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  1. #1
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    The ULTIMATE n00b question...

    I was hoping I wouldnt have to ask this did a few searches on the net and still cant get a good explenation...

    What does C/W mean?

  2. #2
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    Actually I'm not quite sure either, but I think it's a thermal resistance type of thing. I do know lower is better .

    Edit: I typed a word twice

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    yeah from what ive seen on all the reviews of product when they mention the C/W rating lower seems to be better... but what is it exactly?!?!

    Bring on the Cooling Gods!!!

  4. #4
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    Will the copy/paste Gods do?

    C/W means thermal impedance. It's 1 degree C / 1 watt.

    "First, understand what C/Ws are telling you: The difference between a heatsink with a C/W of 0.30 and 0.35 is 5 C at 100 watts under stress. At 50 watts, it's half that."
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  5. #5
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    ok... got that! but a... i still need more info sorry

  6. #6
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    In our context, C/W refers to a measure called Thermal Resistance. It is the standard unit of measure used by the heatsink industry to measure heatsink performance. Thermal resistance is the measure of a substance’s resistance to heat flowing through it. For example, the “R” value rating that you see on home insulation is actually a thermal resistance value. The higher the R value is, the greater the resistance to heat flow, and hence the better the insulation.

    When referring to heatsinks and waterblocks, you want as little thermal resistance as possible. Heatsinks with low thermal resistance allow heat to flow through faster. And since the job of a heatsink is to move heat, lower is better.

    Thermal resistance is calculated by dividing temperature change by heat dissipation. The temperature is measured in degrees Celsius. Heat dissipation is measured in watts. So C/W is Degrees Celsius per Watt.

    So lets look at that previous statement posted by johan851:

    "First, understand what C/Ws are telling you: The difference between a heatsink with a C/W of 0.30 and 0.35 is 5 C at 100 watts under stress. At 50 watts, it's half that."

    A C/W of 0.30 actually means a change in temperature of 0.30 degrees Celsius for every watt of heat that is to be dissipated. If you have 100 watts to dissipate, then you’ll have a 30 degree change in temperature within the first heatsink.

    As before, a C/W of 0.35 means a change of 0.35 degrees for every watt of heat that is to be dissipated. If you have 100 watts to dissipate, then you’ll have a 35 degree change in temperature within the second heatsink.

    The second heatsink is hotter. It’s hotter because it isn’t allowing heat to pass through as easily as the first heatsink does. Why is this bad? Because a hotter heatsink won’t transfer as much heat from the CPU as a cooler heatsink will.



    Temperature = Thermal Resistance * Heat Dissipation
    Thermal Resistance = Temperature / Heat Dissipation
    Heat Dissipation = Temperature / Thermal Resistance

  7. #7
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    Quote Originally Posted by www.overclockers.com
    To calculate what to expect for other CPUs, for every watt the CPU radiates, the heatsink will cool the core by the (C/W x watts) plus ambient temp. For example, at a fan inlet temp of 25 C, a C/W of 0.25 with a CPU radiating 50 watts means that the CPU temp will be 50 x 0.25 = 12.5 C over ambient temp, or 37.5 C.
    Edit: beaten

  8. #8
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    Graystar - I copy/pasted from www.overclockers.com. Their example is correct, no?
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  9. #9
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    That's what I figured it to be. Possibly the "c/w" stands for "celcius *over* watt"? If you take the difference in celcius and devide it by the wattage it's cooling, you get the c/w rating. That's what I've concluded anyway.

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  10. #10
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    Well damn...if you have to ask these kinds of questions then I must have done a poor job of explaining!
    Quote Originally Posted by johan851
    Graystar - I copy/pasted from www.overclockers.com. Their example is correct, no?
    Yes, the example from overclockers is correct. That’s why I referred to it.
    Quote Originally Posted by Yamiyanazz
    That's what I figured it to be. Possibly the "c/w" stands for "celcius *over* watt"? If you take the difference in celcius and devide it by the wattage it's cooling, you get the c/w rating. That's what I've concluded anyway.
    Yes, C/W is a division as per the formula:

    Thermal Resistance = Temperature / Heat Dissipation

    But when used as a rating for heatsinks, the wattage is always assumed to be 1. A C/W of 0.19 is really 0.19 / 1. So a heatsink with a C/W of 0.19 means that for every watt of heat energy applied to the heatsink, the sink will get 0.19 C hotter. If you have a 70 watt processor, then you want to multiply... ( 0.19 / 1 ) X 70 = Max temperature change in heatsink caused by 70W processor.

  11. #11
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    And that temp change is relative to ambient. So to figure out the temperature a heatsink should give you, take the C/W, multiply by processor output wattage, and add ambient. So...

    .19 x 70 + ambient = temp.

    Right?
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  12. #12
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    Quote Originally Posted by Graystar
    In our context, C/W refers to a measure called Thermal Resistance. It is the standard unit of measure used by the heatsink industry to measure heatsink performance. Thermal resistance is the measure of a substance’s resistance to heat flowing through it. For example, the “R” value rating that you see on home insulation is actually a thermal resistance value. The higher the R value is, the greater the resistance to heat flow, and hence the better the insulation.

