Dual Athlon MP 1U units

Bari Ari bari at onelabs.com
Sat Jan 26 09:52:35 PST 2002

W Bauske wrote:

> Velocet wrote:
>>Whats the power dissipation of running dual 1.2 GHz Mp's? How about for
>>1.33Ghz regular athlons in non-SMP configs as comparison? (As well, how much
>>heat comes off typical power supplies to run these systems?)
> My TigerMP XP1600 duals take about 1.7amps at 125v.
> Forgot the formula to convert to btu's. Vaguely remember a factor
> of around 3.42. Not sure if that was for Watt's or VoltAmps. Assuming
> a VA is approximately a Watt, 212.5 * 3.42 = 727 btu per system. 
> At least with that you can calculate your AC load for a rack. Say 40
> 1U's per rack, 29080 btu's. A ton of AC is 12000 btu's. So, 2.5 ton's
> of AC per rack. Course, you have 40x1.7 amps going into the rack for
> a power load of 68 Amps at 125v.
> Those that know the real numbers, please correct. A VA is really around
> .7 - .8 watts, so these calculations are high by maybe 20%. Figure
> the extra allows you to plug in the switches/peripherals/servers in addition 
> to the nodes.

Power is measured in volt-amps (VA) and in watts. Both numbers are 
important in preparing wiring, power conditioning, and cooling.

A system's VA rating is a function of the voltage and amperage of a 
system. A system's watt rating is that system's VA rating multiplied by 
its "Power Factor". You can convert among amps, volts, VA, power factor, 
and watts using the following formulas:

VA = amps × volts
VA = watts ÷ power factor
watts = VA × power factor
amps = watts ÷ (volts × power factor)

"Power factor" is a number between zero and one representing the portion 
of the power drawn by a system that actually delivers energy to the 
system. A system with a power factor of one (sometimes called "unity" 
power factor) is making full use of the energy it draws. A system with a 
power factor of 0.75 is effectively using only three-quarters of the 
energy it draws. Typical PC power supplies are not power factor 
corrected and they can range from 0.7 - 0.9. Power factor corrected 
power supplies typically are rated at 0.99.

All the power consumed by a computer system must end up somewhere. For 
ordinary air-cooled systems, the place it ends up is in the surrounding 
air, in the form of heat. Every watt drawn by a system is eventually 
dissipated as heat. This tends to raise the temperature of the air in 
the room that houses the system. Some method is therefore needed to keep 
the temperature within the required range. The typical method is to 
install additional air conditioning capacity.

Air conditioner capacity is generally measured in Btu per hour (Btu/hr), 
in tons, or in KiloJoules (KJoule).

A Btu, or British thermal unit, is the amount of energy needed to change 
the temperature of one pound of water by one degree Fahrenheit.

One ton of air conditioning removes 12,000 Btu of heat energy per hour.

It is important to calculate the total thermal load of the systems you 
will be installing and determine if the existing air conditioning system 
can handle the additional load. If not, you must provide additional 
cooling capacity.

The thermal load can be determined as follows:

     Add up the wattages of all the items in the room.

     Calculate Btu/hour by multiplying the total wattage by 3.4129.

     Calculate tons of air conditioner load by multiplying wattage by 

       1 KBtu/hr = 1000 Btu/hr
       12,000 Btu/hr = 1 ton of air conditioning load

The calculations described here give results that represent the 
equipment's maximum thermal output.

Even if a system approaches its maximum rated wattage or "worst-case" 
thermal output occasionally, it is highly unlikely it will do so for 
very long. Sizing the air conditioning system for "worst-case" thermal 
output, however, helps to minimize system problems later.

Besides the computer equipment being added to a site, when calculating 
required air conditioner capacity, be sure to take into account the heat 
load from computer equipment already installed at the site, non-computer 
equipment already installed or to be installed, and other factors, such 
as solar gain, outside ambient air temperatures, and even the number of 

One thing I don't get into here is the long term reliability of the 
system based on its temperature. You can also factor in what maximum 
temperature you wish to keep the CPU die below to determine the systems 
mean time between failure (MTBF). Keeping an Athlon die under 40 deg. C 
will greatly increase its MTBF vs its specified maximum of 90 deg C.


More information about the Beowulf mailing list