(--Planning Exercise - Strain gauge Coursework Help!!!!--) [Archive]

Jack_Legacy

May2-04, 12:15 PM

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 :frown:

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.

Please view fig 1.1

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

Please view 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 :redface: )

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.

Vance

May2-04, 03:17 PM

http://search.yahoo.com/search?fr=fp-pull-web-t&p=Strain+gauge

I am sure my brother whose English name is also Jack never changes his major....lol...
or he doesnot dare to do so....

Jack_Legacy

May3-04, 12:33 PM

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

km_michelle

May6-04, 04:10 AM

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

leeez

May8-04, 05:00 AM

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?

titmouse

May9-04, 09:51 AM

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

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.

titmouse

May9-04, 09:53 AM

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.

aimi

May9-04, 10:45 AM

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!

Tosh

May9-04, 10:55 AM

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

Tosh

May9-04, 10:56 AM

i meant that im unsure

titmouse

May9-04, 11:08 AM

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

titmouse

May9-04, 11:12 AM

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

djdunko

May9-04, 12:28 PM

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!!

Tosh

May9-04, 01:23 PM

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

littlerich

May9-04, 01:49 PM

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

titmouse

May10-04, 01:30 AM

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!

aimi

May10-04, 06:41 AM

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! :wink:

moh51n

May10-04, 10:30 AM

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

Byj

May10-04, 11:38 AM

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!

sbenj

May10-04, 04:00 PM

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 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.

asloversgo

May10-04, 04:23 PM

You guys are lucky! For some reason, my head of physics decided it would be a great idea for me to resit my waves/practical paper even though I got a B, so not only do I have to do this one, but I have to do the A2 one as well! And you know what? The A2 one looks so much nicer than this one...

Mine has to be done in class tomorrow or something, I duno, I wasn't listening, but there are some great sites on strain gauges if you just search "strain gauges" on google.com, within the first 6 or so pages.

Good luck guys n gals.

taurinne

May10-04, 05:15 PM

im totally stuck. i havent got a clue how strain gauge work. i only know about resistivity something to do with this formula R = pL/A.

djdunko

May10-04, 05:52 PM

Hey!!
The day is over and i have put a completely wrong answer down on the paper and handed it in and felt stupid!! I just hope that you guys and gals have sum good look and learn how to do it by your deadlines and do well!1 I have no hope now but i hope you get really good grades!! Thanks for all the help and next year when the A2 one comes out we will all probably meet again

Thanks again
See ya next year

aimi

May11-04, 05:32 AM

Well thanks! i now think i have a bit of a clue as to what to do, which is good! i am at college know doing the whole thing in rough! due in tomorrow!!

moh51n

May11-04, 05:50 AM

i don't have a clue what to do? can anyone explain a bit.

aimi

May11-04, 05:55 AM

well the strain gauge is exactly like it is in the paper! im sure all you have to do is apply a load by using a wire and pulley. when you know what you're doing its not that bad!

aimi

May11-04, 05:58 AM

but you need to make sure it wont move, just stretch. 1 thing im not sure of is the dimensions of the material, or the actual material, but as far as i can see it doesn't seem to have that much effect!!

moh51n

May11-04, 06:01 AM

do you have to use a wheatstone bridge

aimi

May11-04, 06:07 AM

oh, ive just found out maybe all you have to do is put the force on top of the gauge, but maybe this wont measure the strain. i dunno what dyu think? oh no, it says tensile stress, this is not right then is it?

i don't really know what a wheatstone bridge is and so i don't think im using 1, 4 that fact!

moh51n

May11-04, 06:10 AM

could you not use an ammeter and voltmeter to work out the resistance using R=V/I

moh51n

May11-04, 06:12 AM

if you apply the force on top of the guage then how will the gauge strech

aimi

May11-04, 06:20 AM

yeah! that would work, i think. so then what about the size of the voltage in the circuit? would it make a difference if it was low or high?

zxltony

May11-04, 06:20 AM

what about the ranges of different measurements used?

aimi

May11-04, 06:21 AM

the gauge would be compressed! i realised this wouldnt b a tensile force then! god this stuff is stressin me! bug style!

aimi

May11-04, 06:23 AM

which measurements/ the voltage and current? i think by adding a range of loads, the measurements can be made each time another is added.

aimi

May11-04, 06:24 AM

hey. i'll leave you to it. i gotta go watch a film for english so... hope i have helped. good luck with it.

