Air Quality Sensor

[promy]

Great project!
I hooked up an MQ135 (sensor on a little pcb with (i think) smd resistor 102 (=1K ohm) between gnd and B )
The readings are however all over the place (135ppm to 1332ppm), so i think i need to calibrate. The raw reading is alwas in the range 71-80 (inside).

I know i have to get it outside and the reading should be 392 or 399 ppm but i am unsure what var i should change?
The sketch says mq135_ro (this has to be tuned 10K Ohm) but that does not seem to be used in the calculations?

I tried the sensor outside, but I am getting some different readings: the measured value changes from 59 to 82. The mq135_ro (i suppose that's the suggested ro) from 3081.00 to 81416 (does not go linear with the measured value?
The sensor was heated for more than 24h and then moved outside (started measuring after 5min, in windy conditions)

[jeylites]

I'm using the standard Smoke sketch from Mysensor library and it seem to be crashing MySensor plugin on Vera edge. Below is a snapshot of the problem. Any ideas?

[korttoma]

I take it that you are talking about the Air_Quality sketch right? The problem is that you do not have the device files for this kind of sensor in your Vera MySensor plugin. There is another thread that contains the files, I'll see if I can find it.

Edit: Found the thread -> http://forum.mysensors.org/topic/661/assistance-needed-completing-gas-air-quality-sensor

[jeylites]

@korttoma

I'm going to try experiment tomorrow... Thank you!

[jroseastro]

@epierre thank you for sharing this code - your work is stellar. I'm working with a team of three other people to build a payload for a near space weather balloon launch in a couple of weeks.

We're using the MQ131 Ozone Gas Sensor and your code has been quite useful to us. One question though, when our data logger reads the sensor we're getting a single number value. We're not exactly sure what that number means. Could you help explain what we're seeing? Are we getting a ppb number? ppm? Or something else altogether?

[epierre]

@jroseastro hello, first of all the MQ series are not so precise as that, use it for a test run, but for serious purpose please consider something better and with calibration.

Ozone is quite chalenging, if you go very high, since the MQ nead heating to work, it will drain fast your batteries.

For any sensor always refer to the datasheet.

MQ131 is given from 5 to 100 ppb (reason you get only a single value) if you use the provided calibration sheet. So the result is the one from the curve. We've had a discussion not yet settled with @hek about units and this is why I've not put it, but maybe I should uniformize everything in ppm since this is the most prevalent value used.

Also the datasheet says you need 6V to heat it, so you cannot heat it from the arduino except with a step up (not that good) or an external power source (best) with a voltage regulator.

[epierre]

I'm now testing the MICS-6814 (3 sensors in one) given for :
Carbon monoxide CO 1 -1000ppm
**Nitrogen dioxide NO2 0.05 –10ppm **
Ethanol C2H5OH 10 –500ppm
Hydrogen H2 1 –1000ppm
Ammonia NH3 1 –500ppm
Methane CH4 >1000ppm
Propane C3H8 >1000ppm
Iso-butane C4H10 >1000ppm

Datasheet maionly speaks on CO, NO2 and NH3:
http://www.seeedstudio.com/wiki/images/1/10/MiCS-6814_Datasheet.pdf

Here is are scripts:
http://www.seeedstudio.com/wiki/Grove_-_Multichannel_Gas_Sensor

http://www.seeedstudio.com/wiki/images/1/10/MiCS-6814_Datasheet.pdf

Some readings:

The concentration of NH3 is 0.99 ppm
The concentration of CO is 1.20 ppm
The concentration of NO2 is 0.15 ppm
The concentration of C3H8 is 1000.04 ppm
The concentration of C4H10 is 999.98 ppm
The concentration of CH4 is 2991.14 ppm
The concentration of H2 is 1.09 ppm
The concentration of C2H5OH is 1.40 ppm

I guess I'll make a script soon...

