baggins2000 wrote:
> 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.
>
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/Deskto...il.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/Produc...82E16811998124
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
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