3 replies to this topic

[Chiko]

Hello,
I have a doubt in mind, for gases, specific gravity is ratio of density of sub. to the density of air which is equivalent to molecular wt. of sub. to mol. wt of air at standard conditions.
But what will be the actual specific gravity. For example if a gas has a pressure of 250 Psig, then will its SG will be density of gas (at 250psig)/Density of air at STP?
OR will it be density of gas (at 250psig)/Density of air (at 250 Psig)?

Please help.

[Art Montemayor]

Chiko:

Good question. I wish more engineers would be as analytical as you have been.

The specific gravity of gases is useful for people who have a specific need for the value as such. If it wasn't for that specific need, there would be no need to have a specific gravity for gases. Therefore, as long as you specify what are the "standard" conditions or the actual conditions of the air that you are referring the gas to, you are perfectly professional and accurate. Let the user handle the value that way. You cannot be a magician and correctly guess what people want to do with the SG value of a gas. But never fail to always state what are your reference conditions (i.e., pressure & temperature).

Those are my recommendations and what I always do when I come to the same situation as you've described.

I hope this helps you out.

[Chiko]

Actually, I want it for sizing of a PCV, specific gravity is required for Cv calculation.
So, what will be the specific gravity?

[djack77494]

Chiko:
I'm afraid you did not recognize the answer. Specific gravity does NOT vary as conditions such as temnperature and pressure vary. It is a property of the substance and not the conditions. There is no such thing as the "actual" and, by implication, the "standard" specific gravity. There is only THE specific gravity.

You seem to be confusing specific gravity with density. The latter of course DOES vary with temperature and pressure. So with a gas with a molecular weight of (say) 44, its specific gravity would be 44/29 (since air, MW=29, is what the gas is being compared to).
Doug