PPM is another measure of concentration. Gases are quite simple to calculate concentrations with, as you can assume, for the most part, that molar concentrations are the same as volumetric concentrations.  From the example you've provided, the concentration of ethylene gas is equal to the flowrate of ethylene over the total flowrate (one divided by fifty one).  To turn this into PPM, simply multiply this ratio by one million, but i'd suggest PPM is not the best way to report this concentration.

I think it's just about calculation and you have all the data you need to calculate this ...

And it will increase the ppm of c2h2 in the chamber only if the chamber is close. Gaz, solids or liquid, calcul are quiet similar just a different state of phases.

Specific to your example, the ethylene concentration is (1/(50+1)) = 1.9608%  To convert this to ppm, just multiply by 10^6 = 19608 ppm.

The answer of David's answer is right. Anyway, the actual unit for gases as he defined (the right unit) is ppmv = part per million in volume (or ppbv, pptv...).

If you keep "ppm",  there is a ambiguity: it may be a mass ratio instead of a volume ratio (Adrien be careful, for liquids ans solids the "ppm" are more often mass ratios not molar ratios) . Ethylene is CH2=CH2 (C2H4).

Appreciate all your feedback...tqvm 

Hi,

For liquids ppm is a mass ration (1 g of substance/106 g of solution) and because the concentration of the substance is very low (ppm are used only in the case of very low concentrations) than we suppose that (density of solution = density of water) and 1ppm=1 mg/L.

For gases ppm is a molar ration (1mol of substance/106 mol of gas), than you must use the molecular weight Mw of the substance and the Mw of the gas (in many case especially in environment problems the gas is Air) to convert ppm to a mass ration.

Good Luck.

Salim is right but I repeat that for gases the molar ratio (in fact volumic ratio, considering perfect gases) should be called ppmv to avoid confusion with other "ppm" values. It is true that for water solution the ppm unit (in mass) is about 1 mg/liter but recall that it depends on the density of the liquid.

ppm only a ratio to total, similar with percentage, %. It could be v/v, w/w, mol/mol, molar/molar, flow rate/flow rate and etc.

It is true that ppm is used in many context, so it is important to precise its meaning when it may be ambiguous. It is also used for the chemical shift in NMR... I know also the use of parts per million atoms = ppma. As ppm use is not obvious for gases, I recommend ppmv when vol/vol is used.

Question: 

if I'm using two VA flowmeters, would I need to use ppmv or ppm (m:m)? 

example:

Nitrogen flow = 1000ml/min 

Ethylene flow = 10 ml/min

this implies that ethylene is flowing at 1% of nitrogen.. but the densities of the gasses are not the same, and its flowing into a flow chamber that is not closed (meaning that the system is open). Under the ideal gas law, then the number of mols of ethylene is equal mols of nitrogen when the flow rates are the same.

If this is true, then using you can just use the percent of the ratio of flows to report ppm (ppmv). But, given the ideal gas law, Vol must be constant (and if my system is open vol may not be constant). If I use mass flow rate which factors in gas density, and cross-sectional area flow and so on, the "ppm" value is drastically different. In my set up, I don't know which one to report.. the ppmv value for my mixture is around 1700 ppm and the ppm (m:m) value is like 24 ppm if I use the percent that the ratio of mass flow rates gives as opposed to the percent that just standard flow rates give.

Obviously I'd like to report the lowest, but I don't know that it is correct. Any ideas on which flow rate to report?

How can i convert 2L CO2 in closed chamber of 1L to ppm??

Noor El-Hassan, It is not clear if you put 2 liters of pure CO2 in a 1 liter volume. Its it just a compression ? If it is not a mixture (CO2 in another gas) ppm is not applicable (only apply to mixtures).

Thank you for your answer. May I say 0.5L CO2 in a glass chember of 1L filled with N2, how can I measure the ppm in this Case

Noor El-Hassan: Assuming you begin with the gases at the same pressure and temperature (say for example , 1 bar, 20 deg.C) and you have the means to pump the CO2 into the 1L chamber. You would then have 1 L of 333,333 ppm(v) of CO2 in N2 at 1.5 bar (after it cooled to 20 deg.C). But, because CO2 is a relatively non-ideal gas, this would NOT equal 333,333ppm(m:m) . To find the molecular ratio, you would have to look up the properties of CO2.

Bert Habicher gives a correct solution to the problem, but usually the gas mixtures are not done this way. For example you start with an empty volume (vacum) then you add some pressures of each gas.

Bert Habicher gives a correct solution to the problem, but usually the gas mixtures are not done this way. For example you start with an empty volume (vacuum) then you add some pressures of each gas. Just mixing volumes is not possible, unless you compress the gases as Bert said, which is not usual in the analytical lab. Additionally it is not practical to use ppm when high concentrations are used.