Specific Gravity for gases

[cherry123]

I want to know how can we express specific gravity for gases at actual conditions.
Is it density of gas/density of air at STP
or
Is it density of gas/density of air at that pressure and temperature

Please clarify.

 

[zdas04]

Specific gravity is not pressure/temperature dependent. A gas with a SG of 0.7 will be 70% of the density that air would have at the conditions of interest. So if you calculate the density of air at 100 psig and 85F, then your 0.7 gas would be 0.7 times the calculated density at those conditions.

David

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[JLSeagull]

Example
Molecular Weight / Gas sg = 21/0.73
Pressure Temperature Density
psig °F lb/ft3
635 53 3.1 dew point
570 49 2.7 dew point
300 70 1.2
300 26 1.4 dew point
50 -32 0.31 dew point
25 -48 0.19 dew point
8 -64 0.11 dew point

 

[cherry123]

I have a case in which I know specific gr. (std conditions) of gas is 1.87
I know the density of gas at 100 Psig = 0.67 lb/ft3
Density of air at STP is = 0.0749 lb/ft3

So, actual specific gr. is = 0.67/0.0749 = 8.94

The gas is so heavy!

 

[dcasto]

NO, specific gravity is a fixed value. It is exactly the mole weight of the gas in question divided by the mole weight of air. Mole weights do not change.

Density of air at 14.696 and 60f = .076313.

You didn't give us the temperature at 100 psig, but solving backwards, the temperature would be about 400 F to give you a density of .67 lb/ft^3 at 100 psig. Of course compressibility will kick in, but we'd have to know the gas species to adjust for that.

 

[pmover]

specific gravity is defined as the ratio of gas density to air density when both are at the same temperature and pressure - referred to as real specific gravity. the "ideal" specific gravity is the ratio of molecular weights.

cherry123,
be sure the SG of the gas referenced applies the above definition or uses the criteria defined above to determine SG.

in nearly all cases i have observed, SG is defined by the ratio of a gas moleweight to that of air.

good luck!
-pmover

 

[tsenthil]

Its sound very basic but please explain me how to calculate Molecular Weight from Relative density of a particular gas. I have a case where relative density of a gas at STP is 2.94 lb/ft3. So what will be the Molecular weight?

 

[speco]

tsenthil,

This is really simpler than you think.

If you use air at MW=29, and the mol weight of your gas is proportinal to the densidy of air, then it's a simple ratio. Using dcasto's density of .076313, the mol weight is your gas is:

29 * 2.94/.076313 = 1117.24

I have a little trouble believing that your density at STP is really that high. I've never heard of a gas that heavy.

Regards,

Speco