Talk:Density of air

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Should this page be merged with Atmospheric pressure (or contrawise)?

Duk 07:25, 28 Oct 2004 (UTC)

I suspect that they shouldn't be merged, because pressure and density are different concepts. As a quick example, please note that the density calculations require that local pressure be known. Alternately, pressure is an integration of density over altitude. However, the distinction may be more technical than useful, so if you would care to mention why you think there should be a merge, I may find myself enlightened.

Cheers, One-dimensional Tangent 00:36, 30 Oct 2004 (UTC)

Tangent, at first glance I thought these articles should be merged. Because in my mind I see the two concepts as mostly (but not completely) the same. But this is a mild opinion. The impetus was a desire to add information on various atmosphere models (MSIS, NRLMSISE ...) , but I wasn't sure were it should go. Maybe a dedicated page is better. Duk 01:45, 30 Oct 2004 (UTC)

Hmm... If you are planning on doing a page on the various models, or a page for each model, then the Density article would likely be redundant. In that case I definately see your point, and certainly have no objections.

The only thing I see in favor of keeping the Density article is that it might be simpler than MSIS-1986 and NRLMSIS-00, especially for people who only want a rough model and who don't need to know temperature, etc. However, all I know about atmospheric modelling is due to a few hours of light research, so I figure you're probably more qualified to judge this than I am.

So... I'll not take any offense at whatever you choose to do. -grin-

One-dimensional Tangent 02:40, 30 Oct 2004 (UTC)

The density formula had the wrong units, so I multiplied by M (to balance the mol and the Temperature) and divided by 1000 (for the gram to kilogram ratio). It now reads:

Density can then be calculated according to the original formula:

$\rho = \frac{p \cdot M}{1000 \cdot R \cdot T}$

However, the external link http://wahiduddin.net/calc/density_altitude.htm also looks confused with its units, the gas constant starts off as

R = gas constant , J/(kg*degK) = 287.05

but later uses:

R = 8.31432 gas constant, J/ mol*deg K

This works out as the link uses M / 1000 to correct the different units, but this article appears to have forgotten them. -Wikibob | Talk 22:21, 2005 Jan 17 (UTC)

Since, in this article, you forgot to specify the units which you end up with, that 1000 is irrelevant.

Furthermore, make sure you specify which of the several units given will be used for the pressure (pascals, hectopascals, kilopascals), and check that kelvins per kilometre and see if that denominator fits in too. (Don't remember if units for height are specified in earlier formula, either—should be kilometres, I suppose, if you are using kelvins per kilometre. Gene Nygaard 22:32, 17 Jan 2005 (UTC)

Good point, I've removed the need for the 1000 by adjusting all the units except pressure to SI base units. This means pressure is in pascals, lapse rate in kelvin per metre (looks odd), and height in metres. Although standard units, the lapse rate, height and molecular weight may look odd to some, so here is an alternative with kilometres, and grams (this brings 1000 back, but that could be removed by using kPa for pressure.)

You should point out what mean air molecular mass you are using in the number of moles -- mass conversion.

[edit] Effects of altitude

To calculate the density of air as a function of altitude, one requires additional parameters. They are listed below, along with their values according to the International Standard Atmosphere, using the universal gas constant instead of the specific one:

sea level atmospheric pressure p₀ = 101325 Pa (= 101325 (kg/m·s²) = 1013.25 mbar or hPa = 101.325 kPa)
sea level standard temperature T₀ = 288.15 K
Earth-surface gravitational acceleration g = 9.80665 m/s².
dry adiabatic lapse rate L = −6.5 K/km
universal gas constant R = 8.31432 J/(mol·K)
molecular weight of dry air M = 28.9644 g/mol

Temperature at altitude h kilometres above sea level is given by the following formula (only valid below the tropopause):

$T = T_0 + L \cdot h$

The pressure at altitude h is given by:

$p = p_0 \cdot (1 + \frac{L \cdot h}{T_0})^\frac{g \cdot M}{R \cdot -L}$

Density can then be calculated according to a molar form of the original formula:

$\rho = \frac{p \cdot M}{1000 \cdot R \cdot T}$

Finally for fun I've uploaded a javascript calculator that uses the above parameters here: http://www.geocities.com/robojojoba/air_density.html (source code in http://wikisource.org/wiki/Air_density_calculator) -Wikibob | Talk 00:24, 2005 Jan 22 (UTC)

In school i am designing a fictional planet that is completly water except for a few scattered volcanoes. Would the air be too dense for earth like animals to survive?

[edit] Relative air density

In my research on corona discharge, I find Peek's law (which I am writing). His book has a variable δ:

δ is the air density factor. It is calculated by the equation:

$\delta = {3.92 b \over 273 + t}$

where

b = pressure in centimeters of mercury
t = temperature in celsius

At SATP (25°C and 760 mmHg):

$\delta = {3.92\cdot76 \over 273 + 25} = 1$

I also come across this version of "relative air density":

$\delta = {b \over 101.3} {293 \over T}$

which looks at first glance to be the same thing with different units (kPa and K), but I am too tired to do the math. Then I find this formula which has several other variables:

RAD = Relative Air Density

BP = Barometric Pressure (inches??)

RH = Relative Humidity (percent)

T = Air Temperature (°F)

SP = Saturation Pressure (this value comes from a table included in the spreadsheet)

RAD = (1737.97 x (BP - (SP x RH/100))/(460 + T))

What is the relationship between these? Is one a standard formula that would be used in chemistry or whatever? Are b and T standard variable names for these quantities? Do the humidity values affect air density negligibly in most cases, so they are left out of the first ones? - Omegatron 02:54, August 8, 2005 (UTC)