Thor86

May11-04, 11:57 AM

Do we have to mention young’s modulus stress/strain?? Why cant you attach the gauge with epoxy and connects different sized masses to an end the measure the current change then work out the resistance?

Thor86

May11-04, 12:03 PM

a voltage is applied across the wires not masses I thought. I heard of that bridge thing but all I know is it makes it more sensitive

aimi

May11-04, 12:21 PM

the voltage is applied to the strain guage. potential difference over this can be mausured, also current to work out resistance (R=V/I).
Your diagram is a good idea Thor86, i thought of putting it on a table and using a pulley with the mass hanging, i reckon either would work. whatever though. i'm past caring. think i know what i'm doing - in a fasion!

the fugitive

May11-04, 02:19 PM

wot i dnt get...(by the ways hey peepz wot u lot sayin?) well basicly me got the sme exercise to do by thursday...n me ain t got a clue either...so me was wandrin ..insted of jus sayin ''me gota do this n sumbody helpp''...can sum1 ACTUALLY help lolz....if u notice yeh....everyone jus rites that me doin that me need help blah blah but no1's helpin lolz....sumbody out ther....PLEASE HELP ME! (me'll pay u if it thas wot it tkes...or u gurls can ha v lil suming else lolz) nah only messin

the fugitive

May11-04, 02:21 PM

so erm ...me biggin to understand this ****...but wot dus u use to giv a voltage suplly? wer dus the current cum from peeps? (excuse me...me reali dumb) soz

the fugitive

May11-04, 02:54 PM

k erm start from the beginning...me figured out the apparatus n the idagram...got ma voltmeter n ammeter around the strain gauge...and a wire connected to the strain gauge with mass hangin from it...me got a redin for the voltmeter n ammeter...then me add asum mor weights and got mor readins from the voltmeter n ammeter...now wot dus me do with the readings???? wots the aim/ i dnt get it proply peepz! please help me please...me cant flop this subject!!!!

Byj

May11-04, 04:45 PM

Hiiii! I put my plan in 2day. Just to make u feel a bit better 'the fugitive' dude, it only counts for 2.5% of your waves theory model, so its a load of bull really. Use a wheatstone bridge, you'll see details about this on a me82 site, R=V/I can be calculated. Every time you add extra load (more Newton weights), you'll have to measure the voltage and current, work out the resistance and tabulate all the readings. You can then plot a graph, the relationship of resistance against force would show u the change in resistance, i'd say it's directly proportional, or resistance increases when force increases most definetely.

I get my plan back on friday so I can alter it, I gave the drapht in to mark. If I learn something new, I'll give u the low down boys & girls.
Good luck and **** happens, I think we should all worry more about revising than this waste of time Plan thing. Stupid OCR examining board!

Byj

May11-04, 04:50 PM

Stick the strain gauge with an adhesive to a wire e.g constantan wire, apply force to the constantan wire hanging over a bench. Don't apply too much force or it will snap. No more than 15 Newtons or it will break under the strain !!!Dangerous!!!
You don't need the youngs modulus, this I don't think has anythins to do with youngs modulus, it's to do with electrical reisitance!

in the words of Cilla Black " Trraaarr everyone!"

n1ck_w87

May12-04, 04:53 AM

What are you meant to attch the strain guage to. A wire would be too thin, wouldn't it? Got to hand this thing in nxt week, no idea what i`m doing!!!! Can any one explain how to use a wheatstone bridge, my physics teacher is probly worse at physics than me!!!

aimi

May12-04, 06:00 AM

Byj, u r so smart cheers dude! i dont give a **** anymor tho. like u said 5% dont mean ****! L8r! x x

susie28

May12-04, 08:50 AM

i'm doing this experiment too, n i av 2 hand it in in 6 days. i havnt even started. can som1 point me in da drite direction? i have no clue. help me please.

