Strain Gage Installation Verification

Zero-drift and zero-shift in a strain gage Wheatstone Bridge circuit can arise from a number of sources, including the gage installation itself.  The following checklist gives the recommended inspection steps immediately after the completion of lead wire attachment, but prior to the application of protective coatings.

After Leadwire Attachment

  • Cold solder joints.  Caused by failure of the entire solder joint to reach the proper soldering temperature, leadwire movement while the solder is in transition from liquid to solid state, or insufficient flux.  Solder may not “wet” the leadwire and solder tabs on the strain gage or terminal strip.  Cold joints are characterized by an uneven, “flaky” appearance and poor solder flow.

  • Solder peaks.  (otherwise known as the Hershey’s Kiss) Sharp peaks of solder are usually the result of insufficient flux (use of solder without flux, loss of flux due to excessive soldering temperature, or failure to remove rosin-core solder wire and soldering iron tip from the solder joint simultaneously).  Peaks may interfere with flux removal and environmental protection.  Large solder masses of any shape are undesirable in tests involving high acceleration or deceleration.

  • Solder bridges.  Solder tabs are often closely spaced.  Care must be taken to ensure that solder joints or excessively long leadwire strands do not form an electrical connection between tabs.  A visual inspection is usually adequate.  However, an electrical resistance check is recommended if any doubt exists.

  • Wiring errors.  Be certain that all leadwires are connected to their intended circuit locations.  Wire markers and color coding are helpful.  Electrical verification of connections is recommended for installations with long runs or large bundles of leads.  Also, in strain gage bridge circuits, wire length in all active arms of the bridge must be equal.
  • Installation resistance.  A properly installed strain gage will usually retain a nominal grid resistance within the tolerance shown on the Engineering Data Sheet supplied with it.  (Gages installed on a small radius or with a heat-curing adhesive may exceed these limits.)  The Model 1300 Gage Installation Tester is an ideal instrument for verifying the deviation of installed resistance (sum of gage grid and leadwire resistances) from the nominal value.
  • Residual flux.  Visually inspect for flux residue after cleaning with rosin solvent.  Bear in mind, in strain gage installations, there is no such thing as “No Clean Flux”.  When cleaning solvents have evaporated, the insulation resistance (leakage) between the gage grid and specimen (if electrically conductive) should be at least 10,000 megohms.  Caution:  Always use a megohm meter, like that incorporated into the Model 1300, that applies a test voltage of less than 100 Vdc.  ASTM prescribes 15Vdc.  Refer to ASTM E-251.
  • Residual moisture.  Condensation and other forms of moisture may also affect insulation resistance.  Dry the installation with warm air and recheck resistance as previously described just prior to the application of any environmental protection.
  • Zero return.  Connect the installation to a strain indicator and zero-balance.  If possible, load the specimen to produce a strain of about the same magnitude expected in the test and unload immediately.  The reading should return to within ±5me of zero.  If loading is impractical, protect the gage grid with drafting tape and gently press the grid with a soft (usually pink) rubber eraser.  Poor zero return may indicate bubbles, inclusions, or unbonded areas in the adhesive layer under the grid.
  • Stability.  A strain gage subjected to a static strain over a period of time should yield an indicated strain that is stable within a few microstrain.  Unexplained changes in indicated strain while the test specimen is under a static load may also be due to bubbles, inclusions, or unbonded areas.

Most gage installations will exhibit none of these faults if the installation techniques outlined in Micro-Measurements instructional materials and training programs are followed.  However, field applications made in harsh environments, or even laboratory installations with difficult gage locations on the test specimen, warrant special attention.  If you have any questions concerning your particular applications, our Applications Engineering Department will be pleased to assist you in establishing a program for verifying the integrity of strain gage installations.  They can be reached at (919) 365-3800 by telephone or by email at

jjohnson's picture

Jim Johnson

Technical Sales Manager