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How to Install Weldable Strain Gauges

When it comes to measuring structural stresses and load, every application is unique, and sometimes that means using a different solution.  In some cases where strain gauges are applied, it can be advantageous or even necessary to use weldable strain gauges instead of chemical bonding the strain gauges to the specimen.

Since many epoxies used to chemically bond strain gauges require specific heat and pressure for curing, it can be difficult or even impossible to use this method on large or irregular structures.  Weldable strain gauges offer an advantage in this situation as they eliminate the need to clamp and cure any epoxies for bonding. In addition, weldable gauges can be installed in a variety of environments and weather conditions which offers additional advantages over traditionally bonded strain gauges. Although weldable strain gauges are applied differently, they function in much the same way as their bondable counterparts.

To properly install a weldable strain gauge, you will need:

Always remember to have the proper safety equipment on hand, such as eye protection and gloves, as well as any PPE required by your environment.

When installing a weldable strain gauge, you must first prep the area. Unlike bondable gauges, the area does not need to be polished to a fine degree.  Simply degrease the gauge area, sand down any paint, coatings or excess debris and ensuring the area is purely metallic and free of chemicals is enough.  A clean metal surface is important to the welding process.

After the part has been cleaned, you can position the gauge.  Most weldable gauges come marked so you can align the grid properly.  When the gauge is properly aligned, spot weld the gauge on either side of the gauge along the centerlines, to hold it firmly in position and prevent shifting as the gauge is welded further.  Once secure, the gauge should be welded all around the carrier surface, as illustrated below.

Recap:

  1. Clean the area to be bonded (Remove all paints, coatings, residue and debris until the surface is smooth).
  2. Mark center-lines for the gauge location with scribe and straight edge.
  3. Place the gauge on the part, lining up the center-lines.
  4. Spot weld the gauge on each line, to secure it in place.
  5. Weld the entire perimeter of the gauge in the pattern shown above.

For more information about our strain gauging and testing services, contact Ryan Welker @ (844) 837-8797 x702.

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Strain Gauge Bridge Completion Modules

The most common strain gauges used to quantify the state of stress on a test specimen’s surface, are uniaxial and rosette gauges.  For accurate measurements of stress and strain, these uniaxial and rosette gauges are independently connected as a Wheatstone bridge in a 3-wire quarter-bridge or half-bridge arrangement.

Today, most high-end data acquisition equipment manufacturers provide signal conditioning options for collecting data from single strain gauges.  Signal conditioning for strain gauges usually includes circuitry for bridge excitation voltage, quarter and half bridge completion arrangements, and shunt calibration.

Sometimes you may need to view or collect data from a single strain gauge using a device that only has analog voltage inputs and no strain signal conditioning.  This was precisely the case when I was working with the HX711 load cell/strain amplifier.  The HX711 requires a full bridge input so I instrumented my test specimens as such.  If I needed to use a single strain gauge with the HX711, I would have had to use an external bridge completion circuit.

What are your options for measuring single strain gauges with a device that only has voltage inputs or full bridge inputs?

Option 1: Buy a commercial off the shelf bridge completion modules.

The list below gives the specifications for some available bridge completion modules.  I plan on adding more completion modules to this list for future reference, so send me any additional completion options.

Strain Gauge Completion Modules
 
Manufacturer NI VPG Campbell Scientific
Model # NI 9926, NI 9945, NI 9944 MR1-10C-129, MR1-350-130, MR1-120-133 4WFBS1K, 4WFBS350, 4WFBS120
Description 3-Wire Quarter Bridge Completion 3-Wire Quarter Bridge Completion 3-Wire Quarter Bridge Completion
Resistance 1000 Ω, 350 Ω, 120 Ω 1000 Ω, 350 Ω, 120 Ω 1000 Ω, 350 Ω, 120 Ω
Strain Gauge Connector Terminal Block Solder Tab Terminal Block
Device Connector RJ50 Solder Tab Pins and Lead Wire

Option 2: Build your own circuit.

If you are building a product or are in the strain business long term, building your own circuit may be a cost effective alternative to the potentially more expensive off the shelf bridge completion option.  I have built a few bridge completion circuits in the past.  Here is a list of things to keep in mind when designing a circuit.

  1. Use high precision, low resistance temperature coefficient resistors
  2. The voltage source used for bridge excitation should be from a stable source like a reference since the output of a Wheatstone bridge is inversely proportional to the excitation voltage Vout/Vex.
  3. Use remote sensing to compensate for errors in excitation voltage from long lead wires
  4. Amplification will increase measurement resolution and improve signal-to-noise ratio
  5. Filter data to remove external, high-frequency noise

For more information about bridge completion or our strain gauging services, contact Ryan Welker @ (844) 837-8797 x701.

