Mech 215 Instrumentation and Measurement

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MECHANICAL MEASUREMENTS
Prof. Dr. Ing. Andrei Szuder Tel.
40.2.1.4112604 Fax. [phone] www.labsmn.pub.
ro szuder_at_labsmn.pub.ro
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Measuring Temperature (Thermocouples)
Prof. Dr. Ing. Andrei Szuder Tel.
40.2.1.4112604 Fax. [phone] www.labsmn.pub.
ro szuder_at_labsmn.pub.ro
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Temperature Sensors

• THERMOCUPLES

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Thermocouples

• If wires composed of two different metals are
  connected together to form a junction, a voltage
  is developed that depends on the temperature of
  the junction.
• This junction, when used as a temperature sensor
  is called a thermocouple.

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The Thermocouple principles

• Operation based on Seebeck effect

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Thermocouples
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• Which results in the thermoelectric emf if the
  circuit is cut in half

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How do we measure thermoelectric emf?

• Any DVM that we might use has copper (possibly
  tin-plated or nickel-plated) therminals!

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• Which is equivalent to

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The Reference Junction
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Thermocouple Calibration
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Another example
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Well yes! Here is how we can handle it
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But Can We Eliminate anIce Bath? Sure!
We can somehow measure (or maintain) the
reference temperature at the second junction and
compensate for it
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Or Even Better
We can combine the two isothermal blocks into
one. Given that J3 and J4 need to be at the same
temperature, they may be at the TREF just as well!
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Simplify, Simplify
The Law of Intermediate Metals states that a
third metal (in this case, iron) inserted between
the two dissimilar metals of a thermocouple
junction will have no effect upon the output
voltage as long as the two junctions formed by
the additional metal are at the same temperature
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Which results in thissimplification
To which we add an absolute temperature sensor
(e.g. a ther-mistor or an RTD) to give us the
reference for the JREF compensation.
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Put all together, the end circuit will look like
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Thermocouples

• Examples of different thermocouple
  construction features.

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Thermocouples
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Thermocouples

• The voltage generated by a thermocouple
  depends on the composition of the wires used to
  form the junction.

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Thermocouples
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Common Thermocouples
Seebeck Coeff uV/C
Type
Metals
Fe-Con Ni-Cr Cu-Con Pt/Rh-Pt Ni/Cr-Con Ni/Cr/Si-Ni
/Si

• Microvolt output is a tough measurement
• Type "N" is fairly new.. more rugged and higher
  temp. than type K, but still cheap

J K T S E N
50 40 38 10 59 39

• 037

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Common Thermocouples
Platinum T/Cs
Base Metal T/Cs

• All have Seebeck coefficients in MICROvolts/deg.C

• 036

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Thermocouples
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Thermocouples
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Linearization
V
Small sectors
T
Tref
Tx
0
o
2
9
3

• Polynomial Ta a V a V a V .... a V
• Nested (faster) Ta V(a V(a V(a
  .......)))))))))
• Small sectors (faster) TT bVcV
• Lookup table Fastest, most memory

0
1
2
3
9
1
2
0
3
2
0

• 035

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Extension Wires

• Possible
• problemhere

Large extension wires
Small diametermeasurementwires

• Extension wires are cheaper, more rugged, but not
  exactly the same characteristic curve as the T/C.
• Keep extension/TC junction near room temperature
• Where is most of the signal generated in this
  circuit?

• 038

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Thermocouples
Sensing Junctions
Reference Junctions
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Thermocouples
Thermopile designed and microfabricated.