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How to Take Your [computer's] Temperature 3:


Calculations: going forwards:

It's necessary to take your temperature-readings at 100% CPU-load. There are various utilities for putting your system under test-loads: I mostly use "CPU Stability-Test" - others include the "torture-test" option in Prime95, & the running of loops of QuakeX Demo.

When your system has settled down to a steady maximal reading - usually after around an hour - note your "CPU-case" [outdoor] & "Intake-air" [indoor] readings. Measurement units used here are metric.

The basic C/W formula is: C/W = [CPU-case C - Intake-air C] Watts - this usually gives you a number in the range 0.2 - 1.5: the lower the better.

The last figure [Wattage] of your CPU can be obtained from this table; &, if you overclock your CPU, you can calculate the overclocked Wattage from the formula:

Overclocked Watts = Default Watts * ([Overclocked Mhz Default Mhz] * {[Overclocked vcore Default vcore] 2 })

Example 1: Intel Celeron 533A FC-PGA - default 533Mhz @ 1.50 vcore [17.1W] - overclocked @ 100% load to 840Mhz @ 1.65 vcore, cooled in a well ventilated case by a Molex FC-PGA Radial-Fin fansink using Circuitworks 7100 Silver Thermal Compound:

C/W = [42 C - 24.6C] 32.61 Watts = 0.53

Example 2: Intel Celeron 366 PGA - default 366Mhz @ 2.00 vcore [21.7W] - overclocked @ 100% load to 605Mhz @ 2.15 vcore, cooled in a well-ventilated case by an HP "turbocooler" [ie a real 'orb'] fansink using Circuitworks 7100 Silver Thermal Compound:

C/W = [35.4C - 20.5C] 41.45 Watts = 0.36

Compensations: H/S compound: C/W measured with this procedure is, it seems, known as: "Rj-a, C/W" - with "Rj-a" standing for "Resistance, junction [or joint] to air."

Rj-a,C/W is two C/W's added together: "Rs-a,C/W" [sink-to-air][the one we want] & "Rc-s,C/W" [case to sink] - which represents the thermal resistance of the joint itself - ie the gap-resistance minimized by the heatsink grease/compound between CPU & fansink: this last can be calculated from the formula:

Rc-s C/W = L [k * A] [where L = thickness, k = thermal conductivity, & A = area]


Example 1: for Circuitworks 7100 Silver H/S Compound, between a FC-PGA & a flat turned fansink base w/ high clamping-pressure , the calculation might be:

Rc-s = 0.003 [estimated thickness in cm] (0.07 [k centimetric, derived from manufacturers' figure of 50 [BTU/hr/in/sq ft/F] x 1.05 [FC-PGA contact-area in cm 2 ]) = 0.04; making the tested Rs-a,C/W of the Molex alone at 32W approximately = 0.49

Example 2: between a lapped polished PGA & a flat polished fansink base [low clamping pressure]:

Rc-s = 0.01 [0.07 x 6.25] = approx 0.03; making the tested Rs-a,C/W of the HP turbocooler at 41W approximately = 0.33

- as you see, the smaller the contact area of a CPU of a given wattage [the .25 process Celeron PGA has a contact-area some 6x the .18 process Celeron2], the greater the influence of high-quality h/s grease & obtaining the high clamping-pressures necessary to minimize joint-thickness.

Estimating this thickness - filled by heatsink compound - is not simple: as a guideline an ordinary handrolling cigarette-paper is approx 0.03mm thick - a possible average thickness of h/s grease between two small polished flat surfaces under high clamping-pressure. Many fansinks come with quite coarse tooling-marks on the base, making the median thickness up to 0.07mm - the very small case-top of FC-PGA Intel & Socket A AMD CPU's can be assumed to be both flat & polished.

Compensations: CPU contact-area: C/W is altered by the size of the contact-area between CPU & fansink; but not as much as many folk believe: the variable is now the "spreading resistance" or internal thermal conductivity of the material from which the fansink base is made, then conduction through the variable-length paths between the heat-source & the radiator-area - fins/pins/vanes - swept by the cooling air from the fan.