susie28

May12-04, 08:56 AM

r u actually allowed 2 give it 2 ur teach 2 look at? wer not, my teach wont even tell me wat appartus 2 use. som 1 tell me from start 2 finish our 2 do dis, n som1 diagrams 2 wil b helpful. av 2 hand it in in 6 days. dats wen i'v got da practiacl examination. or do u no any websites i can get it from? thanx

the fugitive

May12-04, 12:07 PM

erm...fukin hell me so dumb lol...me been told to attach the strain gauge to a pice of metal....and clamp it horizontally so that its in midair n then hang weight of bout 50 kg etc n measure the resistance across the strain gauge...then a graph of resistance against force ....is that rong? but me dnt kno how big a strain gauge is...sum1 tel me plz?

jad_65_2003

May12-04, 02:00 PM

The Experiment Has Nothing To Do With A Crane

the fugitive

May12-04, 02:02 PM

yeh so can sum one plz ansa me...? how big is a strain guage? n how big shud the metal strip be? n wot metal shud be used???? plz tel me !!!!

jad_65_2003

May12-04, 02:03 PM

about 1 cm squared

the fugitive

May12-04, 02:22 PM

wot the metal or the gauge?

sbenj

May12-04, 02:52 PM

Well, aluminium is good cos its young modulus is quite low (that means it is stretchy). a wheatstone bridge is a combination of four resistors, but it is not really necessary, it just makes the measurements more sensitinve than using a multimeter on the resistance range (which is better than using an ammeter and voltmeter and doing loads of calculations), so wheatstone bridge may get an extra mark or two, but not vital. to stik the gauge to the aluminium, use cyanoacrylate. what i am not sure of yet is how to apply the force. any ideas?

sbenj

May12-04, 02:55 PM

quickly ppl, mine is in for 2morro! and i think aluminium foil would b good, as long as it doesnt rip. do u think u could apply the force by clamping the metal horizontally and loading masses on one end? or is that not a tensile force?

the fugitive

May12-04, 02:58 PM

yeh mate thats wot me doin!!!! how big is ur metal strip ???

sbenj

May12-04, 03:01 PM

well, the longer the better, and the thinner, both in width and thickness the better. so it should be quite long, very thin, and as wide as the strain gauge, which i am afraid i have no measurements for

the fugitive

May12-04, 03:05 PM

k...so that dnt reali help me but fanx neway me gota hand it in tomro too....how bout this for measurements for the metal strip: 30mm x 160mm ??? that long enuff?

sbenj

May12-04, 03:16 PM

no idea. probably. how r u gonna attatch the force?

the fugitive

May12-04, 03:17 PM

at the end of the strip...ther gna be a hole for a weight hanger to han g from it n then aply 0.1 kg weights to it one by one...is that stupid or ok?

arizonian

May12-04, 11:24 PM

Stress is a load applied to an object. Strain is the deformation of the object under stress. Pull a taffy candy strip lengthwise. The pull is the stress and the lengthening of the candy is the strain. Both can be quantified. The load divided by the cross sectional area gives pounds per square inch (psi). The (final length of the candy strip being deformed minus the original length) divided by the original length is the percentage of deformation.
Now that you know stress and strain, you need to take readings with your multimeter while applying different loads and at different locations. The different locations will change the loading on a cantilever beam suspended by a cable. Use trigonometry to calculate the loading in the cable.

On edit, I did not realize that there were 4 pages of posts. I hope this helps those that still have time. :redface:

the fugitive

May13-04, 01:07 AM

erm....ell not reali coz it all so complicated n u jus made everyone elses suggestinos that me has had (that sounded good) sound crap...n especially as me understood those ones lolzzzzzz fuk it me jus na stik to wot me originally thought

Fireball

May13-04, 02:56 AM

at the end of the strip...ther gna be a hole for a weight hanger to han g from it n then aply 0.1 kg weights to it one by one...is that stupid or ok?
Thats what I'm thinking of doing and I'm supposed to be doing it this afternoon!!!

