GASTEC

Units for measuring concentration

Common units for measuring concentration

% (percent)	mg/m³ (milligram per cubic metre)
This unit expresses concentration in parts per hundred (percentage) of a substance in 100mL of a medium such as air.	This unit expresses the concentration in one cubic metre of air (equivalent to 1L or 1000mL) of a substance in terms of its mass (measured in milligrams). It is primarily used for particle-like substances, and only rarely for gaseous concentrations.
ppm (parts per million)	mg/L (milligram per litre)
This unit which expresses concentration in parts per million is measured as the volume (denoted in litres [L]) of a substance found in 1L of a medium such as air.	This unit expresses the concentration in one litre of air (1000mL) of a substance in terms of its mass (measured in milligrams). It is generally used for measuring concentrations in liquids, and only rarely for gaseous concentrations.

Converting from one unit of concentration to another

% → ppm

ppm=% x 10000 → 1ppm=0.0001%

mg/m³ → ppm

ppm=mg/m³ x	22.4	x	(273+T)	x	1013
	M		273		P

ppm → %

%=ppm x 1/10000 → 1%=10000ppm

mg/L → %

%=mg/L x	22.4	x	(273+T)	x	1	x	1013
	M		273		10		P

ppm → mg/m³

mg/m³=ppm x	M	x	273	x	P
	22.4		(273+T)		1013

% → mg/L

mg/L=% x	M	x	273	x 10 x	P
	22.4		(273+T)		1013

M	:	Molecular weight of a substance
22.4(L)	:	The volume of 1 mol at 1 atmospheric pressure at 0ºC
273(K)	丗	K stands for Kelvin, the unit used to measure thermodynamic temperature; as 0ºC corresponds to 273.15K. You simply need to add 273 to the Celsius/Centigrade value (273+T) to obtain the temperature in Kelvin
1013(hPa)	:	One atmospheric pressure
P	:	P denotes the atmospheric pressure at the point of measurement (hPa)