Transcript L21_Temperature

MECH 373
Instrumentation and Measurements
Lecture 21
Measuring Pressure and Temperature
(Chapter 9)
• Measuring Pressure
• Measuring Temperature
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Measurement Systems
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Measuring Pressure (Review)
􀂋 Strain gage types
􀂋 Capacitive
very low pressures
􀂋 Piezoelectric
high-frequency pressure
􀂋 LVDT
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Measuring Temperature
• Two traditional electronic output devices used for measuring temperature are
thermocouples and resistance temperature detectors (RTDs).
• Two traditional mechanical measurement devices are called infrared
thermometers (for lower temperature) and pyrometers (for higher temperature).
Thermocouples
• If any two metals are connected together, as shown below, a voltage is
developed that is a function of the temperature of the junction. This junction of
two metals used as a temperature sensor is called a thermocouple.
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What are thermocouples?
• Thermocouples operate under the principle that a circuit made
by connecting two dissimilar metals produces a measurable
voltage (emf-electromotive force) when a temperature gradient
is imposed between one end and the other.
• They are inexpensive, small, rugged and accurate when used
with an understanding of their peculiarities.
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Lecture Notes on MECH 373 – Instrumentation and Measurements
Thermocouples Principle of
Operation
• In 1821, T. J. Seebeck observed the existence of an
electromotive force (EMF) at the junction formed between
two dissimilar metals (Seebeck effect).
– The voltage is generated by a thermoelectric phenomenon
called the Seebeck effect. Seebeck effect is actually the
combined result of two other phenomena, Thomson and
Peltier effects.
• Thomson observed the existence of an EMF due
to the contact of two dissimilar metals at the
junction temperature.
• Peltier discovered that temperature gradients
along conductors in a circuit generate an EMF.
• The Thomson effect is normally much smaller
than the Peltier effect.
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Lecture Notes on MECH 373 – Instrumentation and Measurements
Measuring Temperature
• It was later found that the Seebeck voltage is the sum of two voltage
effects: the Peltier effect, generated at the junction, and the Thompson
effect, which results from the temperature gradient in the wires.
• The general simplicity of thermocouples has led to their wide use as
temperature measuring sensors. However, there are a number of
complications in their use:
• The voltage measurement must be made with no current flow. This is
because current flow will not only result in resistive losses but will also
affect the thermoelectric voltages. Meeting this requirement is not a
problem today since electronic voltmeters and date-acquisition systems
with very high input impedance are readily available.
• Connections to voltage-measuring devices result in additional
junctions. As shown in the diagram, there are actually three junctions the sensing junction and the two junctions where the thermocouple
connect to the digital voltmeter (DVM). The voltage reading is thus a
function of three temperatures, two of which are of no interest.
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Measuring Temperature
• The solution to this problem is shown below. Two thermocouples are used,
the second being a reference junction. The reference junction is held at a fixed,
known temperature, most commonly the temperature of a mixture of pure
water and pure ice at 1 atm (0°C).
• There are still two junctions at the DVM terminals, but each of these
junctions consists of the same two materials, and if the two terminals can be
held at the same temperature, the terminal voltages will cancel out.
• The two terminals can be held at the same temperature by placing them both
in the same thermally insulated box or by mechanically connecting them with
thermally conducting but electrically insulating structures.
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Measuring Temperature
• With the known reference junction temperature, the measured voltage is a
unique function of the materials of the thermocouple wires and the temperature
of the sensing junction.
• Finally, the voltage depends on the composition of metals used in the wires to
form the thermocouple. This problem is solved by restricting the materials
used to construct thermocouples. When wires and wire pairs are manufactured,
standard calibration curves can be used to determine the temperature based on
measured voltage.
• The following table lists common thermocouple pairs.
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Measuring Temperature
• The following table lists voltages as a function of temperature for common
thermocouple pairs.
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Measuring Temperature
• Accuracy of thermocouples depends on the type of the thermocouple, the
quality grade of the thermocouple (manufacturers supply different quality
grades), and the temperature range.
• There are many factors that must be considered in selecting thermocouples
for a given application. It includes
- Sensitivity (voltage per degree temperature change)
- Linearity of output
- Stability and corrosion resistance
- Temperature range
- Cost
• Type R and type S thermocouples are very expensive and not very sensitive;
however, they are satisfactory at high temperatures (up to 1768°C) and are
resistant to a number of corrosive chemicals.
• Type C thermocouples are usable to very oxidizing environment.
• Type T thermocouples are inexpensive and very sensitive but corrode rapidly
at temperatures over 400°C.
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Thermocouple Material Vs EMF
Types T, J, and K are most commonly used thermocouples
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Measuring Temperature
• Type K thermocouples are popular for general use since they are moderately
priced, reasonably corrosion resistant, and usable at temperatures up to
1372°C. They also have relatively linear output, which means that for
applications in which accuracy requirements are not to severe, the temperature
can be computed by assuming a linear relationship between temperature and
voltage.
• Thermocouples can be purchased in a number of forms:
- Purchase wires and form a thermocouple junction by welding or
soldering the ends of the wire.
- Purchase the wires with the junction already formed by the manufacturer.
• Manufacturers also supply lead wire to connect the sensing probe to the
voltage-measuring device. It usually consists of two conductors contained in a
plastic sheath.
• In some cases several thermocouples are connected in series in a device
called a thermopile, as shown in next slide.
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Measuring Temperature
• When arranged in this manner, the voltage output to the display is n times
the voltage of a single junction, where n is the number of thermocouples in
the series. This increases the sensitivity of the system. It also provides a
method to average several thermocouples, which are distributed in a spatial
region.
• Thermopiles are also used in some applications as a power source.
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Measuring Temperature
• For data-acquisition systems, the following arrangement is used
• Each of the thermocouples is connected to a channel of the data-acquisition
system (DAS) in an insulated, constant-temperature connection box. The
thermocouples then effectively measure the temperature difference between the
measurands and the box. One channel is reserved for the reference junction.
Software (e.g. LabView) is then used to evaluate the correct voltage by subtracting
the reference voltage from the voltage read from the individual channels.
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Measuring Temperature
Example: Consider standard chromel-alumel thermocouple circuit shown below.
(a) Find the voltage measured by the DVM;
(b) If the DVM measures 17.51 mV, what would be the temperature T1?
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Measuring Temperature
Example: Consider standard chromel-alumel thermocouple circuit shown below.
(a) Find the voltage measured by the DVM;
(b) If the DVM measures 17.51 mV, what would be the temperature T1?
Solution:
Use Table 9.2 for K-type thermo-couple.
T1= 492
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