Air Quality Sensor

[epierre]

I have had some questions about converting from mg/m3 to ppm gases, here are the weight values and a proposed method:
NH3 17.03 g/mol
CO2 44.01
CO 28.01
H2S 34.08
NO2 46.01
NO 30.01
O3 48.00
C6H6 78.11
C7H8 92.14

you have:
NO2 50 μg/m3 gives 26,5868821 ppm
O3 27 μg/m3 = 0.027 mg/m3 -> (8,31441298,15)/(48101,325)*27=13,7617025 ppm

correct me if I'm wrong

[epierre]

2016 is where IoT for gas sensors are moving to more maturity, Cooking Hacks / Libellium proposed so far Fibaro sensors, but are now proposing more pricey one that are calibrated.

See https://www.cooking-hacks.com/shop/sensors/gas

[epierre]

CO2 with MH-Z14

NO2 MICS-6814

HCHO sensor

[bhavika]

In MQ135 sensor's datasheet , which line on the curve is used to find out the concentration of benzene?

[epierre]

@bhavika said:

In MQ135 sensor's datasheet , which line on the curve is used to find out the concentration of benzene?

MQ135 is sensitive to particle size, benzene is one of these. I rememer someone who used the chinese datasheet and found a chinese to translate it.

[supersjimmie]

@epierre I am testing a stripped-down version of your code from your AirQuality-MQ135.ino. I stripped it to make it run just stand-alone.
What I don't understand is how to do the calibration. When I run it in outside environment (with 10k resistor), I get these values:

valr 223
Vrl / Rs / ratio:13387 / 20875.00 / 0.00

What would be the next step for calibration?

[epierre]

@supersjimmie replace MQ135_DEFAULTRO by your value 20875 in the sketch

[supersjimmie]

@epierre Thanks, I thought it would be something like that, but that didn't work.
I then got

valr 242
Vrl / Rs / ratio:5464 / 8520.00 / 0.00

Nothing like around 400ppm.

[epierre]

yes for the code must be updated, the value is the CO2 (or something else) detected above the 399 in atmosphere

  gw.send(msg.set(MQ135_DEFAULTPPM+(int)ceil(valAIQ)));

[supersjimmie]

Thanks, but when calibrating something strange happens when I put the module outside:

valr 270
Vrl   / Rs       / ratio  / ppm
61355 / 95673.00 / 0.00 / 399.00
valr 265
Vrl   / Rs       / ratio  / ppm
62928 / 98126.00 / 0.00 / 399.00
valr 260
Vrl   / Rs       / ratio  / ppm
64561 / 100673.00 / 0.00 / 399.00
valr 256
Vrl   / Rs       / ratio  / ppm
378 / 589.00 / 0.00 / 399.00
valr 252
Vrl   / Rs       / ratio  / ppm
1773 / 2764.00 / 0.00 / 399.00

As you can see, the valr decreases slowly, but suddenly Vlr and Rs jump over.
Vlr is the value of 'val', which is: uint16_t val = ((float)22000*(1023-valr)/valr);
uint16_t is 16bit, so max 65535. It looks like my environment needs more than that?
(in fact, your code defines the defaultro as 68550, which is also too much for a uint16?)

[epierre]

@supersjimmie said:

Vlr is the value of 'val', which is: uint16_t val = ((float)22000*(1023-valr)/valr);

yes and this is why I did rewrite it to https://github.com/empierre/arduino/blob/master/AirQuality-Multiple_Gas_Sensor1_4.ino but did not updated this one...

[supersjimmie]

@epierre I striped-down that AirQuality-Multiple_Gas_Sensor1_4.ino to use only MQ135.
When used inside, the analogRead is 473, so a reasonble value.
But the MQCalibration function returns 0. Which means the MQResistanceCalculation also returns 0.

