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ME 411
Experiment 1
Winter 2016
Experiment 1: Response Characteristics of Thermocouples
Objective:
In this experiment students explore fundamentals of thermocouple performance. Specifically this
experiment will give students experience constructing and testing thermocouples and also give
them practical experience assessing how the gauge (diameter) of the wire affects the response
characteristics when measuring fluid temperatures. In the process of implementing this experiment
students will gain further experience programming in LabView.
Theory:
Thermocouples are the physical implementation of a phenomenon known as the Seebeck Effect.
Specifically, when two dissimilar metals form a junction, a voltage will be created at that junction.
This voltage is a function of the junction temperature and the two metals involved. If the two metals
are copper and constantan the thermocouple is a “T-Type”. As needed, thermocouple charts can
be used to relate voltage to temperature. In systems where the sensed temperature may be
changing rapidly, it becomes crucial to understand the transient response characteristics of the
thermocouple.
It is important to note that when a thermocouple is directly connected to a voltage measuring
device (multimeter or Data Acquisition board--DAQ) it will register a voltage that is the addition of
the value resulting from the sensing end and any unintentional thermocouples that are created by
virtue of the connections at the voltage recording device (e.g., the constantan wire connected at the
copper terminal of the multimeter). This effect is relatively small if one is sensing very large
temperatures and the multimeter is at room temperature. In most cases, however, it is desirable to
avoid the unintentional thermocouple at the multimeter junction. This can be accomplished by
using “Cold Junction Compensation – CJC” or by introducing a reference temperature bath (at 0
o
C, producing 0 mV) and ensuring that both wires being connected to the multimeter are of the
same metal (e.g., copper).
A thermocouple can be made simply by twisting two dissimilar metal wires together. In most cases
the junction is welded to bond the two wires permanently. The junction can also be formed by
adding a solder bead, although this increases the thermal mass of the thermocouple and
complicates the analysis because of the addition of a third material (solder). When a thermocouple
is inserted in a fluid there is convective heat transfer between the fluid and the thermocouple. If we
consider an idealized thermocouple as a sphere with diameter D we can apply the lumped
capacitance method to determine the transient temperature response of the junction. Specifically,
the energy balance on the spherical thermocouple is given by:
C
D3 
Tsphere   hD2 (Tfluid  Tsphere )
6 t
(1)
where  and C are the density and specific heat capacity of the sphere, respectively. In the case of
a thermocouple that has been created using a solder bead to join the wires these values represent
a weighted average of the properties of the two metals and the solder bead. Further, the bead
shape may be more cylindrical than spherical. In such a case, the lumped capacitance approach
above must be modified to account for the surface area and volume of the cylinder. The heat
transfer coefficient h is a function of the fluid flow conditions. For natural convection in air, the free
convection coefficient is typically on the order of 5 to 15 W/m2K but can be determined explicitly
ME 411
Experiment 1
Winter 2016
from heat transfer correlations found in most undergraduate heat transfer textbooks. Such
correlations are generally accurate to about +/- 20%.
If we define a new temperature variable  = (Tsphere – Tfluid) the above equation can be simplified to:

6
 h  (
)
t
CD
(2)
This first order ordinary differential equation has a simple exponential solution:

initial
t

 6h 
 6ht 

 e xp  
t 
or Tsphere ( t )  Tsphere,initial  Tfluid  Tsphere,initial 1.0  exp  
 CD  t 0
 CD 

