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Author Topic: MQ-135 Value Conversion  (Read 1460 times)
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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 10KΩ to 47 KΩ)

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 (69.05 KB, 278x192 - viewed 53 times.)
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This is basic information : http://playground.arduino.cc/Main/MQGasSensors
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MQ sensors were discussed previous year here - http://arduino.cc/forum/index.php/topic,55780.0.html -
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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
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Sorry, but that is not possible. Every sensor could be different.
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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?


* mq135_test.jpg (58.5 KB, 800x391 - viewed 154 times.)
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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.
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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 smiley
even if i can know it with precision i will approximate it to 394
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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.
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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).
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