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Strain Gauge Installations for Field Testing

iTestSystem Tip: Strain Gauge Rosette Analysis

Wireless Strain Measurements with iTestSystem, LabVIEW, and Arduino

Ohio University Asphalt Cracking Prediction System Project

iTestSystem Tip: Strain Gauge Rosette Analysis

When troubleshooting structural failures or validating FEA models through testing, strain gauge rosettes are used to find the full state of strain at areas of concern around the structure.  iTestSystem’s Rosette Analysis tool is used to calculate the principal strain, principal strain angle, shear strain, principal stress, and other values from strain gauge rosette data. This video shows how to use the Rosette analysis tool.

For questions about using the Rosette Analysis tool or other iTestSystem analysis tools contact Chase Petzinger.

Download your free version of iTestSystem today.

Strain Gauge Installations for Field Testing

Image1: Shaft torque strain gauge installation example for field testing

Our engineers and technicians have epoxied, soldered and spot welded strain gauges for applications ranging from high temperature exhaust systems to miniature load cell measurements. Every application requires a unique understanding of the strain measurement requirements including installation environment.

If the strain gauge installation is to survive in the field you must plan for the conditions it will undergo. Three important variables that you should account for are temperature range, liquid exposure, and potential impact forces. These variables determine the type of strain gauge, epoxy, solder, wiring, coating, and impact/wear protection to use in the application. The table below shows which variables affect your installation choices.

  Gauge Epoxy Solder Wiring Coating Covering
Temperature  
Liquid Exposure      
Impact Forces      

Table1: Strain gauge installation variables

For more information about ITM’s strain gauging services contact Ryan Welker at email: ryan.welker@itestsystem.com or phone: 1.844.837.8797 x702

ITM @ VIATC 2019: Vibration Institute Annual Training Conference

Come see us at the VIATC 2019 Exhibit Hall in Booth 33!

Ryan Matthews and Mark Yeager (CAT III Vibration Analysts) will be there to answer questions about our iTestSystem engineering measurement platform, our on-site testing services, LabVIEW consulting, and strain gauging services.

When: July 24 & 25th

Where: The VIATC 2019 conference and exhibit hall will take place at the Lexington Center, connected by a joint lobby to the Hyatt Regency Lexington.

Lexington Center
430 West Vine Street
Lexington, KY 40507

Strain Gauge Shunt Equivalent Calculations in iTestSystem

When making strain measurements it is important to perform a shunt calibration both before and after the actual measurements are acquired.  Shunt calibrations ensure accurate strain measurements by adjusting the sensitivity or gain of the data acquisition equipment to compensate for leadwire resistance and other scaling errors.

iTestSystem takes advantage of the shunt calibration circuits included in the National Instruments (NI) cDAQ strain modules.  The NI-9235, NI-9236, and NI-9237 strain modules contain an internal shunt resistor that when switched on “shunts” across one leg of the strain circuit’s wheatstone bridge.  When active, the shunt resistor offsets the strain measurement by a constant strain which is calculated using the equivalent shunt calculation.  The equivalent strain/shunt value is dependent on the strain gauge configuration, gauge resistance, shunt resistance, gauge factor, and material properties.

In the latest version of iTestSystem, we added a built-in strain gauge shunt equivalent calculator that can be accessed from the strain configuration page.  This calculator has allowed us to speed up the calibration process and eliminate hand calculation errors.

For a free trial of iTestSystem and the equivalent shunt calculation tool, contact chase.petzinger@itestsystem.com.

ITM Live Demo Series: iTestSystem with NI FieldDAQ

As a part the ITM Live Demo series, we’re excited to release this video featuring iTestSystem and National Instruments FieldDAQ™ hardware.

At ITM, we’re always on the lookout for tools that will help us take tough measurements.  When we heard that NI was introducing a new device that features IP65/67-rated ingress protection, we were very excited.  In many industrial or off-way environments, there is potential for fluids to splash or leak onto exposed hardware.  Could the NI FieldDAQ™ be a solution for these types of environments?

Watch now as I demonstrate how to use an NI FieldDAQ™ Strain Device with iTestSystem and answer the question; Will it work while submerged in water?

If you’re interested in learning more about iTestSystem or FieldDAQ™, feel free to contact one of our test specialists by e-mail at info@itestsystem.com or phone at (844) 837-8797.