MQ-135 Value Conversion

i've a mq135 - http://www.futurlec.com/Datasheet/Sensor/MQ-135.pdf

please anyone correct me if i'm wrong:

what i've undestand from datasheet is that

ppm = Rs/Ro

Rs = mq135 readed resistence
Ro = sensor resistance at 100ppm of NH3 in the clean air (according to datasheet 30Kto200K)

i wire the sensor according to datasheet (see attached image, downloaded is from http://www.staceyk.org)
set RL = pulldown resistor (in the example 39K, according to datasheet any between 10K? to 47 K?)

so Rs = 1024*RL/adc - RL

the point here is to setup Ro, which means calibrate the sensor.
any help?

thank you,
davide

MQ135.png

This is basic information : Arduino Playground - MQGasSensors

MQ sensors were discussed previous year here - http://arduino.cc/forum/index.php/topic,55780.0.html -

yes, i've read the information you link.
but the key point is to find a valid Ro, i do not have a calibrated sensor, so i ask you if someone can tell me a valid Ro values for this chip.

thanks

Sorry, but that is not possible. Every sensor could be different.

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,

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

can this be right?

so i can obtain Ro value for the chip in open air.
can this be right?

Assuming you're not in Bejing with the current smog levels the answer is yes.

robtillaart:

so i can obtain Ro value for the chip in open air.
can this be right?

Assuming you're not in Bejing with the current smog levels the answer is yes.

i hope here near milan is not so high :slight_smile:
even if i can know it with precision i will approximate it to 394

But the problem is the MQ-135 sensor is not for CO2. It is for Ammonia, Sulfide, Benzene etc.
I can't calibrate the sensor under the normal atmosphere.

i'm comparing it against a mh-z14 (using a xively datastrem).
it seams to words good also for Co2 (from 300 to 2000ppm)
results will be published soon on my blog (as soon as i will have time to write the post, and analyze graph).

Did you ever get this working with CO2? If so, can you post the resistor value and a code snippet? Thanks!

yes i get this working.
this is the blog post speaking about how did i get it working, and how does it works.

short but well written!

thank! :wink:

The Mq-135 sensor detects ammonia but when I look up the Mq-135 in the Mq lib it states that it's Sensitive for Benzene, Alcohol, smoke am I getting confused?