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Conversion PPM(wt), PPM(vol), g/m3 (gas phase)

Conversion PPM(wt), PPM(vol), g/m3 (gas phase)

Hi there all,

We are doing frequent measurements of contaminants like H2S, Hg, As in our gas streams, as well as for the concentration of same in air around out plant.  Unfortunately, units used to specify limits, readings from meters, etc are rarely consistent, and in many cases open to interpretation.  E.g. PPM, which could be either PPM (wt) or PPM (vol), or g/m3, which does not specify whether the gas is at actual or standard condition (which makes a big difference in case of sales gas).

Originally, I was told that:
ppm (wt) x 1000x Density  =  mg/m3, and
ppm (vol)x1000  =   mg/m3

However, this does not take into consideration the density of the gas the contaminant is measured in.  I tried searching on the net for conversion factors/formulas, but without success.

Any help available?

RE: Conversion PPM(wt), PPM(vol), g/m3 (gas phase)

trond,Check the following site,
it has a rather extensive concentration conversion calculator. just as an aside this is the conversion base:

1ppm=1 mg/l=1 mg/kg

1ppb=1 micro g/l =1 micro g/kg



Hope this helps.

RE: Conversion PPM(wt), PPM(vol), g/m3 (gas phase)

Typically a ppm value specified in a gas stream is intended to be ppm by volume.

Since ideal gas behavior of all constituents is also typically assumed a correction to standard temperature and pressure isn't necessary.

RE: Conversion PPM(wt), PPM(vol), g/m3 (gas phase)

Trond is correct in assuming that the molcular weight of the "injected" gas and the "carrier" gas are both needed to convet say ppmv (volume or mole) to ppmw (weight). Check out On questions like this the industrial gas manufacturers sites may be helpful. Matheson (at and (British Oxygen).


RE: Conversion PPM(wt), PPM(vol), g/m3 (gas phase)

If you'd like a simple program to convert mixtures between w/w and v/v, I'd be pleased to send it to you. It was written for % concentrations but is easily used in ppm, remembering that 10,000ppm = 1%. You'll need to send me your E-mail address, or E-mail me at

RE: Conversion PPM(wt), PPM(vol), g/m3 (gas phase)

I've been having difficulty sending an E-mail to you. You will be able to get the downloads at Go to the "Our Products" page, then the "FREE ... downloads" page, and you'll find links there. Leave it a day before you try - I'll upload the modified pages tonight, ISP and hosts willing! (GMT here, presently 8am.)

RE: Conversion PPM(wt), PPM(vol), g/m3 (gas phase)

How does one convert from ppm (vol or weight) to moles or g/l etc in a LIQUID ?

RE: Conversion PPM(wt), PPM(vol), g/m3 (gas phase)

For liquid solutions, whenever the concentration in volume units must be changed to mass units, or the reverse, density must be known. Thus,

=>From ppmv to g/L one needs to know the density of the solute.

=>From ppmw to g/L one needs the density of the solution, which, if very dilute, can be replaced (with little error) by the density of the solvent.

RE: Conversion PPM(wt), PPM(vol), g/m3 (gas phase)

Concerning the conversion of ppmv to mg/Nm3, they are two factors to consider: the temperature/pressure and the O2%.
Nm3 stands for "normal" or "standard" cubic meter from the perfect gas equation. You therefore have to correct both parameters to obtain an accurate value.

Temperature and pressure: use pv=RT equation to calculate the volume of the compound under the actual and standard conditions.
O2%: use O2 actual = 21/(21- O2 measured)

RE: Conversion PPM(wt), PPM(vol), g/m3 (gas phase)

Usually, for air concentration results or stack sampling results, when one writes ppm  it is often assumed that it is ppmv (volume (or mole). It is common to have certain pollutants, like Hg and heavy metals ewpressed as mg/Nm3, dry basis, corrected at a reference value for O2 or CO2 (stacks)
the correction formula is
O2 corrected figure= non corrected figure*(21-O2reference)/(21-actualO2)  ; O2 reference can be 11%, sometimes 7% or other values).
pollutants like dust SO2 HCl are often mg/Nm3, dry, O2 corrected
pollutants like NOx can ne either

It is important to understand the dramatic effect of O2 correction, if you have actual O2 figures exceeding 17-18%.

RE: Conversion PPM(wt), PPM(vol), g/m3 (gas phase)

Why not use the following:

[mg/m3] = (MW x PPM) / vol

where; MW = molecular weight,  PPM = parts per million, vol = volume in liters of one mole of gas.

You can then correct the volume to actual process conditions and as such carry through the correction to your concentration conversion.

Take example:

H2S = 120 PPM, gas consituents are 70% N2, 5% H2O as vapour, 15% CO2 and 10 % O2, therefore density at NTP is approx 1.352 kg/Nm3.

Lets say P = 110 kPa and temp is 50 degrees C

Therefore density (actual) = 1.242 kg/Am3

Now calculate actual volume:

Vol of air at actual conditions = 24.43 L, density air (acutal) with 5% moisture = 1.163 kg/Am3.

Therefore actual vol = (1.163/1.242)*24.43 = 22.876 L

As such [mg/m3] H2S = 34.0818 x 120 / 22.876 = 178.78


RE: Conversion PPM(wt), PPM(vol), g/m3 (gas phase)

NeedAHoliday, I think you don't need to know the main gas composition.
If the gas is considered "ideal", 120 ppm v/v of H2S at actual conditions would still be 120 ppm v/v at NTP = 120 mL/m3.

To convert 120 mL H2S into mg at NTP (0 deg C, 1 atm abs.):

120 mL/m3 x 1.5392 mg/mL = 184.7 mg/m3 at NTP.

However, the same 1 m[sup]3[/sub] of gas at NTP would expand under "actual" conditions (50 deg C, 110 kPa) to:

1 x [(273+50)/273] x 101.3/110 = 1.09 m3.

Thus since its mass doesn't change, the adjusted H2S concentration would be:

184.7/1.09 = 169 mg/m3

under actual conditions.   

Please correct me if I'm wrong.

RE: Conversion PPM(wt), PPM(vol), g/m3 (gas phase)


You have asked for too many different conversions and that has led to the confusing array of responses.  

This response of mine applies only to converting ppmv of a gaseous pollutant in the ambient air to mg of the gaseous pollutant per cubic meter of ambient air at a pressure of 1 atmosphere and any temperature:

ppmv = (mg/m3)(273.15 + °C) / (12.187) (MW)

ppmv = volume of gaseous pollutant per 106 volumes of ambient air
mg/m3= milligrams of gaseous pollutant per cubic meter of ambient air
MW = molecular weight of the gaseous pollutant
°C = ambient air temperature in degrees Celsius

{Also, note that the ambient air pressure has been incorporated into the above equation as being 1 atmosphere)

For a more detailed discussion of this and related conversions involved in air pollution, visit

Milt Beychok

RE: Conversion PPM(wt), PPM(vol), g/m3 (gas phase)

The factor 12.187 is the result of dividing 273.15 K by 22.414 m3/kg-mol.

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