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### Topic: MQ-135 Value Conversion (Read 11705 times)previous topic - next topic

#### hozone

##### Dec 27, 2012, 07:55 pm
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

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

#### Krodal

#1
##### Dec 27, 2012, 09:27 pm
This is basic information : http://playground.arduino.cc/Main/MQGasSensors

#### robtillaart

#2
##### Dec 28, 2012, 12:01 am

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

Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -
(Please do not PM for private consultancy)

#### hozone

#3
##### Jan 16, 2013, 03:47 pm
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

#### Krodal

#4
##### Jan 16, 2013, 04:08 pm
Sorry, but that is not possible. Every sensor could be different.

#### hozone

#5
##### Jan 16, 2013, 04:55 pm
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?

#### robtillaart

#6
##### Jan 16, 2013, 07:09 pm
Quote
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.
Rob Tillaart

Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -
(Please do not PM for private consultancy)

#### hozone

#7
##### Jan 16, 2013, 07:24 pm

Quote
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
even if i can know it with precision i will approximate it to 394

#### LQVE

#8
##### Dec 02, 2013, 12:40 pm
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.

#### hozone

#9
##### Dec 02, 2013, 02:09 pm
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).

#### Luckman

#10
##### May 07, 2014, 09:14 pm
Did you ever get this working with CO2?  If so, can you post the resistor value and a code snippet?  Thanks!

#### hozone

#11
##### May 08, 2014, 10:02 am
yes i get this working.
this is the blog post speaking about how did i get it working, and how does it works.
http://davidegironi.blogspot.it/2014/01/cheap-co2-meter-using-mq135-sensor-with.html

#### robtillaart

#12
##### May 08, 2014, 08:28 pm
short but well written!
Rob Tillaart

Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -
(Please do not PM for private consultancy)

#13
thank!

#### lukeparker

#14
##### Dec 04, 2014, 09:58 am
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?

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