# As level coursework

hi, for my as level physics coursework i need to write a plan about investigating the expansion and contraction of wood in relation to temperature.
I think i'll use a constantan alloy strain gauge to measure the strain of the wood since i do not intend to measure a temperature range more extreme than -20 to +70 degrees centigrade, so the varying temperature should not affect the strain gauge too much. (right??) But i've been trying to research what would be the best way to heat the wood. Obviously i don't want to heat the connecting wires too much. Also i'm not sure whether i should be using a wheatstone bridge or not. i've read about it, and to be honest i don't understand.
any help much appreciated!

laura xxxxxxxxxxxxxxxxx

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dlgoff
Gold Member
Welcome to PF

Just guessing, but I would think the strain gauge would be deformed more than the wood.

I agree. That seems to be true after reading through the information.

i am stuck with the same problem.
some of my friends suggested using hotplates. but dont know if the strain gauge will withstand the heat or might the wood will burn.
i was also thinking of lowering the temperature than increasing it.
anyone else got any ideas!!

dlgoff
Gold Member
Could you put it in a box with a window then measure the length optically?

I wonder if change of humidity would have more effect than the change of temperature?

Humidity would cause changes across the grain, whereas I assume that temperature might cause a change along the grain.

However, that's sheer speculation on my part.

Could you put it in a box with a window then measure the length optically?
we need to use a strain gauge.
got any ideas on how to heat the wood

i have a fine plan!! how about u use an incubator from biology to heat ur wood??? use a wheatstonebridge circuit too due to small changes in resistance!!

sounds good. But will it keep the humidity constant as we are only meant to change the temperature if i am not mistaken

ah but use silica gel, the stuff u get in ur new shoes, to control humidity;)

I'd be thinking of measuring the strain optically, using a travelling microscope with a vernier scale. Your school/college will probably have some of these. Sorry, just saw you _have_ to use a strain gauge. Other than that I would say use as small a mass of wood as you can get away with without being silly. This way you can heat it up easier. With the humidity idea, how about go the other way? Immerse them all in water and use a water bath to heat it up, then do the measurement whilst still in the water. This way they all have the same humidity (if it's the same type of wood and soaked for same length of time). You would also need to carefully get a temperature sensor into the wood somehow. Just some thoughts.

how do you measure the change in resistance?
i was thinking of using an ohm meter parallel to the gauge and also do i need to use a small voltage across the circuit?
i am confused!! Normally don't you just take the material and measure the resistance directly using only an ohm meter??
plz reply soon!! need to finish it up

hiya,., i need help on the best way to heat the wood..
ive found this website
tml.jp/e/product/strain_gauge/list/pflw_plw_list.html
its a strain gage specially for wood,. there others on there aswell,,.
but can anyone think of a good way to heat the wood??

xx
ta