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Maximum room temperature

Discussion in 'Overclocking' started by baggins2000, Feb 18, 2009.

  1. baggins2000

    baggins2000 Guest

    What would be the recommended maximum room temperature for operating
    an AMD Phenom 9950 with a mid-size Zantec fan. I do not intend to
    overclock, but I guessed that people here would know more about the
    thermal aspects of CPU operation.
     
    baggins2000, Feb 18, 2009
    #1
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  2. baggins2000

    Paul Guest

    According to this, the max temp for the 125W 9950 is 61C.
    I'd like to dig up more info for you (like verifying in a
    processor spec document, that what is printed here is
    accurate), but the AMD web site doesn't make that easy.
    I'm not even sure there is a separate thermal spec as such.

    http://products.amd.com/en-us/DesktopCPUDetail.aspx?id=447

    Two aspects of cooling are important in a computer.
    There is the ability of the rear exhaust fan, to pull hot
    air out of the computer case. That is important, because
    otherwise heat would build up inside the case. If you
    sealed the computer case, the stuff inside would be boiled
    alive.

    The CPU fan moves heat from the CPU heatsink, into the
    internal computer case air. The CPU cooler cannot do its
    job, without a well cooled case.

    If your motherboard has a "case temperature" sensor, then
    you can get some idea of how well it is cooled.

    For example, let's do some arithmetic.

    Say the room temperature is currently 25C.

    One article I read, declared that a well cooled computer case
    (one with adequate rear exhaust fan), would allow the internal
    case temperature to reach 7C higher, or 32C. That requires
    a relatively good airflow (fan won't be silent).

    Using those assumptions, we can work out the requirements of
    the CPU heatsink. If 61C is the CPU max case temp, and 32C is the
    internal air temp, the difference is 61C - 32C. We assume the
    CPU is running at 100%, with all cores running at max. Say,
    a copy of the latest Prime95 executable with multithreading
    is running. The above product listing says the TDP of the processor
    is 125W. That is purely an estimate, and the real power can
    be exactly equal to that, or a value which is quite different.
    (For example, my current 65W TDP CPU, never draws more than
    36W, meaning it runs a lot cooler than the spec says. Still,
    we have to make some assumption, to do the arithmetic.)

    The ratio of delta_T to power in watts, is called Theta_R or
    the thermal resistance. We can work out the heatsink required
    now.
    61C - 32C
    Theta_R = ---------- = 0.23 C/W
    125W

    Now, we compare that to a Zalman CNPS9700. If the fan runs at
    full speed on that particular CPU cooler, Theta_R is 0.12 C/W.
    That means, in the above arithmetic, we meet the requirements.
    I found that number on this site - the number is no longer
    listed on the Zalman site. And many companies don't want to
    give this number, because it makes cooling design too easy.

    http://www.crazypc.com/products/50992.html

    We can also work out, what the maximum internal case temperature
    can be, using our new CNPS9700 heatsink.

    61C - X
    0.12C/W = ---------- , 61C - (0.12C/W * 125W) = X = 46C
    125W

    Since we've defined the case to be 7C hotter than the room,
    and X is 46C, we know the room is 39C. Therefore, if we use
    the fancy Zalman 9700 family cooler (like the 9700NT with four
    pin fan connector), we can allow the room temperature to hit 39C,
    and at that point the CPU will be at 61C. But, remembering as well,
    that there must be sufficient airflow through the case, such that
    the internal case air is only 7C warmer than the room.

    By now, you might be saying, "how can I figure out what fan to
    use for the case ?". Well, there is an equation for that too.
    Now, for this one, you need to know all the thermal loads inside
    the computer case. We have the 125W processor, say a 100W
    video card, two 12W hard drives, say a total of 250W. Our definition
    of a well cooled case, is 7C, which is equal to 12.6 Fahrenheit degrees.
    Plugging in the values.

    CFM = 3.16 * Watts / Delta_T_degrees_F

    CFM = 3.16 * 250W / 12.6F = 62.7 cubic feet per minute.

    Now we go shopping.

    The Newegg fan page, Advanced search option, has the ability
    to select a range of CFM. I picked this cheap fan from the
    search result.

    http://www.newegg.com/Product/Product.aspx?Item=N82E16811998124

    Fan Size 120mm
    RPM 1700rpm
    Air Flow 63.85CMF
    Noise Level 29.67dBA

    This fan meets our cubic feet per minute requirement. It allows
    the room to reach 39C while the processor runs flat out. However,
    the noise level is about 30dBA, which is a rough number for
    "man, this fan is annoying". Now, if we use a fan controller
    knob, to slow the fan down, and say run it at 70% of full speed,
    the computer case air gets warmer. Say it gets warmer by 3C,
    then we'd have to drop out maximum room temperature by 3C as
    a result. So you can trade noise, for max room temperature.
    With a slightly quieter fan (or a noisy fan with voltage
    reduction), we might take a room temp of 36C.

    Now, when my room hits that temp, I'm usually not interested in
    sitting in front of the computer, and I head out on the patio.
    You'd have to be paying me, to sweat in that.

    Another question would be, how hot is the hard drive. I cannot
    answer that one. I personally treat hard drives better than I
    treat CPUs, and would be monitoring the hard drive more closely
    than the CPU. The CPU has its own protection mechanisms - if the
    heatsink on the CPU falls off, the CPU will actually shut off
    the ATX power supply without warning (so there is no opportunity
    to save your work - the power just goes off). So the CPU will
    protect itself, if something nasty happens. The same cannot
    be said for the hard drive, which cannot defend itself.

    Note that the above "29.67dBA" noise figure is pure fantasy.
    No two people measure noise the same way, so you cannot
    compare the numbers you find in catalog listings. Instead,
    read the customer reviews and "average" out what they
    say about noise. That is a better indicator than the
    29.67 number. Still, as mentioned previously, if you
    cannot get customer reviews, sometimes you're forced to
    rely on that 29.67dBA number.

    Having written all that, perhaps now you can explain to me,
    what you plan to do. Are you going to put the computer
    in a crawl space or a closet ? Well, don't do that.
    Computers need to breathe, to stay cool.

    HTH,
    Paul
     
    Paul, Feb 18, 2009
    #2
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  3. baggins2000

    Paul Guest

    That's a good point, and something I had to fix on one of my computer cases.

    One area I didn't address, is what happens when you use multiple fans.
    The CFMs don't add together directly. Sometimes, one fan actually ends
    up working against another fan (I had air blow *backwards* through
    a hard drive cooler once - pretty funny). So I lack the technical details,
    of how to deal with the air volume versus resistance curves and the
    like. The CFM printed on the fan is not the whole story. But at least
    it gets you into the right ballpark.

    Paul
     
    Paul, Feb 18, 2009
    #3
  4. baggins2000

    baggins2000 Guest

    Thanks, for the info

    I'd would have to rate this as the best set of responses in 8yrs.
    Very informative.

    Maybe needs another post, but you mentioned Zalman fans. I have one,
    but I am not that confident that I put in on correctly.

    Do you have a recommendation on procedures for installing Zalman fans
    or just about any of these larger fans. I'm always worried I'm going
    to install this thing a little off of the chip. The first time I
    tried to install it looked like I had left a 1/4 of an inch off of the
    chip and the whole chip wasn't covered. And on one fan I'm having a
    heck of a time checking the position (Zalman CNPS 7700 ALCU).
     
    baggins2000, Feb 24, 2009
    #4
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