MQResistenceCalculation only does one calc:
return (long)((long)(1024 * 1000 * (long)rl_value)/raw_adc-(long)rl_value);
rl_value = float RL4 = 0.990
raw_adc = 473 (measured as said above)
(1024 * 1000 * 0.990) / 473 - 0.990 = 2142.266
I checked that raw_adc is filled with the correct value (to be sure it is not lost somewhere).

What looks to fix this, I changed everything to float . So:
return (float)((float)(1024 .* 1000. * (float)rl_value)/raw_adc-(float)rl_value);
Which can be simplyfied as:
return ((1024 * 1000*rl_value)/raw_adc-rl_value);
I went to float, because the function is created as float MQResistenceCalculation.

This now gives some ppm values that are within a reasonable range (about 2000 here).

Now I have to figure out how to do calibration on this method again...

[epierre]

looks like I have to rework something... but not in the todo list right now... Imperihome is now back on top, and Particle on lipo/solar also...

[tantt2810]

Hello,
I don't know how to calculate 3 numbers of LPGCurve[]. Can you explain clearly for me? Thank you so much !!!

[epierre]

@tantt2810 as explained above, if the datasheet has a logarithmic scale, you can use the power regression to approximate the curve. One tool for example to do this calculus:http://www.xuru.org/rt/powr.asp

for a sensor discussed above I read this on the datasheet:
H2
1.3 50
0.8 100
0.28 400
0.16 1000
0.05 4000

The xuru website gives me this:

y = 73.59123879 x-1.355617483
Residual Sum of Squares: rss = 87393.44418

and thus I have::
H2Curve[3] = {73.5912, -1.355617};

[tantt2810]

@epierre Thank you so much !!!
Have a nice day =)) !!!

[jenbaker]

@epierre

hi

i am new here.

sorry i am a little confused by with these new formulate.

is it the key for module?

[tantt2810]

Hello,
I'm don't understand the recipe below. Why RL_Value(Load Resistance)(1023-raw_adc)/raw_adc)? Can you explain for me?
Thank you so much !!!
/***************** MQResistanceCalculation ****************************************
Input: raw_adc - raw value read from adc, which represents the voltage
Output: the calculated sensor resistance
Remarks: The sensor and the load resistor forms a voltage divider. Given the voltage
across the load resistor and its resistance, the resistance of the sensor
could be derived.
***********************************************************************************/
float MQ2::MQResistanceCalculation(int raw_adc)
{
return ( ((float)RL_VALUE(1023-raw_adc)/raw_adc));
}

[epierre]

@jenbaker said:

sorry i am a little confused by with these new formulate.

is it the key for module?

if you have a module the datasheet gives you the values, yes. Remember this is their """""generic"""" calibration, in fact they all copy the other so don't expect your MQ** would follow this by the letter...

[epierre]

@tantt2810 said:

Hello,
I'm don't understand the recipe below. Why RL_Value(Load Resistance)(1023-raw_adc)/raw_adc)? Can you explain for me?
Thank you so much !!!
Input: raw_adc - raw value read from adc, which represents the voltage
Output: the calculated sensor resistance
Remarks: The sensor and the load resistor forms a voltage divider. Given the voltage
across the load resistor and its resistance, the resistance of the sensor
could be derived.
float MQ2::MQResistanceCalculation(int raw_adc)
RL_VALUE(1023-raw_adc)/raw_adc));

in fact it is above described, you have a voltage, you want a resistance.
https://learn.sparkfun.com/tutorials/voltage-dividers

datasheet needs a value which is the Rs/Ro (called here RL) where
Ro: sensor resistance at 100ppm of NH3 in the clean air.
Rs:sensor resistance at various concentrations of gases

here the formula is a simplification of this:

float Vrl = val * ( 5.00 / 1024.0 ); // V
float Rs = 20000 * ( 5.00 - Vrl) / Vrl ; // Ohm
int ratio = Rs/Ro;
ppm = 37143 * pow (ratio, -3.178);