[bhavika]

I think MQ131 need 24 hours preheating time.Because of preheating time it will be costly. and for MiCS 6814 how do you calculate or measured other values of gases?? Because sensor's data sheet it hard to understand

[epierre]

@bhavika said:

I think MQ131 need 24 hours preheating time.Because of preheating time it will be costly.

no MQ is expected to run on battery for they always need pre-heating before getting a value.

and for MiCS 6814 how do you calculate or measured other values of gases?? Because sensor's data sheet it hard to understand

So fat I only use the provided library. Sensors are said to be factory calibrated, maybe a reason they are so costly. I'm not running it constantly at this time, but I saw no change in the few time I ran it so I'm still expecting to see something.

[haro]

@epierre Hi...it is ameasing your work

I am wondering about Mq-135. We have troubles with reading differents gases.
From the datasheet we read the slopes
Gas X" X0" "Rs/R0 "Rs/R0 pendiente (log y- log y0)/(log x-log x0)"
Air 200 10 3.6 3.6 0.00000
co 200 10 1.3 2.9 -0.26783
nh4 200 10 0.79 2.7 -0.41024
c02 200 10 0.8 2.4 -0.36673
3/4/ 200 10 0.72 1.9 -0.32391
1/4/ 200 10 0.64 1.5 -0.28432
+- 200 10 0.59 1.4 -0.28845

the data are in the next worksheet https://docs.google.com/spreadsheets/d/18Z6XyxwXVsWdMIk9vbUuCucaLJDdmR0QUFSTYG79ftw/edit?usp=sharing

¿may you share with us your code for this sensor in order to see your slopes and Rs/R0 values ?
¿do you know the meaning of the name of gases with 3/4 or 1/4 or +-

thank you in advance

[epierre]

@haro

based on a datasheet, this one:
https://drive.google.com/file/d/0B8OvrwVDp8fCcWpHSjZNVzB1TjQ/view

I apply a power regression (tool: http://www.xuru.org/rt/PowR.as ) on the following points (I've not marked down values but theses are close match):

0.8 200 
1 100
2.3 10

Result: y = 103.6748512 x-2.81699418
Residual Sum of Squares: rss = 45.01031346

 ¿do you know the meaning of the name of gases with 3/4 or 1/4 or +-

looks like you had a chinese datasheet just remember those are correlated gases based on particle size for those captors, so it is not very accurate...

You can find many more sensors hare : https://github.com/empierre/arduino/blob/master/AirQuality-Multiple_Gas_Sensor1_4.ino or simply in https://github.com/empierre/arduino/blob/master/

[haro]

@epierre
Hi, May be I do not explain my point correctly (or maybe I misundertood ), let me try again.
We adopt a code, which is the next. The blue lines are the points (for each gas we need two points) that we need.
https://docs.google.com/document/d/1CevnhBLL9uoyZ31kkrMn-SXeOF_4e9YFGmchj46pGPY/edit?usp=sharing

Specifically we read from the datasheet the pairs of points for CO
point1 (lg200 , lg1.3)
Point2 (lg10, lg2.9)
then, we construct the data format:{ x, y, slope};
COCurve[3] = {1.0,0.462,-0.268};

We are wondering about the pair of points for the others gases or the { x, y, slope} data for them, could you share them?.

and the other hand, the name of gases with 3/4 or 1/4 or +- appears as well in your datasheet (figure 3)... do you know why?

thank you so much

[epierre]

I prefer the power regression than the curves.

From your datapoints :
1.3 200
2.9 10

I find:
y = 532.6744764 x-3.73371402
rss=0

I don't remember where I found the datasheet with the chinese names that I translated through google.

[haro]

@epierre Now I see your point of view and of course it is better way to reduce the error.

in the other hand, I found the datasheet in chinese an then a friend translate it.
the meaning of the name of gases is for 3/4 (Alcohol), 1/4 (Tolueno) or +- (acetona)

Thank you so much.

