Lec TC Strain Gauge

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    Introduction to Thermocouple and Strain Gauge

    Dr. Chi-fu Wu

    ME 4903-Special Problem in MEReference: www.omega.com, www.vishay.com, zone.ni.com

    GTREP, Savannah

    Oct. 21, 2004

    http://www.omega.com/http://www.vishay.com/http://www.vishay.com/http://www.omega.com/
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    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?

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    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 twodissimilar 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)

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    Comparison of Temperature Sensors

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    What is Thermocouple?

    What problems do occur when we connect the thermocouple to a

    voltmeter or make wire extension ?

    Answer: cold junction compensation from software correction.

    ( is the Seebeck coefficient for small delta T)

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    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 testpoint 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, additionalEmf 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.

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    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 inlength.

    , is strain

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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

    1. Grid length (250 = 0.250 inch)

    2. Grid pattern and tab geometry (uniaxial, biaxial, three-element rosette, shear pattern)

    3. Type of grid alloy and backing (strain gauge series EA, Constantan grid, polyimide

    backing)

    4. Option features: protection, termination (L = pre-attached Leads, E= Encapsulated withexposed tabs.)

    5. Strain gauge resistance (350=350 ohms)

    6. Self temperature compensation (STC # = 06 for steel substrate)

    Gauge selection

    http://www.vishay.com/brands/measurements_group/strain_gages/mm.htm

    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.

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    How to Measure from Strain Gauge?

    Wheatstone bridge with strain gauge R3

    When Vo is zero, R1*R3=R2*R4

    When the strain gauge is strained,

    R3=R3+R

    Vo=

    1

    2 3 1 4

    RR

    RRRR

    2 3 2 4

    13 s

    RRRR

    RRG

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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 rubberpressure 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.

    Substrate

    WiresTermination

    Strain gauge

    Adhesive

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    Structure of World Trade Center WTC Model (12 x12x 24)

    Floor plane

    Where to Install TC and SG to WTC Model? (I)

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    Where to Install TC and SG to WTC Model? (II)

    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?

    Backing with aluminum foil

    Tiles (weight)

    Heat

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

    Tasks:

    Fo

    l

    l

    o

    w

    t

    h

    e

    e

    x

    am

    p

    l

    e

    d

    oc

    Resources:

    http://www.vernier.com/labview/

    http://www.vernier.com/labview/nielvis.html

    http://www.ni.com/academic/companion_products_vernier.htm

    http://www.ni.com/pdf/datasheet/us/vernierflyer.pdf

    http://www.vernier.com/accessories/access.html?bta-elv&btd-elv&template=basic.html&title=Proto%20Board%20Connectors

    BTA interface for Veriner TC (K type)

    http://www.vernier.com/accessories/access.html?bta-elv&btd-elv&template=basic.html&title=Proto%20Board%20Connectorshttp://www.vernier.com/labview/http://www.vernier.com/labview/nielvis.htmlhttp://www.ni.com/academic/companion_products_vernier.htmhttp://www.ni.com/pdf/datasheet/us/vernierflyer.pdfhttp://www.vernier.com/accessories/access.html?bta-elv&btd-elv&template=basic.html&title=Proto%20Board%20Connectorshttp://www.vernier.com/accessories/access.html?bta-elv&btd-elv&template=basic.html&title=Proto%20Board%20Connectorshttp://www.vernier.com/accessories/access.html?bta-elv&btd-elv&template=basic.html&title=Proto%20Board%20Connectorshttp://www.vernier.com/accessories/access.html?bta-elv&btd-elv&template=basic.html&title=Proto%20Board%20Connectorshttp://www.vernier.com/accessories/access.html?bta-elv&btd-elv&template=basic.html&title=Proto%20Board%20Connectorshttp://www.vernier.com/accessories/access.html?bta-elv&btd-elv&template=basic.html&title=Proto%20Board%20Connectorshttp://www.ni.com/pdf/datasheet/us/vernierflyer.pdfhttp://www.ni.com/academic/companion_products_vernier.htmhttp://www.vernier.com/labview/nielvis.htmlhttp://www.vernier.com/labview/
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    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.

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