# Strain gauge for wood

I have a project in mind where I want to measure the pulling force of each finger while doing a climbing exercise. My original plan was to build a wooden climbing hold that used 4 load cells under a hold that looks more like 4 piano levers with the cells mounted under each lever. After a bit of research I found these strain gauges and was wondering if it's possible to mount these on the wooden levers (approx 20x80mm surface area) to make the the piece of wood itself into a load cell and get good enough data. My criteria for each load cell is a max load of 75-100kg and accuracy to be within +/-1kg. Is this possible with wood using strain gauges or should I go for a more complex build using movable parts pressing down on a load cell?

Short description: Is it possible to turn a wooden stick 20mm wide and 80mm long into a load cell capable to measure force up to 100kg using a strain gauge?

Update: Here is a image of what I had in mind:

As alephzero said, its better to use plastic, but how can I find a type of plastic and calculate the correct thickness/height that can hold 100kg without breaking or deform over time?

• the important material property is the max strain it can take without yielding/failing. That will let you design for more strain and so more resolution in the gage measurement. From that point of view aluminum would be much better than wood. Some plastics might be good too. Commented Oct 30, 2016 at 13:44
• I just need to find the right type of plastic then and also the right strain gauge for the job. By the way, can I get the same result/accuracy if I mount the strain gauge underneath to measure compression? This will make my design more compact. Commented Oct 30, 2016 at 17:54

Strain gauges on wood are useful for detecting failure conditions like cracks developing, by picking up changes in strain over time when the loading is constant.

But if you want to convert the strain into a force or stress measurement, you need to know the elastic properties of the wood. Wood is anisotropic, and the properties will vary depending on the exact position of the gauge relative to the wood grain and any "defects" like knots, etc. They will also be affected by humidity, unless you seal the wood against moisture - but doing that may make it difficult to bond the gauges to the wood.

If you want "accurate" force measurements (i.e. to within say 5% rather than 50% accuracy) a load cell attached to a statically determinate structure (so the load path from your hand to the load cell does not depend on the stiffness of the material) would be a much better design.

If you want to use strain gauges, you would be better to use a man-made material where the elastic properties are known and consistent - for example some type of plastic, if you don't want to use metal.

• Thank you for answering, I've updated the question with a sketch and more details. I think I will go with plastic like you said, but I'm not sure how to calculate the amount of plastic (and type of plastic) to use. Maybe I should just try and order some strain gauges and some plastic and start prototyping. Commented Oct 29, 2016 at 10:47
• you actually dont need to know the properties at all, just directly calibrate the device. Commented Oct 30, 2016 at 12:17
• Calibrating directly will not help you if the properties keep changing as you climb. Even if you make the bulk of it using wood, consider aluminum for a portion where you measure loads with strain gauges- incorporating a load cell into the design.
– Abel
Commented Jun 12 at 10:29