Co2 sensor connexion and calibration

Hello,
I am trying to make my co2 sensor module work.
The CO2 module can be found at the following link: http://www.ebay.com/itm/CO2CarbonDioxideSensorModuleTGS4161VoltageProbe/311429272431
So far, I only found the datasheet of the inner sensor and not of the whole module:
0_1484124919516_HOTA100069 (5).rar
I set the conversion curve to be : EMF[350ppm]EMF[xppm]=M ln(PCO2[xppm]/PCO2[350ppm] then retrieved the linear factor M from the datasheet curve of the inner sensor . M turned out to be equal to 27.62.
The conversion equation became:
πΈππΉ[π₯]=πΈπF[350]β27.62 lnβ‘(πππ2[π₯]/350) where EMF[350] is the voltage given in ambiant air (350 ppm of CO2).
Now I am struggling to find the value of EMF at 350 ppm CO2. I connected my sensor module to Vcc=5V and to ground . Then i tried to measure Aout.
Everytime I connect the sensor it gives a different value!
Shouldn't this be constant since the air contains the same CO2 concentration ? In the datasheet they say that we should expect 220mV490mV. Is this after the amplifier or before?
I have been stuck on this for days. If you can please help me.
Thanks

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