(3)
Given a plot of sphere temperature (thermocouple temperature) vs. time one would then expect an
exponential approach from an initial value to a final limit of the fluid temperature. The time constant
for this response is readily determined from the exponential term, noting that
 6ht 
CD
exp(  t / )  exp  
 or  
6h
 CD 
(4)
There are several methods that could then be used to estimate the time constant and h. For
example, if one plots the natural log of theta vs. time the slope can be related to the time constant.
Equipment:
Each lab station is equipped with a variable temperature heater, a Pyrex beaker, a 16-bit
data acquisition card (built in the computer), two different gauges of thermocouple wire and a
standard glass thermometer. Figure 1 shows the heater unit with the beaker that contains water.
Figure 1. The heater unit with a beaker of water.
A thermocouple can be constructed by twisting the two dissimilar metals (e.g., Copper and
Constantan in the case of a T-type thermocouple) and then welding or soldering the tip. Any
excess wire beyond the bead junction can be trimmed to make the thermocouple more spherical. A
thermocouple can be directly connected to a high precision multimeter and any applied
temperature condition will result in a change in the measured voltage. Typical thermocouple
voltage signals are on the order of millivolts, however, making it difficult to measure accurately
small changes in temperature, or to record temperature data in a data acquisition system. Many
data acquisition systems will consist of internal amplification, signal conditioning circuitry, and high
ME 411
Experiment 1
Winter 2016
resolution (e.g., > 18 bit representation of voltage input). In this experiment students need to
acquire temperature with 16-bit data acquisition system. The bit resolution implies that the range of
possible continuous voltage input (+/-10 volts) is represented by only 2n (4096 for n=12) discrete
levels. A cold junction compensation is accomplished by measuring the room temperature using a
thermometer and placing the measured value in the DAQ assistant VI.
Procedure Outline:
First, fill a beaker/flask about 2/3 full with water and immediately start to heat the water up to
approximately 80 oC. While it is heating up you will construct thermocouples and write the VI for the
experiment. Keep an eye on the water temperature and adjust the heater down so that the water
does not start to boil. To ensure you have sufficient time for the experiment – one or two students
in each group can create thermocouples while the other(s) work of the virtual instruments.
Construct thermocouples
Each group will work with one fine gauge and one coarse-gauge thermocouple. The fine gauge
thermocouple will be either 30, 36, or 40 gauge. Students will be offered the option of welding their
own fine gauge thermocouples (likely 30 gauge) or selecting one that is prefabricated.
For the coarse gauge thermocouple (either 20 or 24 gauge) the bead should be made by first
twisting the wires together and then applying a drop of solder just where the wires meet. When cool
(~20 seconds) the excess wire can be snipped off using wire cutters. While use of a solder bead is
not ideal, it does a good job of holding the wires together and helps to highlight the time constant
issues associated with larger bead thermocouples. Later, when analyzing the transient response
you will need to estimate the average thermal properties of the bead –composed of solder, and the
two thermocouple wire materials.
The coarse-gauge thermocouple wire is likely to be 24 gauge (0.020” dia), 20 gauge (0.032” dia) or
larger. The fine gauge wire will be either 30 gauge (0.01” dia), 36 gauge (0.005” dia), or 40 gauge
(0.0031” dia). It is important that you record the wire diameter and estimate the effective
dimensions (and volume) of the thermocouple bead. The shape of the bead MAY be best
approximated as a cylinder, in which case the analysis presented above must be suitably modified.
Connect both thermocouples to the data acquisition unit screw terminals. Be sure to connect them
with the correct polarity (for T-type thermocouples the Copper wire – clad in blue insulation - is
positive). Create a virtual instrument that is capable of reading both thermocouple channels
(probably Ai0 and Ai11) at the same time. The VI should be set to use the Cold Junction
Compensation using the measured room temperature (i.e. a constant temperature value that you
enter in the DAQ assistant).
The experiment involves exploring the transient response of the thermocouples and estimating
the free convection in air. The larger thermocouple should demonstrate a slower response to
changes in the temperature of the surroundings. You can apply equation 4 to estimate the heat
transfer coefficient for the thermocouple as it cools in air. If you determine that the thermocouple
junction is better modeled as a cylinder you will need to modify equation 4 as discussed earlier.
1
Note: Ai0 corresponds to device pinouts 68 and 34, while Ai1 corresponds to pinouts 33 and
66.
ME 411
Experiment 1
Winter 2016
Prepare the VI
You will need to create a VI that reads both thermocouples at as fast a frequency as possible. The
samples should be stored in a file for later analysis. Be sure that the sampling VI obtains sufficient
data to fully explore the transient response (~ 30 to 60 seconds).
Determine transient response
With the hot bath at ~80 oC and the thermocouples submerged in this bath (for 15+ seconds),
initiate the sampling VI. Gently remove the thermocouples from the bath as the VI samples the
temperatures. Expose the thermocouples to ambient air until the sampling is complete. Be sure to
hold the thermocouples away from where they could be influenced by the heat rising from the bath,
and be careful not to shake the thermocouples.
Save the resulting data file with a unique name and take notes regarding the experiment conditions
that correspond to this name. Replace the thermocouples in the bath. Wait at least 15 seconds and
then repeat the sampling procedure just described several more times so that you have a total of 4
test cases—each with a unique name.
Now remove the beaker from the hot plate. Set the hot plate temperature to 100 oC. Being careful
not to touch the hot plate with your hand place both thermocouple beads on the plate so that they
heat up. Leave them in contact with the plate for at least 30 seconds. Start the VI again and
sample as you remove the thermocouples from the hot plate and hold them still in air. Be sure that
you sample for at least a few seconds before removing the thermocouples from the plate. Give the
resulting file a unique name and repeat this measurement several times so that you have 4 sets of
data for this condition.
Take a quick look at the resulting text files to assure the data appear reasonable. Repeat any
experiments that appear to have any questionable data.
Turn off equipment
Be sure to turn off the heater. The teaching assistants will empty the flasks of hot water from
each station and/or prepare them for the next group.
ASSIGNMENT:
For this experiment you will write a complete report along the lines of the requirements specified in
class. This report should be no more than 7 pages including report text and all figures. A separate
appendix including sample data and calculations should also be included, but should be no more
than 4 pages long. If you have data files that are long you should include just a sample.
The following list of questions is intended as a general guide for students as they consider their
report write-up. Students should NOT set out to answer this specific set of questions, but rather use
them as a guide as they consider the analysis of their experimental data.

What is the effective bead diameter (and volume) of both thermocouples and how should
the time constants be related? Is the thermocouple best modeled as a sphere or a
cylinder?

What are the time constants for each thermocouple and what are the implications for the
types of transient measurements that can be taken with both?

What is the best estimate of the free convection coefficient in (still) air? How do the results
of cooling a wet thermocouple differ from those for cooling a dry thermocouple? Why?
ME 411
Experiment 1
Winter 2016

Was the sampling frequency fine enough to determine the convection coefficients? How do
these estimates compare with published values for natural convection?

If the same experiments were conducted in reverse, what might you expect? That is, if the
thermocouple was taken from air and heated rapidly by placing it in the water, could our
system capture the transient behavior?