U-Values

Suaimhneach

My mate is freaking here beside me as we speak. How do you calculate U-Values??

subway_ie

Whoa. Deja vu. Didn't you ask this before, google it and get an answer?

Suaimhneach

I thought it was an answer, but on closer inspection it was all technical and ment for builders. Its not the forumla she needs!

And I couldnt find that old thread!!

Melp!

Boston

U -Value: The rate at which energy is conducted through unit area of a side structure for a certain temperature difference between the two sides of the structure. units are Watts per meter squared per kelvin.

thats all they can ask. They might give you a time and an area and an energy but just remember the difinition and units and your grand.


divide energy by time then again by temp diff in kelvin. Though I doubt they will ask this.

Subway don't be a spanner.

subway_ie

Seamus was doing some house cleaning... it probably got swept away into the uber-physics thread. I can't help you out though, don't have a clue about u-value calculations. Maybe try googling it again? The links on this[/EMAIL] page should be of interest.

[edit]How exactly was I being a spanner Boston??[/edit]

Boston

Originally posted by subway_ie
How exactly was I being a spanner Boston??

five seconds it took me to explain it to her, telling someone to look up something like U -Value is of little help, due the the amost pathetic amounth of detail its given in the leaving (its a side note on some note of mine somewhere) If you can't contribute, then really don't.

subway_ie

Just out of interest, what subject do you think she's talking about?

Suaimhneach

Thank you for your help Boston. My friend is grateful.

Boston

Originally posted by subway_ie
Just out of interest, what subject do you think she's talking about?

Probably the one I got an A1 in for the leaving cert. Damn that really doesn't narrow it down though, does it...

No Problem Jesjes, I don't think they ask maths questions on U-values, Just remember if in doubt trust the definition and units to work it out, and most of all don't loose the head. Still works that way in thrid level.

Suaimhneach

It's for Construction Studies, as I told Subway in the PM.

Any ideas what's likely to come up, shes a tad desprate as her teacher sucks!

Boston

Originally posted by Jesjes
It's for Construction Studies, as I told Subway in the PM.

Any ideas what's likely to come up, shes a tad desprate as her teacher sucks!

Haven't a clue. Construction studies eay, awh well, looks like Subway beat me with his mind powers again.

subway_ie

Originally posted by Boston
Probably the one I got an A1 in for the leaving cert. Damn that really doesn't narrow it down though, does it...

Lol. Ouch. :rolleyes:

Jesjes - there's a link on that google search I posted (got a really slow connection here so I didn't want to go giving you advice about something I wasn't 100% sure on) - but it seems to have what you're looking for. It's a page on U-Value calculations for Construction Studies. You know, just incase it's not on Bostons A1 list and you need more than just a definition, like on the 2000 exam paper. Here's the link to the 2000 exam paper and marking scheme incase your friend wants to check it out for herself. And, just incase anybody isn't clear - you need to be able to do u-value calculations for construction studies

Edit: Forgot the link :rolleyes:

Suaimhneach

Originally posted by subway_ie
The links on [EMAIL=http://www.google.ie/search?hl=en&ie=UTF-8&q=u-value+calculations&btnG=Search&meta=cr=countryIE]this[/EMAIL] page should be of interest.

This link doesnt work. Thanks though, she's figuring it out anyway.

subway_ie

Originally posted by Jesjes
This link doesnt work. Thanks though, she's figuring it out anyway.

Link should be fixed now... I must have clicked "@ instead of http://". She might want to take a look at the actual marking scheme as well, just to see how the department did it and the marks distribution.

Suaimhneach

Were going to www.examination.ie and then comparing it to the link you gave us. Thanks again. Nearly there.

Boston

Right My answer wasn’t in-depth enough.

Looking at that paper you seem to need to first off convert conductivity into thermal resistance

Thermal Resistance= Thickness/ Conductivity

When materials are place in series, their thermal resistances are added. When materials are placed in parallel, their thermal conductances (Conductance equals conductivity multiplied by thickness) are added. The total resistance includes all of the resistances of the individual materials, as well as both the internal and external resistance of the free space. U= the reciprocal of total termal resistance.


Now this is basically Third level for moi, I’ve no idea weather or not you need all of this, but looking at that example (exam 2000) you should be able to work it out from the above.

This is what I gave you before E = U x A x Delta Temp, which you will probably need to answer part 2) of the 2000 question.

Suaimhneach

tthis is jes jes' friend thank you very very much for all the effort your puttig into this. one question though what relevance has the conductance to the resistivity when it comes to calculatig the overall u value?

Boston

Sorry, you see I don't know if things are always in series or not for your subject, if they are in parallel you add the reciprocals of resistance (just like in electronics) which is the same as adding the conductance. note conductance = 1/resistance.

Basically the one trick they threw at you(in 2000) was you had to convert from conductivity to resistance. conductance = 1/resistance = conductivity/tickness.
once you did that it was a piece of cake.

Suaimhneach

thank you, thank you, thank you, you very nice person you!!!

Boston

made a minor mistake (knew it didn't look right) its

Thermal Resistance= Thickness / Conductivity. Sorry about that, should make more sense now.

Boston

Actually hang on a second. Someone tell me is it reistance or resistivity you use for calculating U. Cause they didn't bang in all that extra data for the crack.

Edit its grand we use resistance, just scared the **** out of myself. Damn yanks and their resistivity.

On a side not I've found an error with the sample answers to the 2000 question on U value. It would seem someone in the department can't convert conductivity into resistance either.


Remember that if you get a double glazed question there are two windows, so be sure to double the tickness.

Suaimhneach

Thanks again lads.

She's sat the exam and all now. In the pub getting hammered so she is. I'm sure she did fine! But when....I get my results, I'll be sure to mention her's in building construction too!

Agh.

Boston

Did U Values come up? It's been awhile since I helped anybody around here, trolling is more fun. Anyway have a good one.