Re: What does the unit mg/Nm3 mean

Question: What does the unit mg/Nm3 mean

Hello Jan, Thank you for your question. I have tried to solve your puzzle but I’m entirely not sure I have. I needed to make an assumption or two.

The following web site is useful for looking up definitions of units. http://physics.nist.gov/cuu/Units/units.html

It includes the “SI” (from the French Le Système International d'Unités), units which are used in your mystery unit mg/Nm3 which, presumably is same as mg / ( N*m*m*m ) or milligrams / ( Newtons * metres * metres * metres )

It is often useful to reduce a complex unit to its basic components using Dimensional Analysis. This is especially the case when dealing with equations where the units’ dimensions need to be the same on both sides. It isn’t too difficult in most cases. You just need to identify the dimensions “length”, L, “time”, T and “mass”, M and express the units in those terms. In your case we can translate the Newton, N into :-

m.kg.s-2 ( m*kg / (s*s) ).

The mystery unit then becomes:

mg*s*s / ( kg*m*m*m*m)

or in terms of dimensions only, because the mass is cancelled out,

T*T / L*L*L*L

If we now translate this back into something more understandable we get 1 / ( acceleration * volume )

I have to admit that it didn’t help on this occasion ! but I thought I’d go through the process in case it might be useful for you in future.

In the form it reached me it included the phrase “In terms of emissions” and was categorised as in the Environment area. Emissions are often expressed in terms of amounts per unit volume of air so we can rearrange the mystery unit as:

(mg / m*m*m) / N or

mg per cubic metre per Newton. This would mean that the unit is expressing an amount of emission in terms of weight per cubic metre for each Newton wherever the Newton comes from. It could be for example, because the Newton is a unit of force, come from something which produces a useful force but causes harmful emissions, then the mystery unit would describe what concentration of emission is produced for the amount of useful performance delivered.

I hope I have guessed correctly !

Added 8/21/2005 by Esa Kotala:
Nm³ most likely means normal cubic meter, i.e. the amount of gas in one m³ at 1 atmospheric pressure and standard temperature.