In the short term, copper/silver or other low-thermal-resistance metals are/will be used to form fansink bases or inserts - at present, of course, most fansinks are relatively crude extrusions formed from alloys optimized for the extrusion process.  The exceptionally clever design of the Molex Radial-Fin Fansinks lends itself to the use of differentially optimized materials: see our Radial Cooler Test where one is compared to a "Golden Orb." Upcoming 0.13 process CPU's like the PIV will have built-in "heat-spreaders" [ie low-resistance enlarged platforms for fansinks to contact] which'll diminish this problem.

The "ArtiCoolerCA" [developed for FC-PGA/SocketA CPU's from the original HP 'orb'] has a most interesting & intelligent design approach to equalizing & minimizing as far as possible the conduction-distance between source & vanes: this [circular] fansink has an essentially conical solid base, deeply cut into by the radial vanes.

Working out compensations for different-sized contact areas is non-trivial, since many fansinks are made of anonymous alloys; as a guide, a HP turbocooler 'orb' prototype had a tested C/W of 0.21 with a 25 sq.cm contact area, rising to 0.23 with a 6.25 sq.cm contact area [Celeron1 size]; a compensation method published at www.montac.com derived a C/W for a FC-PGA [1.05 sq.cm contact area] using the following steps:

Ratio 25 : 6.25 = 4 : 1 [for 0.2 C/W drop];

Therefore Ratio 6.25 4 = 1.5625 sq.cm [for another 0.2 C/W drop]

Ratio 1.5625 : 1.05 = 1 : 0.672

[interpolating] {[0.25 0.672] * [0.02]} = 0.00744

Therefore area 1.05 sq. cm has a C/W of 0.25744

[note - the model tested here has a calculated Rs-a,C/W of 0.32 using the same methods]

Compensations: CPU-case temperature: With the equipment used here, the measurement of "CPU-case" temperature will always be understated - simply because the resistance losses between the heat-source [the core proper] & the bead-thermistor will differ from core-to-case-top losses. If the thermistor is tightly pressed to the back of a socketed CPU these losses are over a very small physical distance - however any estimate I give here of an allowance to make is no more than that.

My original guess was perhaps 1C for a FC-PGA/Socket A & 2C for a PGA. It has been pointed out - notably by Nevin of coolingstore.com, that 8C differences across the plane of the tiny core itself can be measured. While these figures are not neccessarily related to variation in heat-loss at right angles to the core [upward to the case-top & down to the base], they suggest the difficulty of estimating a realistic compensation. As far as a fansink's C/W is concerned it is this case-top temperature which matters; we are working on a practical way to obtain this directly, & on obtaining compensation figures.

Calculations: working back to the real-world:

Given the C/W number for a fansink, you can quickly see how well it should work in your system:

CPU temp [at 100% load] = Air-Intake Temp + [Rj-a,C/W x Watts]

You'll have to find from the link, & calculate from the formula - both at the top of this page - the Wattage of your CPU at the speed & vcore you intend to run it; & to assume, if you don't have an adequate temperature-measuring system, that your intake-air temperature is around 1.5-3C above ambient [room] temperature only if you have a very well ventilated system - in normal systems, the difference could easily be 5-10C.

Again; you should make an allowance for the h/s grease - using a FC-PGA, top quality Silver Grease against a dead-flat fansink base could add 0.04 to the C/W number, ordinary silicon grease or one of the half-assed heat-curable compounds beloved of fansink manufacturers on an unpolished base could add 0.1 +

Example 1: Your room's reliably air-conditioned to 20C, you want to tell your friends you, too, have overclocked a TBird 650 to 900 [OK, you'll probably need to turn up the wick in that nice 'n easy SOFT-BIOS to 1.85 vcore], you've no special case-ventilation, & you're going to use whatever h/s grease comes with the fansink.