CKLi

May13-04, 06:32 AM

Hi! Try this website to find out more about Strain gauges. www.dur.ac.uk/richard.scott/gauges.html

devil_uk

May13-04, 11:21 AM

i have untill wednesday 19th to get mine in, and i am cluless to what to do.......i know what a strain gauge is and that the crain has nothing to do with it, but i have no idear how big the gauge is, or how we have/allowed to apply the force to the material, which i have no-idear to what im using..............lol..................we are all doomed..............

Fireball

May13-04, 01:17 PM

Hi! Try this website to find out more about Strain gauges. www.dur.ac.uk/richard.scott/gauges.html
Thanks for that link, I had no idea what this was all about :confused: before but reading that sorted everything out in my head and helped me to understand the theory behind it all. i now know exactly what I have to do! :smile: :biggrin: :approve: :rofl: :smile:

ashpot86

May13-04, 01:40 PM

I have to write mine up in exam conditions tomorrow(13/5/04), I am trying to plan it now but not much luck, the question asks for any relationship between the force applied to the surface and the electrical resistance, so all that strain and stress doesn't need to be worked out I don't think. I said to use a standard metre rule made of beech wood (i think). Whenever you look for a relationshipe between two things it means plot them in a graph, so i guess that will be a good idea to include. My idea for how the strain gauge is attached to the surface, i just think using a strong adhesive will suffice. For plotting a graph you want a good range of forces to apply, however you want to keep other variables constant. The only thing I havent thought about yet is what safetly precautions to include. Ah well...

firstkings

May14-04, 06:55 AM

ibnwando

May14-04, 07:50 AM

Thanks for that site CKLI .iT WAS totally relevant!!
Im in teh same situation as well. Whats wrong with these OCR people??

m0dm0

May14-04, 09:29 AM

Done mine today. thanks for the help everyone!

marieke

May14-04, 01:26 PM

how do you attach the weight to the strain gauge and connect it to the circuit at the same time? is there a paritcular way to plug wires in a strain gauge? please answer quickly! :frown:

Morph

May15-04, 07:39 AM

hiya im doin this exp too. I think ur meant to use some kind of epoxy adhesive. But first u apply some of it to the metal ruler, then stick down a piece of felt for insulation.Then add another layer of epoxy adhesive and stick down the strain gauge. Correct me if I'm wrong people!

MightyMeanie

May15-04, 12:59 PM

ibnwando

May15-04, 09:00 PM

Is there anyone who still has tim left?? ive got a lot of tips for u:-
precautions-safety goggles,in case something snaps
use a low voltage
make sure the metal rod doesnt slip
repeat reading to get an average
air the room to prevent temperature from rising too much

James101

May16-04, 05:53 AM

Hey i am doing this experiment as well and have got to hand it in for wednesday has any1 got any idea of then range of the meters we need to use? or the dimensions of a strain gauge and the material on which u mount it????????? This is all sssssssssooooooooooooo confusing??????????????????????????????

thespacedevil

May16-04, 07:46 AM

the experiment is in two parts....

stretch a piece of wire . Add a mass, measure the resistance of the strain gauge. No wheatstone bridge, no crane. Glue (superglue) the strain gauge to the wire. keep adding masses (weights), keep measuring V and I. etc

never use super glue!!!! it will melt the strain gauge....... that the whole point of superglue..... to melt the two substances together............it will damage the gauge........trust me.....................dont use superglue

thespacedevil

May16-04, 07:50 AM

use a ruler clamped on a clamp stand............. masses atttached to the ruler................move the masses along bout 10cm and take a reading...... again move 10 cm.........another reading...........repeat experiment............draw say u gonna draw table and graph..............hey presto........u done it................caio

