Published on February 16, 2008
Introduction to Thermocouple and Strain Gauge: 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 Outline: 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? How to Measure Temperature?: 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) Comparison of Temperature Sensors: Comparison of Temperature Sensors What is Thermocouple?: 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) Temperature Conversion Table (I): 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. Temperature Conversion Table (II): 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. How to Select Thermocouple?: 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 How to Use Thermocouple (TC)? : 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 (100°C at 1atm) and ice melting point (0 °C at 1atm) for calibration. What is Strain?: 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. What is Strain Gauge? : 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. How to Select Strain Gauge?Use Vishay Strain Gauge as Example: 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 (350=350 ohms) Self temperature compensation (STC # = 06 for steel substrate) 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. How to Measure from Strain Gauge? : 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= 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(t^3)/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) How to Install Strain Gauge? : 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. Where to Install TC and SG to WTC Model? (I): Define the damage zone. Identify the failure mode: truss or bracket Determine strains and temperatures of trusses and brackets. Structure of World Trade Center WTC Model (12” x12”x 24”) Floor plane Where to Install TC and SG to WTC Model? (I) Where to Install TC and SG to WTC Model? (II): 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 Data Acquisition System-NI ELVISNational Instrument Inc., Educational Laboratory Virtual Instrumentation Suite: Data Acquisition System-NI ELVIS National 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.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 Safety Issues in WTC Model Testing: 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.