If you're looking at a sexy little "Chrome Orb for Socket A 1Ghz" the calculation could be (based on Thermaltake's figures, inclusive of the "high-quality Chomerics T725 compound" - ie the thing you buy has an Rj-a,C/W of 0.81):

Overclocked Watts = [32.4 ([900/650]{[1.85/1.7] 2 }) = 53.1W

CPU temp @ 100% load = [20 + [say] 5] + [0.81 x 53.1] = 68C [154.4F]

[OK, so you'll get some case-fans]:

CPU temp @ 100 % load = [20 + [say] 1.5] + [0.81 x 53.1] = 64.5C [148.1F]

[get some silver h/s grease on that ChOrb]:

CPU Temp @ 100% load = 21.5 + [0.73 x 53.1] = 60.3C [140.5F]

HELP . . . . & this thing is meant to cool a Tbird at 1Ghz? . . . at a default 48.7-freakin'-Watts: ie with an Intake-air temp of 21.5C that'll run out-of-the-box at a minimum of 60.9C [141.7F] under 100% load . . . . .

. . . . even if you turn the air-con down, for every one degree the room's cooler the CPU will be one degree cooler, no more. Playing Quake among the frosted carcasses in a butcher's cold-store at 4C the 1Ghz TBird cooled by a "Chrome Orb" would still run - according to the manufacturer's own figures - at 43.5C [110.2F] minimum.

[Read the 08/04/2000 article by Joe Citarella at www.overclockers.com on his [appalled] findings when testing the "Chrome Orb" on a TBird overclocked to 800. For a Texan view, read the contrasting piece at www.hardocp.com of 08/15/2000, where a "Golden Orb" is said to hold a TBird overclocked to 1.1Ghz [60W] at 45C].

Example 2: You read a web-review of a GOrb on a PGA366@550Mhz/2.0vcore; it shows the fansink to hold this CPU to 35C in 22C "ambient" at full load; this seems to compare real well to a test of an Alpha cooling the same CPU: have you just been saved $15+ here?

If both reviews came from the same otherwise reliable place under the same conditions, you can say it shows the GOrb does the same job on this CPU at this wattage. Strictly speaking, though, it's nonsense & cannot be compared to any other test/review:

Rj-a,C/W calculated from this above review would be [35-22][550/366]*21.7 = 0.40

Thermaltake modestly claim a Rj-a,C/W of 0.81 for the GOrb with its own h/s grease; we have tested this fansink out-of-the-box on a 32.5W PGA to have a Rj-a,C/W of 0.58;

CPU-case = ["ambient" [Intake-air] + [Rj-a,C/W * Wattage]; so, in the real world, a GOrb on a Celeron366@550/2.0vcore at 22C Intake-air will run at:

a) Thermaltake's figure:

CPU-case = 22 + [0.81*32.55] = 48.4C [119F]

b) Burningissues' figure:

CPU-case = 22 + [0.58*32.55] = 40.9C [105.6F]

. . . & I'm afraid that's it - that's the best the thing will do in those conditions; anywhere, at any time. If anything [due to our cautious test-procedure] this is an underestimate of temperature.

[As a general rule: [quality] manufacturers' C/W figures are worst-case over a wide wattage-range & taken in a poorly ventilated test-chamber; you can expect to improve substantially upon their products' efficiency where using it in a well-ventilated case, lapped, & using top-quality h/s grease. Most web-reviews are astonishingly inaccurate & always err on the low side: a general exception are figures from www.overclockers.com]

Example 3: You want to buy the fastest socketed gaming-CPU that'll work through the summer without having anything in your computer too hot to touch - say 55C [131F]. You're an avid gamer - play for at least two hours every day - living in Italy [Pisa, say] without air-conditioning: Your local store have 2 Alpha models in stock [hey, this is an example . . .] : 6035MUC & a FCPAL35MU - both with an advertised Rj-a,C/W of 0.37 - & you have lots of case-fans.

Watts = [CPU Temp C - Intake-air temp C] Rj-a,C/W [fansink + H/S grease]

You can expect the local temperature to be 38C [100F] for several days of the year; so the maximum Intake-air temp you must allow for is 39.5C.

