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billsalmon May4-08 12:32 PM

strain gauge to measure wood expanasivity.
Im measuring how much wood expands in different temperatures using a strain gauge.
do i need to use a wheatstone bridge circuit schematic? or else how do i heat up the wood without spoiling the strain gauge. Actually how will i heat up the wood anyway?
Also how would i draw a diagram to show the apparatus? and how do you attach the strain gauge to the wood- is it with ps adhesive?
Your help is much needed and very much appreciated if you know.

Astronuc May4-08 01:03 PM

Have you used a strain gage before? Or do you have one in mind. I would expect the supplier has some instructions on setting up a gage, and perhaps some recommendations on what gage to use with what material.

Let me check around.

jaap de vries May4-08 02:50 PM

Yes normally a Wheatstone works however, many strain gage modules have the Wheatstone bridge build in (Labview, NI). I always had trouble calibrating strain gages or having them to do what the factory said (probably my fault). Be careful because they might be sensitive to temperatures. "Each strain gage wire material has its characteristic gage factor, resistance, temperature coefficient of gage factor, thermal coefficient of resistivity, and stability".

tosina May5-08 06:31 AM

i need help on this too...i dnt knw hw 2 start at all!!!and i dnt knw hw to draw a diagram for the apparatus and even a circuit diagram

tosina May5-08 06:35 AM

has anyone finished this??!!

Astronuc May5-08 06:57 AM

The site provided by jaap de vries is a good reference.

Here is another -
Strain Gage Installation Guide Chapter 1 & 2

It has diagrams of gages and circuits.

Now it seems unusual to measure thermal expansion with strain gages, since the traditional purpose is to measure mechanical strain which is different from thermal strain. Normally, one would match the thermal expansion coefficient of the gage with the material, which effectively eliminates (or at least minimizes) the thermal strain. In the case where one wants to measure thermal strain, then one needs a strain gage with a very low (as much as possible) thermal expansion coefficient. There are apparently strain gages for wood and concrete, but I believe these are primarily designed for measuring mechancial strain.

Presumably the wood is enclosed within an oven? Otherwise I'd recommend an extensometer, which is used in tensile test experiments.

jim mcnamara May5-08 09:15 AM

In general, if it deals with wood properties and mesasurement, it is reported at the FPL.
It is the very best place to start.

'Wood as an Engineering Material' from the Forest Products Lab in Madison WI:

Chapters, index, etc. and/or the whole book are there in pdf.

Astronuc is right: strain gages are not usually the standard way to deal with this, AFAIK.
Some other older references I have floating around:

Kubler, H.; Liang, L.; Chang, L.S. 1973. Thermal expansion
of moist wood. Wood and Fiber. 5(3): 257-267.

Weatherwax, R.C.; Stamm, A.J. 1947. The coefficients of
thermal expansion of wood and wood products. Transactions
of American Society of Mechanical Engineers. 69(44):

ASTM. 1997. Standard methods for testing small clear
specimens of timber. ASTM D143. West Conshohocken,
PA: American Society for Testing and Materials.

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