How to calculate the CO2 concentration using a mg-811 sensor from its analog value or sensor voltage? can you give an idea of logic used or formula as well. Thank you.
This is the datasheet, it has a response graph. http://sandboxelectronics.com/files/SEN-000007/MG811.pdf
1st hit with Google.
What do you expect from this sensor? What accuracy do you want? These sensors are not calibrated.
You can use to calibration graph from the datasheet but don't expect an accuracy better then +/- 100ppm or so in the low range and much more in the high range. Remember that every sensor is a different and the sensitivity will change over time. Also temperature and humidity can have an effect.
You want to have a better accuracy, then you have to calibrate your sensor using several calibration gas-mixtures of CO2 in air around the values you want to measure. You have to repeat this calibration several times a year.