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How to Take Your [computer's] Temperature:


One thing you're guaranteed in a web-review of a CPU, a fansink, a case - any widget in the warm - is that it'll come with a table of temperature measurements. Sadly, with very rare exceptions these are about as much use as if the reviewer had sidled up to their computer & tried to stick a thermometer up its . . . . .

. . . . ahem.

Half the problem is that we don't all live in Anchorage, Alaska or Panama City - so all "absolute" temperature measurements need allowances/corrections/guesstimates for the effects of the local environment; the other half is that the equipment & techniques used to obtain such measurements are - in the main - at the Astrology end of scientific method.

Let's put it another way: Intel - who started all this by putting a thermal diode in their recent processors - allow around 5C variation, PLUS some variable "motherboard correction factor," in their published guidelines for calculating PIII efficiencies. So, frankly, all these absolute measurements you see on the web taken from uncalibrated mobo sensors, are . . . well . . . let's call 'em well-intentioned gossip.

Since we don't all have - never will have - certified calibrated thermocouples - what's needed to share meaningful information is a portable standard which any of us, anywhere, can use quickly to apply web-review-data to our real-world system - & vice versa, for those many folk who want to get some facts out there.

Fortunately, such a standard measure of efficiency exists, & is easy & cheap to measure or apply: it's called "C/W"

Two's better than one:

C/W is a portable & reliable measurement because it's relative - you don't need expensive accurate thermometers, just a pair you know agree with each other: the idea as applied here is that you use a cheap easily-available "Inside-Outside" digital thermometer hacked to provide a matched pair of remote bead-thermistors with independent digital readouts. Using these, you compare the heat going in with the heat going out. What you measure is the case temperature of the CPU & the temperature of the fluid [air, mostly] you're using to cool it, then perform a simple calculation relating these to the amount of power going into the CPU & applying - if you like - a few correction factors.

The number or range you come up with has real meaning wherever you live & whatever your system; you can easily calculate back from a published C/W number to see how well the widget in question will work in your own system. The manufacturers of stuff like fansinks usually have a C/W tucked away quietly somewhere; & you can have hours of fun seeing: a) if that 50CFM fan you've put on their innocent product is worth the fashion-challenge of living in earmuffs, & b) whether their product information belongs in the fiction shelves.


Any digital inside-outside thermometer with similar bead-thermistors acting as the two sensors can be hacked to give you a suitable C/W-measuring device. I very strongly recommend going to www.benchtest.com right now & reading their clear & thorough how-to on hacking a Radio-Shack 63-1024 "Inside-Outside" - this very cheap device works well for our purposes & is so simple to hack that even a ten-thumbed anti-solderer like me can convert one in about half-an-hour.

What you'll end up with is a pair of remote bead-thermistors each on wires about 1.7m[5'6"] long The whole point of hacking the 63-1024 is that the unencapsulated bead-thermistor connected to the upper digital readout is very small indeed - about 1.3mm [3/64"] - & on around 75mm [3"] of extraordinarily thin insulated wires of around 0.3mm - less than [1/64"] in diameter.
Two's better than one [squared]:

Taking air-temperatures with both pairs of sensors, you can see the RH device's top readout [1.3mm bead] is about 0.6C lower than the 3 others - this one always is in this range. At slightly higher temperatures - uncured geekhide or Quake in a well-cooled machine - both sensors of the RH device are around 1C lower - this is normal with cheap devices & OK so long as you know what allowances to make. The important point is that each pair agree well within 1C over the 20-40C range. They may not be perfect; but they're the same imperfect - making them perfect for relative measurement. RadioShack claim the device to be within 1C each way of absolute over its range, & I see little reason to doubt this & enjoy it as a useful bonus - here I'm using two to check against each other & test cooling strategies on dual CPU machines. The "outdoor" [unencapsulated] thermistor will read up to 70C - important in the Tropics of AMD.


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