How to calibrate MQ 135 for CO2 PPM

[eaquinte]

Hello

I have been watching some videos and reading information on how to calibrate the MQ135 to be able to measure CO2 in PPM. Currently I already have the formula to calculate it, which is: 5.1839*pow(Rs/R0, -0.348)

Where:
R0 = 82 //initial MQ135 reading, found from the average of 100 readings
Rs = 1000*((5-Voltage)/Voltage)
Voltaje = MQ135_ADC*(5/255) //255 is the maximum value of the variable and MQ135_ADC is the sensor reading


I have some problems, which are:

1. All the information that I have found is for arduino and, what I have seen, is when they use the MQ135, the initial measurements, you get hundreds or thousands, while when I do the ADC reading, I get an average of 87. Could someone tell me why this is? I did the datasheet, the thing about having the MQ connected for 24 hours, so I don't think it's something like that. If I do my calculation with that value, the amount of CO2 I get from the calculation (at least in a spreadsheet like Excel) is 0.5132, which is clearly a very low value for CO2 PPM.

2. I don't know how to calculate numbers with a decimal point and negative exponentiation, could you help me? I am using the PICAXE 20M2

Thanks in advance

 

[hippy]

All the information that I have found is for arduino and, what I have seen, is when they use the MQ135, the initial measurements, you get hundreds or thousands, while when I do the ADC reading, I get an average of 87. Could someone tell me why this is?

It could be that you are reading the ADC with READADC which will only return a 0-255 value. Reading with READADC10 will return a 0-1023 value.

Any actual average of readings will be in those ranges but scaling will make those larger if needed to match the values others are using.

I don't know how to calculate numbers with a decimal point and negative exponentiation, could you help me?

That can be tricky with a PICAXE which but it may be possible. The first thing to do is to ensure the raw data you are getting is as expected before trying to process that further.

I am not familiar with the MQ135 but I did find this article which seems to cover a number of things -

How To Use MQ-135 Gas Sensor - Codrey Electronics

A Gas sensor serves to detect and measure the concentration of gas in the environment. Nowadays you can find many different types of easy to use gas sensors, especially in MQ series. In this article, […]

www.codrey.com

Probably the best way to proceed is to provide us a link which shows which MQ135 module you are using, a description of how it is wired to your 20M2, and show us the code you are currently using.