The life and times of a strain gauge.
It’s common to hear unqualified statements that strain gauges fail or fall off. This can lead to rumours that gauges are unreliable, but in fact installation failures are caused by insufficient planning, incorrect products or procedures, poor workmanship, a lack of understanding, incomplete information, or environmental attack not considered when defining the specification. Installation life can be reasonably predicted and therefore maximised though good practice.
Life in excess of 30 years can be achieved by following these simple steps:
1. Understand the specification of the test – material to bond to, installation and operational environment, strain level and number of cycles. This will enable you to select the appropriate products and processes.
2. Select the proper surface preparation technique – refer to our surface preparation bulletin, B-129.
3. Follow the adhesive instructions carefully – storage (shelf and pot life), mixing, application, clamping and cure/postcure requirements.
4. Correct use of a soldering iron to avoid dry joints, keep joints as small as possible, and thorough cleaning of the flux residue upon completion.
5. Ensure that the appropriate protective coating adheres to the installation by ensuring it’s clean, and overlap the adhesive line by at least 20mm if space allows. Etch or prime leadwires for optimal bonding.
6. Thorough visual inspection at every stage, looking for lumps or voids in the adhesive, solder spikes/dry joints, contamination, cable anchoring, coating integrity. If in doubt, start again!
7. Robust electrical tests – resistance/deviation, insulation in excess of 10Gohms, stability at load and excellent zero return. Do this before and after applying the protective coating.
8. Inert gas purge and hermetic sealing is recommended for the best guarantee of long life in extreme environments.
It should be added that good technique must be implemented while using strain gauge-certified products such as residue-free degreasers, high-quality gauge handling tapes, uncontaminated cleaning supplies, and ensure it’s all in date.
Saving small amounts of money during installation will often lead to expensive failures.
VPG Micro-Measurements is a one-stop shop for everything you need to be successful, including comprehensive practical strain gauge installation workshops and free technical advice from the most experienced strain measurement experts in the industry.