# (-Planning Exercise - Strain gauge Coursework Help -)

1. May 2, 2004

### Jack_Legacy

(--Planning Exercise - Strain gauge Coursework Help!!!!--)

Hi ive been stuck for several days in the early stages of my coursework i would have asked my tutor but I have been forced to stay home due to a very bad chest infection

I will first provide a full transcript of the question (please view figure 1.1 and 1.2 they are attacthed to this post) before i tell you where i am stuck

Fig1.1 illustrates a crane beign used to move a load. As the distance from the load to the control cabin increases, the force in the supporting struts of the metal frame work or the crane increases. It is important to engineers to know how these forces change as the load moves.

One way of investigating the forces in a beam is to attach to it a device known as a strain gauge. A Strain gauge is illustrated in Fig 1.2

If the length of the strain gauge is increased, wires in the gauge become longer and thinner, and therefore the electrical resistance of the gauge increases.

You are required to design a laboratory experiment to investigate how the electrical resistance of a strain gauge attached to a flat surface depends on the tensile force applied to the surface.

You should draw a diagram of the arrangement of your apparatus, and in your account you should pay particular attention to:

(a) the procedure followed
(b) the material on which the strain gauge is to be mounted and the dimensions of this material *
(c) the method of attaching the strain gauge to the material *
(d) how the force would be applied
(e) the approximate range of forces which you would use
(f) how the resistance of the strain gauge would be measured, including the range of any meters used
(g) any safety precautions you would take when carrying out the investigation
(h) particular features of the desing that would ensure accuracy and reliability of your results.

I have put a star next to the ones where i am completely lost

(b) the material on which the strain gauge is to be mounted and the dimensions of this material * - I think is reffering to the plastic backing sheet if so what would be the dimensions used for the plastic backing sheet would'nt a larger backing sheet exert more force on the strain gauge.
(c) the method of attaching the strain gauge to the material * - would i have to use some form of adhesive/blu tac maybe to attatch the the strain gauge to the sheet.

I have come up with a diagram see apparatus.bmp

My procedure is simply to move the strain gauge plus weight/load accross the ruler at 0.1m intervals and measure what the resistance is at each of them. Do i need a weight or should i just use the strain gauge and the backing sheet? I plan to measure resistance with a multimeter is that correct?
if my diagram is wrong could you please provide another (doesnt have to look good )

Also ive never used a strain gauge in my life could you post a link to a site where i can see what one looks like and how it is used.

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2. May 2, 2004

3. May 3, 2004

### Jack_Legacy

thanks... atleast i know what a strain gauge looks like now

4. May 6, 2004

### km_michelle

i also need some help.. i m going yo do da same experiment as you..m please help

5. May 8, 2004

### leeez

I'm doing this as well.

I think your sposed to just use the plastic backing to stick it onto the surface. I think something like double sided sticky tape. Nothing hot, or you could damage the wires, and I don't think blu-tack is strong enough.

Regarding material, something like a metre rule, with a hole drilled at one end to put weight on?

6. May 9, 2004

### titmouse

The apparatus diagram by Jack Legacy is completely flawed. All it would be measuring is the strain on the wire due to the mass on the end of it. Moving it along the meter rule will change nothing, and the fact that it looks remotely like a crane means nothing. You are supposed to test the strain in the STRUTS of the crane, not the cable attaching the mass to to the crane. I would therefore suggest something like the diagram I have attached.
But I'm pretty sure thats not the right thing to do either.

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7. May 9, 2004

### titmouse

The pictures not terribly clear; the diagonal is acting as the strut, with the strain gauge on it. The mass can then be moved along at set intervals, taking readings from the gauge along the way.

8. May 9, 2004

### aimi

Im so stuck, i don't know how to work this web site, n i haven't a clue what a strain gauge is! help!

9. May 9, 2004

### Tosh

well the idea is that the strain gauge has to be glued so securely that the entire strain on the material beneath it is equally strained. The change in resistance in the strain gauge is relative to the strain being put on it... im sure of a lot of the other stuff

10. May 9, 2004

### Tosh

i meant that im unsure

11. May 9, 2004

### titmouse

But what do you stick it to?

It would be very helpful if someone had an idea of the size of the strain gauge, and wether it shows horizontal or vertical strain

12. May 9, 2004

### titmouse

And also on what part of the crane the strain is being measured

Come one, someone must have done this experiement, or have an idea how to do it

13. May 9, 2004

### djdunko

I am doing the design tomorrow and i have no idea what to do!! But i do know that the experiment dosn't have to have anything to do with a crane thats just there to give you a reson to be working out the strain. It needs to be a experiment that can be done in a lab so u can't use a model of a crane! My teacher told me!! I just can't work out how to attach the material to a piece of string or something to attach the load to to strech the material!!

14. May 9, 2004

### Tosh

Thanks for that ... I thout the crane was important.

Does anyone know any safety precautions?

There is an adhesive called cyanoacrylate that is commonly used. I think that it is suitable to assume that this can be made available in a school lab but im unsure.

Well your not working out the strain either ( sorry if you didn't literally mean that), you are actually workin out the increases in resistance in the strain gauge caused by the strain on the material it is attached to. The resistance increases because of the factors included in the equation [resistance= (resistivityxcross-sectional area)/length].

I'm rather stuck and mine is due in 2moro ....eeeeeeeekkkkkkkkkkkkkkkkkkkkk

15. May 9, 2004

### littlerich

HIya me too, i am completely stuck with this whole plan thing, it is absolutely stupid, me too in 4 2moro, please help

16. May 10, 2004

### titmouse

Well the day is upon us, and I havnt slept at all (damn DT coursework..grumble grumble)
Anywhoo good luck, and remember its only 5% of your total As.

And if it helps, use GLUE and a low voltage stable power supply, and GOGGLES ;)

(oh and ignore my apparatus I posted, utter nonsense it was)

Let us know how you all got on!

17. May 10, 2004

### aimi

Damn this physics!!

I feel so bad 4 you! mines due in thurs, so i got a while to think about it! I sorta know that the whole gauge stretches, even theplastic its mounted on, but no use now if youve already done your work! ope you did ok!

18. May 10, 2004

### moh51n

I am doing the same exercise and its in 4 2moro can anyone help?

19. May 10, 2004

### Byj

My Physics plan is supposed to be in tommorrow, if I don't do it, im in deep ****.

All I know is that a strain gauge has a resistance of approximately 100 ohms. It is 15mm long and inside it is a looped copper/nickel foil. Newton weights must be hanged off the end of the wire the strain gauge is attached to, we would need to plot a graph of force against resistance with various forces applied on the wire e.g fron 5-15 newtons. The actual value of resisivity of a strain gauge is 2.

For the resistance to vary, the strain gauge must be really stuck to the wire so that any change in the length of the wire, will strech the copper/nickel foil inside the strain goauge, making the foil longer, therefoer it's resistance increases. But, the change in resistance is too small to be measured with a standard classroom ohmeter, an alternative needs o be used, and I don't know what!! Something about a wheatstone bridge, anyone heard of that??

IF anyone has any details, plllleeeeeeeeeeeaaaaaaassssssseee fill me in!

20. May 10, 2004

### sbenj

you have to use something called a wheatstone bridge circuit, to measure the resistance accurately. look it up, it is too complicated to explain. look in http://zone.ni.com/devzone/conceptd.nsf/webmain/C83E9B93DE714DB08625686600704DB1?OpenDocument [Broken] and u need to attach the gauge to foil, i think. what i am worried about is how u apply the force to the foil in measurable amounts.

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