For a vapor, the SG is simply the ratio of the molecular weight of the gas to the molecular weight of air. It is dimensionless and so is not dependent on the system of units you choose to use.
I thought it was the density of the vapor versus the density of air, is this not the case? Either way, I would assume dry air at standard conditions, correct? If so, what IS the appropriate value of dry air at standard conditions, in USC units?
Air @ 60 deg F 14.7 psia would be approx. 0.076 lb/cu ft.
See 'Control Valve Handbook' Chapter 10 Engineering Data, and Chapter 12 Conversions and Equivalents, for other useful properties of air. CVH published formerly by Fisher Controls, now Emerson Process Management, in 3.1MB pdf.
Thanks to all that posted a reply, this helps a lot. I wouldn't have put two and two together about the molecular weight and density being directly proportional. I would have thought that the density would be affected by pressure on compressible fluids, and molecular weight would not be affected. I am an electrical guy, and these concepts are very fuzzy for me.
Maybe I am wrong, but I don't thing that the Specific Gravity of a vapor is ratio of the gas MW over the MW of air.
I think it is the ratio of the vapor density at its operating conditions (diferent Press and Temp) over the density of the air at Normal or Standard conditions...
homayun, the MW ratio may not be logical, but it is the definition. I prefer to avoid all equations that are formulated around a vapor SG. It is much better to work with proper dimensions like mass density, compressibility etc.
Gas specific gravity isn't necessarily the most useful when considering fuel gases. There it is the relative density that is often used, that is, the ratio of the test gas density to the density at the same conditions of a reference gas.
In other words, the (MW of the test gas)/(MW reference gas).
Melon,
The ratio of molecular weights goes back to the ideal gas laws.
One (gram) mole of any ideal gas displaces 22.4 liters at standard conditions.
So if you have 22.4 liters of a gas with a molecular weight of 15, and 22.4 liters of air with an effective molecular weight of 29, the density of the light gas is 15G/22.4L and the density of air is 29g/22.4L.
The SG of the gas is (15/22.4)/(29/22.4)
Cancel the 22.4s and you get 15/29; the ratio of the molecular weights.
I don't really have a choice on using specific gravity. I am trying to calculate the maximum flow through a regulator at wide-open conditions. Fisher controls has a website full of equations dedicated to this:
They use specific gravity, but I want to be clear on how to calculate it. I think that is what athomas236 was meaning when they said, it depends on what info you know to start with.
JimCasey:
My problem with your info is that gas is a compressible fluid, i.e. a gas under standard conditions will displace a different amount of liquid under pressured conditions. If what you say is correct, does that mean that the specific gravity of a gas doesn't change under different pressure/temperature combinations? Can anyone else concur?
The flow of the gas through the orifice is directly proportional to the specific gravity. This is the ratio of the mole weights as described earlier. Fisher's regulator charts compensate for the differing pressures. Simply put, you will be able to flow more fuel gas at a 0.65 gravity than air at a 1.0 gravity.
Gas SG can't be the ratio of gas density at actual conditions to air density at standard condition. In that case gas SG closely follows its actual density as the air density at standard conditions is about 1.2 bar.
Did some calculations for 10 gases at pressures ranging from 0 to 100 bar g (more or less close to ideal conditions) and SG values by pressue definition (i.e actual gas density/air density at gas conditions) differ after second decimal when compared with SG values by molecular weight definition.
