lec tc strain gauge
TRANSCRIPT
-
7/31/2019 Lec TC Strain Gauge
1/18
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/ -
7/31/2019 Lec TC Strain Gauge
2/18
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?
-
7/31/2019 Lec TC Strain Gauge
3/18
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)
-
7/31/2019 Lec TC Strain Gauge
4/18
Comparison of Temperature Sensors
-
7/31/2019 Lec TC Strain Gauge
5/18
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)
-
7/31/2019 Lec TC Strain Gauge
6/18
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.
-
7/31/2019 Lec TC Strain Gauge
7/18
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.
-
7/31/2019 Lec TC Strain Gauge
8/18
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
-
7/31/2019 Lec TC Strain Gauge
9/18
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.
-
7/31/2019 Lec TC Strain Gauge
10/18
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
-
7/31/2019 Lec TC Strain Gauge
11/18
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.
-
7/31/2019 Lec TC Strain Gauge
12/18
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.
-
7/31/2019 Lec TC Strain Gauge
13/18
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
-
7/31/2019 Lec TC Strain Gauge
14/18
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
-
7/31/2019 Lec TC Strain Gauge
15/18
Structure of World Trade Center WTC Model (12 x12x 24)
Floor plane
Where to Install TC and SG to WTC Model? (I)
-
7/31/2019 Lec TC Strain Gauge
16/18
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
-
7/31/2019 Lec TC Strain Gauge
17/18
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/ -
7/31/2019 Lec TC Strain Gauge
18/18
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.