Fundamentals of Thermocouples

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Fundamentals of Thermocouples
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Seebeck Effect
Discovered accidentally by the German-Estonian
physicist in 1821 In a closed circuit consisting
of dissimilar conductors, a current is generated
when the junctions of the conductors are at
different temperature. The direction of current
is reversed if the temperature is reversed
V (SB-SA) (T2-T1)
S Seebeck coefficient, thermoelectric power
Comments a thermocouple works by measuring the
difference in potential caused by the dissimilar
wires. It can be used to measure a temperature
difference directly, or to measure an absolute
temperature, by setting one end to a known
temperature.
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Peltier Effect

• The reverse of Seebeck effect, a creation of a
  heat difference from an electric voltage.

Observed in 1834 by Jean Peltier, 13 years after
Seebeck's initial discovery When a current is
passed through two dissimilar metals that are
connected to each other at two junctions (Peltier
junctions). The current drives a transfer of heat
from one junction to the other one junction
cools off while the other heats up depending on
the current direction
Q ?AB I (?B -?A) I
? is the Peltier coefficient
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Thomson Effect
Named for William Thomson, describes the heating
or cooling of a current-carrying conductor with a
temperature gradient. Any current-carrying
conductor, with a temperature difference between
two points, will either absorb or emit heat,
depending on the material.
dq ?A I dT
?A Thomson coefficient for material A
TdT
T
I
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The Thomson Relationship

• The Seebeck effect is actually a combination of
  the Peltier and Thomson effects. In fact, in 1854
  Thomson found two relationships, now called the
  Thomson or Kelvin relationships, between the
  corresponding coefficients. The absolute
  temperature T, the Peltier coefficient ? and
  Seebeck coefficient S are related by the first
  Thomson relation
•           
• ? S T
• which predicted the Thomson effect before it was
  actually formalized. These are related to the
  Thomson coefficient ? by the second Thomson
  relation
•               
• ? TdS/dT

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Applications Empirical Laws
Laws of Magnus For a given pair of homogeneous,
isotropic conductors forming a closed loop, the
seebeck coefficient depends only on the
temperatures of the junctions, and not on the
temperature distribution along the length of the
conductors
Laws of Intermediate Temperatures The Seebeck
effect of a thermocouple with junctions at
temperatures T3 and T2 is the algebraic sum of
the Seebeck effect emfs on the same thermocouple
with junctions first at temperature T3 and T1,
and then at temperatures T1 and T2.
Laws of Intermediate Metals In a thermocouple
consisting of three conductors, A, B, and C, the
seebeck emf of the thermocouple AC with junctions
at temperatures T2 and T1 is the algebraic sum of
the Seebeck emfs of the thermocouples AB and BC
with their junctions are at the same temperatures
T2 and T1.
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Thermoelectric Thermometry
The Seebeck emf of a circuit composed several
different homogeneous conductors is determined
solely by the temperatures fo the junctions, and
is independent of the temperature gradients
along the conductors.
Typically, a thermocouple consists of two
annealed metallic wires of different materials,
encased in insulating sheath. One end of the
wires is welded together to form a homogenous
junction, called the hot junction or measuring
junction, and the other ends of the wires called
cold junction or reference junction, are
connected to copper leads. The thermocouple-copper
junction is typically maintained at zero
temperature.
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Types of Thermocouples
Base-metal thermocouples -200 to 1000 degrees
inexpensive Noble-metal thermocouples Stable in
oxidizing atomosphere Refractory-metal
thermocouples measure temperatures beyond the
upper limit
of base-metal thermocouple Resista
nce thermocouples Most accurate, most sensitive,
and most stable
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Computerized Data Acquisition Systems
Transducer
Signal Conditioning
A/D Conversion
D/A Conversion
Computer
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Typical PC-based DAQS

• Data Acquisition
• Data saving
• Data manipulation and display
• Process control by making use of the results

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SCX Signal Conditioning

• Amplification
• Attenuation
• Multiplexing
• Filtering
• Excitation
• Linearization

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Signal Conditioning

• Amplification
• Amplify low level signal
• Max of amplified signal equals to max input range
  of the analog to digital converter (ADC)
• Attenuation
• Reduce high-level signals
• Multiplexing
• Measure multiple signals
• Filtering
• Remove undesirable frequency information
• Noise filter
• Antialiasing filter
• Excitation
• Generate excitation for some transducers strain
  gauge
• Linearization
• Account for nonlinear response to change in
  phenomena being measured, thermocouples

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Signal conditioning board
SC-2345
SC-2075
CA-1000
SCB-68
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Plug-In Data Acquisition Boards

• Key Benefits
• Lowest cost
• Large selection to choose from
• Established platform

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Example of Computer DAQ System
DAQ Board
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Inside the Case of a Computer

• It contains a system board (mother board). This
  board hosts most of the circuits connecting
• CPU,
• memory,
• I/O ports.
• buses,
• storage
• Power supplier

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Sampling Concepts

• Sampling Rate
• How often the AD conversion take place
• Multiplexing
• A common technique to measure several signals
  with a single ADC

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Aliasing
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Representing Numbers in Computers
Everyday life Decimal
Computers Binary
One-to-one correspondence
A bit has bistable states on (1) and off (0) 8
bits 1 byte
01011100 0?20 0?21 1?22 1?23 1?24
0?25 1?26 0?2792
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Representing Negative Numbers
MSB is 0 for positive numbers MSB is 1 for
negative numbers
2s complement

• Convert the integer to binary as if it were
  positive
• Invert all of the bits
• Add 1 least significant bit to produce the final
  result

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Major DAQ Components

• Multiplexers (MUX) multi-channel handling
• Analog-to-digital converters
• Digital-to-analog converters
• Simultaneous sample-and-hold subsystems

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Basics of A/D Converters

• The output has 2N possible values quantization
• N number of bits.
• Characteristics
• N determines the resolution of the output
• Input range unipolar and bipolar
• Conversion speed

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Data Acquisition Terminology

• Resolution - Determines how much different
  voltage changes can be measured
• Higher resolution ? more precise representation
  of signal
• Range - minimum and maximum voltages
• Smaller range ? more precise representation of
  signal
• Gain - Amplifies or attenuates signal for best
  fit in range
• Code width (LSB) smallest detectable change in
  voltage depend on range, resolution, and gain

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Resolution Error
Input resolution error Example A 12-bit A/D
converter has an analog input range of -10 to 10
V. Find the resolution error of the converter for
the analog input.
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Single-slope integrating A/D Converter
Analog input voltage
voltage
Integrator voltage
Input range
Time (counter)
Counter
Integrator
Comparator
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Successive-approximation converters

• Employ an interval-halving rule
• Example
• An 8-bit AD unipolar successive-approximation
  converter has an input range of 0 to 5V. An
  analog voltage of 3.15 V is applied to the input.
  Find the analog output

Reference voltage increment (?V) input span/2N
0.01953 V
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Demo AD Conversion
saturation error
Quantization error
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D/A Conversion
Convert n-digital binary word into an analog
voltage For controlling purpose