MaxWatts = [55 - 39.5] 0.37 = 41.9

The highest-wattage FC-PGA available as of September 2000 [1 Ghz cB0 stepping] draws 33W; the TBird 850 uses 40.2W, the 900 44.6W.

The highest-speed TBird many folk can use throughout the year air-cooled by a copper-insert Alpha is the 850, with the same cooling, you can use a PIII 1Ghz. It's your call which of these you reckon to be the faster gaming chip.

[Please note: none of the above is intended as: "Anti-AMD" - but it is clear that 'bargain' CPU's may well need non-bargain cooling if you put your system under anything like full loads. Also note that both example calculations were based on the manufacturers' own figures: we have not yet tested these exact models of fansinks & have not seen a reliable review]

Calibrating your system-monitoring utility:

The best of these utilities - like MBProbe & MotherBoardMonitor [both free] allow the option of resetting the temperature measurements from the various sensors on your mobo upwards or down. Sadly, one of the most interesting [CPUCool - cheap, with an inbuilt HLT cooler for W9x, a SOFT-FSB-like utility & with really good graphical logging], cannot yet be negatively calibrated.

To give an example of how necessary calibration is; I've had to reset the readouts from the onboard Winbond monitoring chip on a Soyo6BA+IV to read 7C higher [for the CPU] & 5C lower [for the "system" temp]. These kind of errors - 12C in total if used to contrast case-air temp & CPU temp - are fairly typical of the sort of thing you can expect from an uncalibrated utility, which, sadly, are generally used as the source of the absolute temperature measurements in cooling widget web-reviews.  Even after calibration, you'll possibly find that your motherboard's sensing hardware is disturbingly non-linear, & will be passably accurate only over a very small range - typically 5C, from observations on several modern motherboards.

Cooling range:

Any fansink has a range of watts within which it will work to a tested C/W efficiency -even a model which uses its tiny 9.5CFM fan with the efficiency of the Molex FC-PGA has a limit beyond which it simply cannot cool stably. The maximum wattage it will cool is determined by several factors: radiator-size [area of fins/pins/vanes swept by the fan]; internal conductivity [how quickly heat is wicked from base to radiator]; airflow management [how efficiently what flow there is scrubs heat from the radiator]; & lastly fan-size/fanflow. To some extent, simply putting a larger/higher-flow fan on an existing fansink can improve its C/W - but this appears less true the more efficient the basic design of the fansink is - ie the better the above factors have together been optimized.

For a very brief look at this topic, see the section in our Radial Cooler Review where we examine the cooling density of three fansinks of very different design - ie how much cooling work can be done within a given volume occupied by the fansink.


Despite all the proviso's made in this piece, I believe that temperatures & C/W measures constructed from them using the techniques outlined here will be accurate well within 5% - enjoy . . . . . & please share your knowledge - there's not enough fact out there.

Notes on hardware used in this piece: From our test-data I believe the Thermaltake line to be at their efficient limit at around 40W, & the Molex FCF-PGA at just over 30W - the HP turbocooler 'orb' was designed for .35 process RISC processors running at up to 80W [with a large contact-area & very firm bolt-down clamping between CPU & cooler].

If heat cannot be conducted away quickly enough from the area of contact, through the body of the fansink to the fins/pins/vanes sitting in their local breeze, the fansink will literally choke on its own internal resistance. Playing vast quantities of air over poorly designed heatsinks made of insufficiently conductive materials is as fashionable as it is silly; our next cooling review will concentrate on case-cooling & practical solutions to getting an adequately cool environment for your system to work in.


We hope this piece has been useful & encourages other users to adopt the C/W efficiency standard: any manufacturers or distributors seeking to have cooling products thoroughly reviewed to the burningissues standard please contact: Hardwarereviews@burningissues.net  or Webmaster@burningissues.net

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copyright shoarthing September 2000 for Burningissues.net - all rights reserved


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