altitude vs. temperature

[rodneyb]

I am modeling an electronic heat sink specified to 10,000 ft. How should I change the pressure to handle this? Will lowering the environmental pressure correct the elevation change?
-R

 

[sailoday28]

Lowering the environmental pressure will correct for it.
However are you considering the effect of temperature changes?

 

[rodneyb]

That is the main question I am asking. Should I change the temperature effects? So say I am looking for 10k ft and 65 C.

 

[IRstuff]

So what's the issue? You're given the temperature.

TTFN

 

[JStephen]

Temperature gets colder higher up, that is the reason to check temperature effects.

 

[IRstuff]

Electronics in military equipment are not necessarily operated outside at 10,000ft. Additionally, solar load can bring the temperature up to 65ºC regardless of external temperature.

TTFN

 

[JStephen]

My point was that if temperature at altitude affected the design, it would be from colder temps. If the widget was heated or insulated, sea level conditions would control, and temp at altitude wouldn't matter.

 

[ko99]

Reducing air pressure in your model may not give the correct results (depends on your analysis tool).

You'll probably need to enter the correct air density. This can be calculated from temperature and altitude. If you don't have the formulas, you can try this little calculator:

http://www.denysschen.com/catalogue/density.asp

ko (

http://www.ecooling.biz)

 

[rodneyb]

That is really what I wanted. I needed to know the air density.. Now do you have one that adds humidity?
Thanks,
Rodney

 

[EngJW]

Tables are available of temperature, pressure, and density at increasing altitudes. They are standard values and are not constant as they fluctuate with weather changes. I saw them in an aeronautical handbook and wish my memory was better so I could tell you the name. The data comes from one of the Federal agencies, maybe NWS or NOAA.

 

[mbeychok]

.
The relationships between temperature, pressure and altitude that are used in air pollution dispersion modeling are presented in the book "Fundamentals of Stack Gas Dispersion", Fourth Edition, 2005 available at

http://www.air-dispersion.com,

and the following is a direct quote from that book:

Atmospheric pressure decreases with increasing altitude, and any air rising from the warm surface of the earth will expand as it rises to lower atmospheric pressure levels. Taking the atmospheric pressure at sea level to be 14.696 psia, we can obtain the atmospheric pressure at any altitude from this approximation:

(1) P_a = 14.696 (0.963)^a

We can obtain the temperature which will be acquired by dry air rising from sea level to any given altitude from this expression which assumes that the rising air expands adiabatically:

(2) T_a =T_s (P_a/14.696)^(k-1)/k

where:
a = altitude in 1000's of feet
P_a = atmospheric pressure at altitude a, in psia
T_a = air temperature at altitude a, in degrees R
T_s = sea level ambient temperature, in degrees R
k = 1.4 for air

Using the above relationships, one finds that from sea level to 8000 feet, the atmospheric pressure decreases 0.5 psi per 1000 feet ... and the temperature of dry air will decrease 5.5 degrees F per 1000 feet.

Milton Beychok

 

[ko99]

Rodney,

You need a psychrometric chart to find the relation between humidity and density. In most environments (5-95% humidity, 15-50C) humidity has a negligble effect on heat sink performance.

A tutorial on psychrometric charts:

https://www.uwsp.edu/it/tlrn/LOs2003/paperlo/

ko (

http://www.ecooling.biz)

 

[sailoday28]

The below NASA website might be of interest.

http://www.grc.nasa.gov/

http://WWW/K-12/airplane/atmos.html

 

[rodneyb]

Thanks,
R

 

[BrianR]

I think that although temperature decreases as altitude increases, the drop in air density has a negative effect on heatsinks. MIL-W-5088 has info on this in relation to current ratings in electrical wiring which relys on ambient air cooling and it applies a 5% derating factor at 10,000ft.

 

[ko99]

Rodney mentioned 10,000 feet at 65C, so I'm guessing his heat sink is not airborne.

His worst-case scenario might be a heat sink at the top of an electronic cabinet on a hot day with an A/C failure in a crowded computer room in La Paz, Bolivia...


ko (

http://www.ecooling.biz)

 

[BrianR]

Point taken, but the efficiency of heatsinks will be less than at sea level and I would start with at least a 5% derating factor.

 

[ko99]

Rather than a straight derating, Rodney should be able to change his model to simulate density at 10,000 feet & 65C, which is 40% less than sea level at 20C.

ko (

http://www.ecooling.biz)

 

[Heckler]

Rodney,

Here is a good article on heatsink selection. Check out the box at the bottom of the page it has derating values for heatsinks at altitude.

http://www.electronics-cooling.com/Resources/EC_Articles/JUN95/jun95_01.htm