Go Down

Topic: MQ-135 Value Conversion (Read 3675 times) previous topic - next topic

hozone

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

Krodal

This is basic information : http://playground.arduino.cc/Main/MQGasSensors

robtillaart


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

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

Krodal

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

hozone

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

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


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

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

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

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

hozone

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

Rob Tillaart

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


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?

Go Up