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Old 14th February, 2002, 01:57 PM
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Simulated Heat Source - Help with Physics!

Hi,

I'm doing a bit of watercooling for my Physiscs Coursework. I want to see if cooling is more/less effecient at temperatures varying from around room temperature up to about 80c.

Anyway, I need to cool something. I was thinking a resistor of some sort? They give out quite a bit of heat, right? Or perhaps I should use a peltier? But the thing I'm worried about their is killing the peltier as the temps go up.
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Old 14th February, 2002, 02:08 PM
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Peltiers are heat pumps and shouldn't be used as a heat source. They are limited in the temperature difference they can achieve without complaining.

A large wirewound resistor should provide a reasonable heat output. Manufacturers will specify the heat output produced.
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Old 14th February, 2002, 02:12 PM
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So If I get a say 25W resistor, thats how much heat it will give out right?
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Old 14th February, 2002, 03:49 PM
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In theory, yes. You'll get 25W +/- the design tolerance of the resistor, minus some heat losses due to environmental conditions and setup.
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Old 14th February, 2002, 07:41 PM
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a view of such is here

the power input is determined from the (regulated, eh) volts and amps applied

the actual is the input less all the other parasitic losses, etc etc

be cool
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Old 14th February, 2002, 08:31 PM
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I don't need to measure what the heat output is. I just need a heat output that will stay the same.

So I'll see how big a resistor I can get hold of
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Old 14th February, 2002, 08:45 PM
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jeezz
this is a Physics class ?

the resistance WILL vary with (or inversly wrt) temperature

how do you (presume to) know what the power is if you are not measuring it ?

enjoy
be cool
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Old 14th February, 2002, 08:50 PM
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dammit. I knew this wasn't quite as simple as I thought.

As heat rises so does resistance right?

All I need is a heat source that gives out the same ammount of heat at different temperatures. All I want to measure is the temperature difference between the heat source and the water.

Do you think it might be better to also measure the currant and voltage, work out the power and then take a C/W and use that as my data instead?

But not all power is going into heat is it?
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Old 14th February, 2002, 09:25 PM
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uummmmmmmm, I don't think you understand quite what you're getting into

an ongoing thread discussing the actual application of this is here

I use a nichrome wire heater cartridge whose resistance drops as it gets hotter
BUT not always the same amount, or the same rate (a real PITA)

the ONLY way to resolve this is by controlling the inputs

note also that the same heater will "respond" differently to different (regulated) power supplies
- you'll have to get an EE to explain all that

I can give you a number of past threads on this (heat) topic
also try my radiator article

be cool
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Old 14th February, 2002, 09:43 PM
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Your right, I don't think I do :S

Is this something I'm going to be able to EASILY do?

All I'm trying to show is if water is less effecient at higher temperatures than at lower temperatures at cooling.

Is their an easier way of doing this?
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Old 14th February, 2002, 09:45 PM
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And would it just be easier to use a CPU?
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Old 14th February, 2002, 10:02 PM
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pardon my French, but JESUS CHRIST !!

you are going to measure the differential heat capacity of water as a function of temperature ???

first off, look it up in a Handbook of Chemistry

it is a very, VERY, VERY small number

there is NO simple way to measure values so small

and a CPU is the worst possible heat source
did you read all of those links ?

sorry, but you've a task beyond my ability

good luck
be cool
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Old 14th February, 2002, 10:11 PM
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Okay, here is what sparked off the idea.

Watercooling a cpu. use 24c water and get 27c temps, thats a 3c delta.

If you use 80c water. WILL Your temperatures be 83c or is it less effecient at 80c and you get say a delta of 5c ?

that was my thoughts. If your saying this is stupid, then I better be thinking of something else to do.
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Old 14th February, 2002, 10:32 PM
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no no, no question is stupid

but some questions may be QUITE difficult to quantify, even if you already know the answer

what you want to do is not at all difficult
what is your budget for equipment ?

now be a sport, READ those effing links
LOOK at the test equipment lists and capabilities

you DO need a heat source
and temp measurement capability to at least hundredths of a degree
then a recirculating lab chiller for a temp controlled coolant source

be cool

read those links, eh
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Old 14th February, 2002, 10:39 PM
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dont u just hate it when somethign like this happens?

i think i got the answer tho! use a simple flame, heatiing a pan of water. now in this pan have a copper cube, this is what u will cool, keep water level at a set height all the time, and strap the waterblock to this cube
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Old 15th February, 2002, 04:14 AM
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Quote:
Originally posted by Spode
Watercooling a cpu. use 24c water and get 27c temps, thats a 3c delta.

If you use 80c water. WILL Your temperatures be 83c or is it less effecient at 80c and you get say a delta of 5c ?
The delta is constant so long as the load is not changed.

I have used a thermistor in the waterblock inlet to calculate the C/W of my own waterblocks as well as various industry leading waterblocks.
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Old 15th February, 2002, 05:36 PM
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Quote:
you are going to measure the differential heat capacity of water as a function of temperature ???

first off, look it up in a Handbook of Chemistry

it is a very, VERY, VERY small number

there is NO simple way to measure values so small
Oh it get's worse! The differential heat capacity of water is polynomial - it's of the form:

Cp = a + bT + cT^2 + dT^(-2)

The values can be found in Smith, Van Ness and Abbott "Chemical Engineering Thermodynamics", 5th edition. I'll try to remember to bring my copy down when I drop off those heatsinks.

Without very specialised equipment, it can't be measured, since it must be a completely closed system with pressure, temperature and volume known absolutely.

Quote:
Watercooling a cpu. use 24c water and get 27c temps, thats a 3c delta.

If you use 80c water. WILL Your temperatures be 83c or is it less effecient at 80c and you get say a delta of 5c ?
I doubt that the difference in delta would be large enough to measure. You definitely wouldn't be able to work out the controlling variable.

Since just about every component of Nusselt's equation reduces to a function of temperature, it might be possible to work out the change in heat transfer coefficient with changing temperature, at known fluid speed, for water in this specific system. I'd hate to try to solve the resulting mess, though!
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Old 15th February, 2002, 06:09 PM
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ahh Kaitain
I've not encountered you before
do like your comments

would you care to chew a bit on this (misnamed/hijacked) thread

be cool
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Old 15th February, 2002, 08:33 PM
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Quote:
would you care to chew a bit on this (misnamed/hijacked) thread
TVM for the link - I spotted that in an earlier post of yours. I'm impressed: you're self taught? Can I ask what made you pursue this avenue of research, and what your normal specialism is?

Not sure what I could contribute to the thread you linked to that's not already covered... it's after work on a Friday here, and I've got a head full of solvents from work. Maybe once the nasty chemicals have left the brain

Quote:
ahh Kaitain
I've not encountered you before
do like your comments
Thanks. I'm a permanent resident of Spodesabode and AOA, with neither the time nor the inclination to get involved in other forums. (I tend to neglect the work I have here, as it is )
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Old 16th February, 2002, 05:24 AM
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Kaitain
got involved a year and a half ago due to my total disgust with the foolishness of the "reviewers",
a situation unchanged today (other than Joe Citarella, for whom I write an occasional article)
- I was even banned form a site for being unrelentingly technical

I spent 20 years in engn and consulting related to composite piping - and testing
but no thermo or fluid mechanics

you touch upon a "problem" I've seen before
individuals tend to "hang out" on one, sometimes two, forums
and I, being a pickey bastard, am most interested in the comments of one or two here, another couple there, etc

I tend to use forums to solicit criticism of my thinking, so this "isolationism" makes it harder to do

be cool
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