devil_uk

May16-04, 07:52 AM

The material seems to be at the candidates discretion, and as to the dimensions, i hav read that is only small being about 16mm long but dont quote me on that.............mine also has to be in on wednesday (i go to Joeseph Ruston college in Lincoln and got an extension to the time :smile: )
i think that i will use a standard metre rule of beech wood, and mount the stress gauge with adhesive suggested earlier in this topic called cyanoacrylate, mounted at the highest stress point, this should give sufficeint strain on the gauge for decent results, i will use a 10g weight set with a hook-base attached via string through a hole in the end of the rule to apply the strain. also i will suggest saying that ur using the quarter brige wheat-stone bridge circuit with a variable resistor for adjustment and a stress gauge in the circuit for the measurement of the resistance(obvious reasons apply..........lol).
Also the crain in the introduction to the question has no relevance to question.
Please if any one has suggestions for improvement can than post them plz?

Morph

May16-04, 08:51 AM

hi i have until wed 19th as well, I have no clue wat a strain gauge is either!- Alot of use i am to u ey! do u no wat the dimensions r meant to be? Is that meant to be the metal ruler? the strain gauge has to be as wide as the ruler rite?

Morph

May16-04, 08:53 AM

my teacher said it has to be a metal ruler instead of a beachwood one. Is that rite?

longhairedbean

May16-04, 09:09 AM

i would have thought that using any kind of ruler would be less effective cos they'd have a large cross sectional area compared to something like metal foil. Considering that it doesn't state the material you need to use, and your only measuring the extension of the strain gauge applying less force, ie less weights would be easier to manage therefore a material with a low young's modilus (is that right? i can't spell) should extend more that a material with a higher young's modilus with the same force applied. the same is true with the cross sectional area.

You'd then also be able to talk about the safety aspecs of the foil/wire snapping rather than working your arse off thinking up something totally irelivant.

on a side note, our actual practical is on wednesday and neither of my physics teachers have 'made' a hassle about buying in strain gauges yet the practical will have something to do with the planning so maybe thats where the crane comes in? i don't know but it might be.

one last thing. does anyone know if we Have to use a wheatstone bridge or will it only icrease the accuracy? is it possible to measure the resistance just by measuring the voltage and the ampage?

devil_uk

May16-04, 09:12 AM

as i said its at the candidates discretion, i'm already changing my plan i'm changing to a aluminium strip about 20mm x 150mm and i will attach the waits from the bottom with a drilled hole about 10mm from the bottom of the strip i will hange the strip vertically aso it dangles, and attack the gauge about mid way (the gauge dosnt have to be the same width i dont think..........)

longhairedbean

May16-04, 09:19 AM

a drilled hole about 10mm from the bottom

if you drill a hole then all the force will be applied on a small length of the strip. what you need to do instead is use a clip or something along the whole of the bottom of the strip so that the force is applied evenly over it. All this is just scroundging for that extra mark or two though.

Morph

May16-04, 09:41 AM

seems like aluminium is the best to use then! Yes I'm going to talk about using a wheatstoe bridge. It isnt nessecary according to other people measuring the voltage and current, But I'm talking about it just for the sake of getting an extra mark or 2. I have looked on many websites, but I cant seem to find one which shows how the gauge is connected into a wheatstone bridge circuit. Or maybe its jus me being thick and not being able to understand it! :confused:
A wheatstone bridge circuit is more accurate to use coz a) it measures really precisely b) shows that the voltage which shows up to force applied. A full wheatstone bridge is always best to use for these reasons. The other ones like half, quarter etc seem to have more disadvantages for some reason, which I cant remember which helps of course!

Morph

May16-04, 09:47 AM

according to my teacher, the actual practical has nothing to do with our planning exercise, strain gauges or cranes even!but has got somthing to do with stress and strain.
Anyone who has done theirs give us a hint will ya! pleeze!

