Introduction to Thermocouple and Strain Gauge

1
Introduction to Thermocouple and Strain Gauge

• Dr. Chi-fu Wu
• ME 4903-Special Problem in ME
• Reference www.omega.com, www.vishay.com,
  zone.ni.com
• GTREP, Savannah
• Oct. 21, 2004

2
Outline

• How to measure temperature?
• What is thermocouple?
• How to select thermocouple?
• What is strain?
• What is strain gauge?
• How to select and install strain gauge?
• How to measure strain using strain gauge?

3
How to Measure Temperature?

• What is temperature?
• What effects may temperature cause on material or
  structure?
• Examples
• Resistance
• Volume
• Emf (Electromotive Force, Thomas Seebeck effect
  1821 from the joint of two dissimilar metals)
• Thermal radiation (astronomy, the temperature of
  a star)
• Appearance (color)
• Existing sensors
• RTD (Resistance Temperature Device), thermistor
• Thermometer (thermal expansion device, use
  mercury or organic fluid or bimetallic device)
• Thermocouple
• Infrared radiator
• Liquid crystal (change of state device)

4
Comparison of Temperature Sensors
5
What is Thermocouple?
(? is the Seebeck coefficient for small delta T)

• What problems do occur when we connect the
  thermocouple to a voltmeter or make wire
  extension ?
• Answer cold junction compensation from software
  correction.

6
Temperature Conversion Table (I)

• The type of thermocouple is associated with
  the type of metal joints.
• Constanan is a generic name of copper-nickel
  alloy. Its composition
• is not the same when used in types E, J
  and T.

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Temperature Conversion Table (II)

• Hand-held portable thermocouple readout has
  done this voltage-
• temperature conversion in the circuit for
  users.
• Have to match the right type of thermocouple
  specified in the readout.

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How to Select Thermocouple?

• Junction protection sheath or not.
• Tip size big or small
• Thermocouple type (T, J, K, E, S, R)
• Temperature range (cryogenic or high temperature)
• Corrosion (noble metal is more inert to chemical
  attack)
• Termination wire or connector
• Cost

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How to Use Thermocouple (TC)?

• The tip of TC should be buried inside the
  material and gets to the test point as close as
  possible.
• Use thermally conductive epoxy or thermal
  interface material to mount TC.
• The extension cord should match the type of TC.
  Otherwise, additional Emf errors would be
  produced at junctions.
• To reduce the noise, the TC wires can be wrapped
  with aluminum foil as a EMI shield.
• To calibrate any signal drifting, use fixed
  points such as water boiling point (100C at
  1atm) and ice melting point (0 C at 1atm) for
  calibration.

10
What is Strain?

• Strain is the amount of deformation of a body due
  to an applied force. More specifically, strain
  (e) is defined as the fractional change in
  length.

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What is Strain Gauge?

• A strain gauge, a device whose electrical
  resistance varies in proportion to the amount of
  strain in the device. The most widely used gauge
  is the bonded metallic strain gauge.

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How to Select Strain Gauge?Use Vishay Strain
Gauge as Example

• Grid length (250 0.250 inch)
• Grid pattern and tab geometry (uniaxial, biaxial,
  three-element rosette, shear pattern)
• Type of grid alloy and backing (strain gauge
  series EA, Constantan grid, polyimide backing)
• Option features protection, termination (L
  pre-attached Leads, E Encapsulated with exposed
  tabs.)
• Strain gauge resistance (350350 ohms)
• Self temperature compensation (STC 06 for
  steel substrate)

Note The S-T-C number is the approximate
thermal expansion coefficient in ppm/ deg F of
the structural material on which the
strain gage will display minimum thermal output.
http//www.vishay.com/brands/measurements_group/st
rain_gages/mm.htm
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How to Measure from Strain Gauge?

• ? ? ?C / I
• E ?? ? ??, where
• C t/2, C is the distance to the neutral axis.
• M is the bending moment PL
• I is the moment inertia for a cross section, I
  b(t3)/2
• G gauge factor, (?R/R)/(?L/L)
• The gauge factor for various alloys,
• A 2.05, K 2.1, D 3.2, P 2.0 (Vishay
  strain gauge)

• Wheatstone bridge with strain gauge R3
• When Vo is zero, R1R3R2R4
• When the strain gauge is strained, R3R3?R
• Vo

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How to Install Strain Gauge?

• Objective
• Have strain gauge to produce the same amount of
  strain as the substrate.
• Procedure
• Align the grid direction with the uniaxial
• Solder wires to termination which provides stress
  relief.
• Apply adhesive to substrate and attach strain
  gauge with clamps and rubber pressure pads.
• Cure the adhesive at room temperature or elevated
  temperature.
• What happens to our plastic substrate when cure
  at elevated temperature?
• Tape or secure wires to avoid stress from heavy
  instrument.

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Where to Install TC and SG to WTC Model? (I)
Structure of World Trade Center
WTC Model (12 x12x 24)
Floor plane

• Define the damage zone.
• Identify the failure mode truss or bracket
• Determine strains and temperatures of trusses and
  brackets.

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Where to Install TC and SG to WTC Model? (II)
Tiles (weight)
Heat
Backing with aluminum foil

• If the truss (floor) fails and causes the model
  to collapse, where should the maximum deformation
  be ?
• If the bracket fails and causes the model to
  collapse, where should the maximum deformation
  be?
• Remember that the accuracy of strain gauge is
  affected by temperature. How to minimize heating
  effect on strain gauges?

17
Data Acquisition System-NI ELVISNational
Instrument Inc., Educational Laboratory Virtual
Instrumentation Suite

• Tasks
• Follow the example document to construct the half
  bridge circuit for strain gauge.
• Tests existing NI VI programs for temperature and
  strain single-channel measurement.
• Integrate VI programs for simultaneous
  temperature and strain multi-channel
  measurements.
• Contact the lab manager
• Clint Jones at 912-966-7932

Resources http//www.vernier.com/labview/http/
/www.vernier.com/labview/nielvis.html http//www.
ni.com/academic/companion_products_vernier.htmht
tp//www.ni.com/pdf/datasheet/us/vernierflyer.pdf
http//www.vernier.com/accessories/access.html?bt
a-elvbtd-elvtemplatebasic.htmltitleProto20Bo
ard20Connectors
18
Safety Issues in WTC Model Testing

• Potential Safety Issues
• Fire caused by overheating
• Toxic fume from melting plastics
• Short circuit
• Pre-caution
• Stop heating right after the WTC model starts to
  collapse
• Break the circuit by using a switch right to stop
  heating
• Keep a good ventilation by running the exhaust
  blower or conduct the test outside the building.
• Wear safety goggle, glasses and heavy glove when
  handle the heated model.
• Do not conduct testing without supervision.