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?
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.
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.
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).
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?