    When referring to heatsinks and waterblocks, you want as little thermal resistance as possible. Heatsinks with low thermal resistance allow heat to flow through faster. And since the job of a heatsink is to move heat, lower is better.

    Thermal resistance is calculated by dividing temperature change by heat dissipation. The temperature is measured in degrees Celsius. Heat dissipation is measured in watts. So C/W is Degrees Celsius per Watt.

    So lets look at that previous statement posted by johan851:

    "First, understand what C/Ws are telling you: The difference between a heatsink with a C/W of 0.30 and 0.35 is 5 C at 100 watts under stress. At 50 watts, it's half that."

    A C/W of 0.30 actually means a change in temperature of 0.30 degrees Celsius for every watt of heat that is to be dissipated. If you have 100 watts to dissipate, then you’ll have a 30 degree change in temperature within the first heatsink.

    As before, a C/W of 0.35 means a change of 0.35 degrees for every watt of heat that is to be dissipated. If you have 100 watts to dissipate, then you’ll have a 35 degree change in temperature within the second heatsink.

    The second heatsink is hotter. It’s hotter because it isn’t allowing heat to pass through as easily as the first heatsink does. Why is this bad? Because a hotter heatsink won’t transfer as much heat from the CPU as a cooler heatsink will.



    Temperature = Thermal Resistance * Heat Dissipation
    Thermal Resistance = Temperature / Heat Dissipation
    Heat Dissipation = Temperature / Thermal Resistance

    This is money...

    Thanks Guys

  13. #13
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    Quote Originally Posted by johan851
    And that temp change is relative to ambient. So to figure out the temperature a heatsink should give you, take the C/W, multiply by processor output wattage, and add ambient. So...

    .19 x 70 + ambient = temp.

    Right?
    No. But that was a nice try, though.

    Thermal resistance is simply a property of the heatsink. The final performance of a heatsink depends on things like contact surface area and what kind of fan you put on it. Maybe even a few other things as well.

    If a C/W rating for a heatsink includes those factors (like the Overclockers.com C/W ratings do) THEN you can probably use that simple formula to get really close to actual performance.

  14. #14
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    I've always found these ratings confusing, if not worthless. A C/W rating is dependent on ambient temperature; Newton's Law of Cooling dictates that a hot body will cool exponentially with the temperature difference. Obviously a heatsink with a certain power load at a lower ambient temperature will stabilize at a lower temperature than one in a hotter ambient environment. I can work out the math if someone really wants me to, but I'm trying to say that the C/W rating changes with the wattage applied and the ambient temperature.

    Though I suppose an average over 20C-30C ambient and 70W-100W might be a decent representation.

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  15. #15
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    Quote Originally Posted by Restorer
    I've always found these ratings confusing, if not worthless...
    I hate to be the one to break it to you, but Newton’s laws don’t dictate anything because they’re all in the realm of classical physics. Quantum physics is the way things really are (well, at least until something else comes along.) Sorry.

    Anyways...the C/W rating is not dependent on ambient temperature; nor is it dependent on the wattage applied. That's the reason why it's so useful for comparing heatsinks, and the reason why it's the industry standard for heatsink comparisons.

    The rating itself may be confusing, but it’s use is not...just a simple direct comparison between ratings will tell you which heatsink is better. No need to do any math to figure out any sort of “true” rating. If the C/W is lower, the heatsink is better.

    So sit back and enjoy how simple C/W ratings make your life!

  16. #16
    Member jaydog's Avatar
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    Graystar, You are the king

  17. #17
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    Quote Originally Posted by Graystar
    I hate to be the one to break it to you, but Newton’s laws don’t dictate anything because they’re all in the realm of classical physics. Quantum physics is the way things really are (well, at least until something else comes along.) Sorry.
    String Theory

  18. #18
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    Quote Originally Posted by Tovas
    String Theory
    Yeah, that's what they keep saying...but they just can't get it to work! Personally, I think string theory is hangin' by a thread.

  19. #19
    Member TheWesson's Avatar
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    Anyhow, heat transfer is a linear function of the temp difference.

    That's why you can simply multiply C/W times the watts to get the [approximate] [expected] rise in C over ambient.

    You could view the formula as specifying the *equilibrium* temperature of the system - the temperature at which heat transfer out (via conduction and convection) is equal to the heat transfer in (the wattage that the CPU or other heat source is consuming.) The wattage is constant but the heat transfer rises with temperature, so C/W helps tell you at what temperature heat being generated will be equal to heat being removed.

    It's also worthy to note that C/W of a HSF depends to a large degree on the rate of airflow across the heatsink ... low airflow will contribute the bulk of the C/W in many situations.

    Finally, it is also possible to specify the C/W for a given airflow. 1.685/CFM gives a C/W figure that lets you figure the [expected] [approximate] difference between intake and outflow temperatures if you have a box with airflow through it and a heat source inside it. This doesn't really tell you the temp at any point inside the box, but it gives you some idea what to expect. Somewhat useful for thinking about case fans. Doesn't apply to heatsink fans too well; the geometry of the heatsink becomes very important there.

    the wesson

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