James101

May16-04, 10:28 AM

If you attach the gauge to a ruler and then add masses to the end of the ruler how do you work out the forces on the strain gauge??

longhairedbean

May16-04, 10:33 AM

http://www.allaboutcircuits.com/vol_1/chpt_9/7.html
http://www.entran.com/gages.htm

the first of these two i found very useful, it shows how the gauge is connected but you've still got to realize that if the amage is kept constant like it is for this experiment (because your not changing the power supply) then the voltage will vary with the resistance, it also shows the full, half and quarter wheatstone bridge.

the second web site is an actual strain gauge; we were all talking about them but most of use were ignorant, this page won't help most, me included but i does show that they come in packs of 4 so are designed with a full wheatstone bridge in mind, they also show that the GF is not 2 but 140 or 155 unless i can't read it properly which is very interesting seeing as we were going on about them being only 2 or so, the resistance it shows so is useful but beyond that i can't work anything what any thing means.

can't think of any questions but can't think of anything to write either so something must be wrong...ah well

ah yep; i find the starts the hardest, how are you/how have you started writing you plan cos i can't seem to manage that.

longhairedbean

May16-04, 10:49 AM

this is a question for you morph; in the full-bridge, where are all of the strain
gauges attached to and WHY is this more sensitive than, say a quarter-bridge?doesthe full bridge only test for bending rather than streching because otherwise all of the strain gauges will read the same resistance and so the bridge will always stay balenced. this is just my reasoning so tell me if i'm wrong.

taurinne

May16-04, 04:28 PM

does any1 know how or where to connect the ohmeter and voltmeter on the strain gauge?

wolf_song

May16-04, 04:29 PM

isn't it just in a circuit, so the volt metre would be parallel

chidinma

May16-04, 05:30 PM

The Actual Practical Is On Moment And Electrolysis..actually This Site Helped Me Alot..i Just Started Mne Today...
Thanx To You All

chidinma

May16-04, 06:05 PM

i dont undertand the two experiments...i used a metre rule and attached the strain guage at the centre of the metre rule with the aid of a super glue..and attached a load under the gauge..i dont know whether it is right..we are told to use copper not aluminum as it gives much strain..im now asking..where do we fix the copper..do we put it round the gauge or what...
secondly,do we have to put down two drawings: that of the meter rule and also the second expt...i was told by ma teacher to use a voltmeter if using a wheatstone bridge is gonna be complicated.If youve done this sort of expt let me know or youve got better idea..im stuck i need to submit it on wednesday

wolf_song

May16-04, 06:13 PM

At least i'm not the only one up at this time doing the work... has anyone here finished and handed it in already?

ibnwando

May17-04, 06:26 AM

So far,it seems you guys are making a mistake :surprise: . In the planning sheet it says to investigate how the resistance of a strain guage depends on the *tensile force*
Tensile forece is:-A stretching force (tension) pulling at both ends of a component or structure along its length.

so basically ,you have to stretch the metal strip or whatever it is you are using, not bend it like in the crane .The crane was jsut given as an illustration of a forec but they specified what kind of force to look out for:-tensile force. So just to tell u guys not to bend ur metal strip cos it wont be giving u a tensile fore. heres a bit of my diagram tho.

And a strain guage is a tiny thing, as small as a post stamp so u can just stick it on your surface.

n1ck_w87

May17-04, 09:45 AM

Morph

May17-04, 10:35 AM

[QUOTE=longhairedbean]http://www.allaboutcircuits.com/vol_1/chpt_9/7.html

Well I read that the full wheatstone bridge is more sensitive on the exact same website that u mentioned. Its stated towards the end. As for how it is connected, i cant draw it 4 u at the moment my paint is ****ed :mad: so I cant draw at all which helps but I'll try & describe it to u. I am only using one strain gauge and 4 resistors. Set it up in a full bridge circuit except replace the gauges with resitors so u get r1,r2,r3,r4 and there will also be a volt meter in the middle to pick up slight changes in electrical resistance and a voltage supply. If one of the resistors experience anything, this results in an unbalance and something comes up on the voltmeter.
This probably sounds confusing :confused: but I was never good at explaining!

