i've made a mistake before, i consider Rs/Ro = ppm, but from fig.2 of datasheet is not so.

it seems that it is a power function (see attached).

so i made some power regression based on the values, and i obtain a scaling factor and an exponent

given those values

a = scaling factor obtained by datasheet figure, for the gas you need

b = exponent obtained by datasheet figure, for the gas you need

ppm = a*(Rs/Ro)^b

now, to obtain a valid Ro value, we have to know the ppm of a gas, for C02, we know the CO2 value for atmosphere,

http://www.esrl.noaa.gov/gmd/ccgg/trends/#mlo

so i can obtain Ro value for the chip in open air.

can this be right?