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# Specific Gravity Of Gas

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4 replies to this topic

### #1 linamus

linamus

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Posted 16 December 2008 - 01:10 AM

All,
I have this confusion over specific gravity of gas. Since it is well known that for liquid, specific gravity is density of liquid over water (1000 kg/m3), I assume that for gas, it density of gas over air (1.29 kg/m3). But somehow, there's also theory that for gas, specific gravity is Molecular Weight of gas over air (28.9 kg/kmol).

So, which is which?

### #2 ankur2061

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Posted 16 December 2008 - 02:35 AM

QUOTE (linamus @ Dec 16 2008, 02:10 AM) <{POST_SNAPBACK}>
All,
I have this confusion over specific gravity of gas. Since it is well known that for liquid, specific gravity is density of liquid over water (1000 kg/m3), I assume that for gas, it density of gas over air (1.29 kg/m3). But somehow, there's also theory that for gas, specific gravity is Molecular Weight of gas over air (28.9 kg/kmol).

So, which is which?

Linamus,

Specific gravity of gas = density of gas / density of air (at the same temperature)

Being a ratio of similar properties, the specific gravity is dimensionless.
Thus the specific gravity of a particular gas may be stated as 0.65 relative to air at 60°F. Sometimes,
specific gravity is abbreviated to gravity and may be stated as follows:
Gravity of gas = 0.65 (air = 1.00)

Calculation of specific gravity of a gas, when it is not known:

Using molecular weights, we can define the gas gravity as the ratio of the molecular weight of the gas to that of air. The molecular weight of air is usually considered to be 29.0 and therefore, the specific gravity of gas can be stated as follows:

G = Mw / 29.0

Where
G = specific gravity of gas, dimensionless
Mw = molecular weight of gas

Hope this helps.

Regards,
Ankur.

### #3 linamus

linamus

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Posted 16 December 2008 - 03:32 AM

Thanks ankur2061,

Are you saying that for specific gravity, if the gas temperature is at 60°F, specific gravity is defined by ratio of density of gas over air and if the temperature is not known, the ratio now is molecular weight of gas over MW of air?

### #4 ankur2061

ankur2061

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Posted 16 December 2008 - 04:28 AM

QUOTE (linamus @ Dec 16 2008, 04:32 AM) <{POST_SNAPBACK}>
Thanks ankur2061,

Are you saying that for specific gravity, if the gas temperature is at 60°F, specific gravity is defined by ratio of density of gas over air and if the temperature is not known, the ratio now is molecular weight of gas over MW of air?

To be explicit, specific gravity for the purpose of definition is in comparison to air with the same reference temperatures for both air and gas. When I say specific gravity of a gas is 0.65, it means that the reference temperature for both gas and air are same. This is for the purpose of understanding the concept of specific gravity of gas, and even if you omit the reference temperature values it is understood that the reference temperatures are the same.

However, calculating the specific gravity of any gas does not require the temperature as indicated in my earlier post. All it requires is the molecular weight of the gas in consideration. For example the specific gravity of pure methane gas (molecular wieght: 16.04) would be:

16.04 / 29 = 0.553

Obviously, nowhere the temperature comes into the calculation.

I have a suspicion that you are confusing the specific gravity of the gas with the absoulte density of the gas at the given pressure and temperature. Please note that the absolute density of a gas is a function of basically three parameters: Pressure, Temperature and the compressibility factor Z. The following equation calcualtes the absolute density of a real gas:

ρ = P*M / R*T*Z

where:

P = absolute pressure, kPaa
M = molecular weight, kg / kg-mole
R = gas constant, 8.3143 kPa*m3 /kg-mole*K
T = absolute temperature, K
Z = compressibility factor (dimensionless) ( =1 if the gas is ideal)

Hope this helps

You can call me "Ankur".

Regards,
Ankur.

### #5 linamus

linamus

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Posted 16 December 2008 - 10:22 PM

Ok, Ankur...
I got it!