MightyMeanie

May17-04, 12:11 PM

never use super glue!!!! it will melt the strain gauge....... that the whole point of superglue..... to melt the two substances together............it will damage the gauge........trust me.....................dont use superglue
:frown: are you sure that will happen, as my physic's teacher hinted at using superglue :confused:, - if this is the case techniqually, would we loose marks in planning to use this as it is really just a theoretical experiment (as it were) as we arent really carrying it out... :confused: :confused: :confused: :confused: oh well i will keep it as it is for now and ask my teacher tomorow

littlerich

May17-04, 01:31 PM

ibnwando's right, thats exactly how i've set out mine, and apparently thats rite, all i need to know is where i can get more info on what to attach the strain gauge to the metal strip wiv, and what are the safety precautions, if someone wud like 2 help me wiv these 2, i will be glad to email my full plan to that specific person.

little me

May17-04, 01:58 PM

superglue shud be fine as it shouldn't actually get hot anyway. I thought of using epoxy adhesive but I think that it is not ok to use as it has give and is flexible. Safety precautions: safety goggles, nothing on floor to trip over, no water near electricity, use low current as less harmful but still be careful

little me

May17-04, 02:08 PM

i think that some of u r making it more complicated than necessary. You dont need to worry about wheatstone bridges, a voltmeter and an ammeter will be fine and calculate the resistance from V=IR.

little me

May17-04, 02:33 PM

My physics teacher has just sed that epoxy adhesive is good as it is strong but also has give which is good for this experiment! (I'm at a boardig school so my physics teacher is still here!!!)

Morph

May17-04, 03:00 PM

Hey guys,
What did u all put down for your dimensions?
This is the only thing im stuck on now.

James101

May17-04, 03:17 PM

what ranges of forces are u using????

Morph

May17-04, 03:31 PM

As they obviously havent stated, I'm sayin use anywhere between 1-10 N as anything bigger than this could result in whiplash- v dangerous!
what r u sayin about the dimensions?

sampreet

May17-04, 04:24 PM

Can't be more then 0.1mm as strain gauge will snap

tom_shardlow

May18-04, 04:05 AM

i think that some of u r making it more complicated than necessary. You dont need to worry about wheatstone bridges, a voltmeter and an ammeter will be fine and calculate the resistance from V=IR.

yeah but remember that you get awarded marks for things in your experiment that improve its accuracy and reliability

so a wheatstone bridge will give you a couple of marks id have thought

little me

May18-04, 04:24 AM

yeah but then you can put the wheatstone bridge etc into the improvements section and get marks for it there.

ibnwando

May18-04, 05:06 AM

But if the wheatstone bridge is too complicated for you tio make..one thing is to get it right then, a voltmeter and ammeter will do just nicely....
but u cud do the wheatstone bridge if u still have time..

ibnwando

May18-04, 05:10 AM

does any1 know how or where to connect the ohmeter and voltmeter on the strain gauge?

u dont need n ohmeter(whatever that is)
connect an ammeter in series and a voltmeter in parallel with the ciruit :approve:

tom_shardlow

May18-04, 05:25 AM

yeah but then you can put the wheatstone bridge etc into the improvements section and get marks for it there.

what improvements section?

are some of you actually doin this experiment? coz we dont hav to

i only hav to do the plan, with (a) to (g)

and then we have a different experiment tomorrow

ibnwando

May18-04, 05:26 AM

If you bend a strip of material, you are still applying a tensile force along its top edge. If you hang a material like in ibnwando's diagram, you will have to use a lot of force to be able to get an appreciable change in length, and therefore resistance. Bending will be much easier in a labotary situation.

I agree with you, Nick but u dont have to use a lot of force to get a change in lenght in my diagram, instead of adjusting the force, you could adjust the dimensions of the material,and it would stretch nicely. :uhh:

ibnwando

May18-04, 05:30 AM

:frown: are you sure that will happen, as my physic's teacher hinted at using superglue :confused:, - if this is the case techniqually, would we loose marks in planning to use this as it is really just a theoretical experiment (as it were) as we arent really carrying it out... :confused: :confused: :confused: :confused: oh well i will keep it as it is for now and ask my teacher tomorow

I dont think superglue will melt the strain guage cos if u do a google search, most laboraotory experments use superglue ,even in biology,when testing animals cos its a lab experiment and its only a temporary adhesive that can serve the purpose without separating the 2 surfaces in contact.