuring the bath temperature, we will assume that
the temperature indicator of the bath registers the true bath temperature.
A. RTD
The resistance of an RTD circuit should vary linearly with temperature. Find the constant α for
your data assuming that T0 is 0°C and R0 is the value of the resistance when T = T0. Plot the
values for the RTD resistance versus temperature and use a linear fit. Compare value of α with
that of Platinum given in your text.
R = R0 [1 + α (T − T0 )]
B. Thermistor
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The resistance of a thermistor should vary non-linearly with temperature. Find the constant β for
your data assuming that T0 is 0°C and R0 is the value of the resistance when T = T0. Plot the
values for Thermistor resistance versus (1/T-1/T0) and use an exponential fit.
⎡ ⎛ 1 1 ⎞⎤
R = R0 exp ⎢ β ⎜⎜ − ⎟⎟⎥
⎣ ⎝ T T0 ⎠⎦
C. Thermocouple
Find attached a table of emf (voltage) versus temperature for a K-type thermocouple. Compare
your voltages with those in the table for your measurements with a single TC, a cold
compensated circuit, and the thermopile circuit.
For transient measurements, an unsteady heat transfer analysis shows that:
⎛ ⎛ 3h ⎞ ⎞
T − T∞
t
⎟⎟t ⎟ = exp⎛⎜ − ⎞⎟
= exp⎜⎜ − ⎜⎜
⎟
Ti − T∞
⎝ τ⎠
⎝ ⎝ ρc v R ⎠ ⎠
where:
h = the heat transfer coefficient
cv = the heat capacity of the sensor material
~ 520 J/kg-K
R = radius of thermocouple shield
Ti = the initial sensor temperature
τ = the time constant
t = time (sec)
ρ = the density of the sensor material
~ 8.7 gm/cm3 = 8700 kg/m3
T = the sensor temperature
T∞ = is the ambient temperature into
which the TC is inserted (water or
air)
Rthick = 1.59mm Rthin = 0.80mm
See the helpful guide on reducing the raw data. Plot the natural log of your normalized
temperature data (left hand side of equation above) with respect to time and find τ through a
linear curve fit. Then use τ to find the heat transfer coefficient (h) for water and for air using the
constants given and the radius of the thermocouple shield (measured previously).
VI. REPORTS.
Items you might include in your lab report
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Discuss and calculation of hot/cold junction circuit
Discussion and calculation of thermopile circuit
Comparison of recorded thermocouple voltages with given emf values
Constants β and α from the thermistor and RTD measurements
Estimation of thermocouple heat transfer coefficients
Thermocouple time constants (τ)
Variation of temperature with time for transient thermocouples tests
Comments on time response of all types of gages tested
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This report should have the format as described in the class guidelines described online: General
Guidelines in Section 2 of the general guidelines.
IMPORTANT:
The Laboratory reports are LIMITED to FOUR PAGES! ANY MATERIAL presented on
additional pages will not be considered part of the report. Laboratory reports may contain a
maximum of 5 FIGURES and 2 TABLES! What you include in these figures/tables is your
choice; however, use the lab handout and class lecture notes as your guide to determine what the
most important measurements you’ve made are. Then create figures which best describe and
present the data you have collected as well as support your conclusions.
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