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Thread: help with project - strain gauges

  1. #1 help with project - strain gauges 
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    i have to plan an experiment to test the how much wood expands/contracts with temperature change. it is suggested i use strain gauge to investigate resistance change.

    i have a few queries:

    how can i heat wood with changing moisture content?

    also i have looked at strain gauges but i still don't really understand how they work, if someone could explain that would be great!!!


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  3. #2  
    Forum Masters Degree bit4bit's Avatar
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    Hi, if you are going to use an electrical strain gauge, then its resistance will also change with temperature (the stain gauge is placed onto a surface, and as the object is strained, the wire in the straingauge is stretched, changing its resistance, which is then measured by another circuit). Depending on your temperature range this may or may not be significant. I found this article:

    http://zone.ni.com/devzone/cda/tut/p/id/3642

    figure 5 shows that you can use two gauges to reduce the effects of temperature somehow.

    Are you suggesting to test pieces of wood which have different controlled levels of moisture, or are you asking how to accomodate for changes in moisture level during heating?


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    I actually don't see how a electrical strain gauge would work in this application.

    A typical use of a electrical strain gauge would be to make a load cell, say for example to measure how much weight is on a crane. You would have the strain gauge attached to a steel bolt of known material and diameter. The electrical resistance of the strain gauge would be proportional to how much the bolt stretches due to the load. The strain gauge material is made to match the bolt material. This would minimize the effect of thermal growth, since it would be nearly the same for the bolt and the strain gauge. Attaching a strain gauge to wood, you would just measure the differential expansion of the gauge and wood.

    What you want is a mechanical strain gauge.
    http://www.shambhaviimpex.com/demoun...ain-gauge.html

    You are right to be concerned about moisture. Wood will shrink and swell due to moisture, and you just want to isolate the temperature effect. Maybe you could seal the wood with some kind of moisture barrier.
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    both the articles are great, as i have little knowledge of how they work and how they can be used.

    i'm trying to keep the moisture level the same throughout the test and therefore need to find of heating the wood, without changing this.

    i know this is a lil random, but if i heated oil and placed the wood in it would this work???

    i have found that wood expands most against the direction of the grain (width ways) so should i place the strain gauge against the grain?

    thanks for your help much appreciated.
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  6. #5  
    Forum Masters Degree bit4bit's Avatar
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    Probably the best way to control moisture, is to start off with dry wood. I'm sure you can dry this in an oven/kiln, and I'd be surprised if there wasn't any chemical methods of doing it either. (using some water-absorbing chemicals). I'm not sure about treating it in hot oil. I know this is used in certain metal treatments, but it seems unlikely it'd work for wood.
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    The people who cure wood, like for gunstock blanks, wrap the blank in tar paper while it cures to keep the blank from drying out too fast causing cracking and checking. You could also varnish the ends of the grain.

    Whether you measure the grwoth along the length of the board or width, just depends on what property you want to measure. If it is a structural member you might not be concerned about swelling, but for a door frame, you would be.

    The mechanical strain gauge is just a glorified micrometer. I'd put one end of the board up against something solid, like a wall, then attach the strain gauge between the other end and the floor or wall just to measure how far the end moved.
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