Lec TC Strain Gauge

7/31/2019 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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Lec TC Strain Gauge