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Name: Dannie
Status: Student
Grade: Other
Location: FL
Country: United States
Date: June 2006


Question:
I just recently started an internship in an air separation plant, I have known that a normal cubic meter is an unit to measure gas volume. In this plant, they measure the liquid argon production and other liquid gases with this same unit (Nm3) which has gotten me really confused. Is it possible to measure a liquid volume with this unit? What does it mean when used like that?



Replies:
Dannie,

A Normal Cubic Meter of a gas (Nm3) is the volume of that gas measured under the standard conditions of 0 degrees Celsius, and 1 atmosphere of pressure. Specifying the "Normal" conditions of measurement must be done with gases, since the volume of a gas is inversely proportional to its pressure, and proportional to absolute temperature. In other words, because gas volume changes so dramatically with temperature and pressure, we have to establish some "normal" or "known" temperature and pressure conditions in order to be able to compare one person's measurement with another.

Liquids are also universally measured in cubic meters, but not "Normal" cubic meters, since liquids are not subject to the dramatic changes in volume that gases are when temperature and pressure are changed.

So the answer to the first part of your question is, yes, it is not only possible to measure a liquid volume using cubic meters, (or the unit called "Liters" that is 1/1000th of a cubic meter) nearly everyone in the world uses this as an everyday measure of liquid volume, just as in the US, quarts, gallons, cubic feet, cubic inches, and fluid ounces are somewhat confusingly used for the same thing.

The answer to the second part of your question, namely can liquid gases be measured in *Normal* Cubic Meters, is no. The reason is simply that it is required to do the "Normal" measurement at 0 degrees Celsius, and as you no doubt know, any normal liquefied gas (such as liquid argon) is no longer a liquid at this high temperature. It would not be possible to measure its volume since it would have all boiled away already. I suspect that you may have misunderstood and your employer must be using Cubic Meters as a liquid measure.

Regards,

Bob Wilson.


Dear Dannie,

The unit Nm3 stands for normal cubic meter. When measuring gas volumes, the N means the gas is at normal temperature and pressure, usually taken to be 20.0 C and 1.01 bar (one atmosphere).

I presume the meaning is the same for liquids, though the unit should probably be just m3. This would probably refer to the volume at one atmosphere and whatever temperature the liquid argon is at. It could also mean at the pressure the argon is subjected to. For completeness, the temperature and pressure should be specified, although liquid densities do not vary much with temperature and pressure (compared to gases).

Best, Dick Plano, Professor of Physics emeritus, Rutgers University



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