[Yuki M]

Hi,

I've started working with an MQ135 sensor, attached to a board like the disucssion you had from a year ago. I measured some of the resistances between the Aouts and the sensor pins and seems similar with the 1kohm resistance between AO and Gnd, and the pot not changing the output.

I was wondering if anyone managed to calibrate their board using the AO of this board, and if so how? I can't seem to get a good resolution, as it has noise of at least in the 10's of ppm.

I'm considering whether its simpler to by pass the board and its amplifier, and directly hook set up onto the sensor itself with the correct resistors.

[epierre]

@Yuki-M said:

Hi,

I've started working with an MQ135 sensor, attached to a board like the disucssion you had from a year ago. I measured some of the resistances between the Aouts and the sensor pins and seems similar with the 1kohm resistance between AO and Gnd, and the pot not changing the output.

I was wondering if anyone managed to calibrate their board using the AO of this board, and if so how? I can't seem to get a good resolution, as it has noise of at least in the 10's of ppm.

I'm considering whether its simpler to by pass the board and its amplifier, and directly hook set up onto the sensor itself with the correct resistors.

Calibration is not possible as it varies between the same sensors of this construction class. Check datasheet on minimal sensibility also on low and max values.

Also without a gas concentration it is not possible to calibrate it. The best guess is that co2now.org gives you that outside (aka clean air) it is 399 for CO2, but that would be only for CO2 and not the other gases !

[Yuki M]

@epierre Thanks for a rapid response!
Yes I've been trying to calibrate it assuming out door air is at 399ppm. I understand each one has variances but would like to know how you guys worked it out.
I'm using
ppm = 116.6020682 (Rs/Ro)^-2.769034857
from davide's blog http://davidegironi.blogspot.ch/2014/01/cheap-co2-meter-using-mq135-sensor-with.html#.VinvWX4rIuU

Since the board show RL, I took that to calculate Rs. This lets me calculate Ro, thus calibrate it.

Not quite sure where I'm going wrong. I had it working 8 months ago, and came back to it and couldn't figure out how i had it working so well.. I was able to get pretty nice graphs and now its very unstable.

I'm having a play around with it, you can see my current points here https://thingspeak.com/channels/31162

[epierre]

@Yuki-M said:

@epierre Thanks for a rapid response!
Yes I've been trying to calibrate it assuming out door air is at 399ppm. I understand each one has variances but would like to know how you guys worked it out.
I'm using
ppm = 116.6020682 (Rs/Ro)^-2.769034857
from davide's blog http://davidegironi.blogspot.ch/2014/01/cheap-co2-meter-using-mq135-sensor-with.html#.VinvWX4rIuU

Since the board show RL, I took that to calculate Rs. This lets me calculate Ro, thus calibrate it.

Not quite sure where I'm going wrong. I had it working 8 months ago, and came back to it and couldn't figure out how i had it working so well.. I was able to get pretty nice graphs and now its very unstable.

would say: let it burn for 24/48 hours to leave it clean itself

[bhavika]

IN MiCS 2614 , sensitivity factor is defined by "Rs at 100 ppb of O3 divided by Rs at 50 ppb of O3". From where you measured Rs at 100 ppb or 50 ppb.???

[Heinz]

Adjusting my mq-135 sensor took several atempts as the CO2 value of the atmosphere is not a constant value of 399ppm.
This value is measured in Hawaii ( see http://co2now.org/ ) and is probably significantly lower than at your home. In germany for example you should calibrate your sensor when the wind comes from west over the atlantic which brings fresh air along. See http://www.donnerwetter.de/deutschland/co2.htm

Everytime my sensor passed values below 400 I adjusted the R0 value. In the end I modified the sketch to be able to send this R0 value from FHEM to the sensor to avoid recompiling the sketch everytime.
See
https://github.com/windkh/mysensors/tree/master/CO2Sensor/CO2sensor.ino Line 440ff

[Heinz]

The sensor is in my kitchen. The value in the screenshot is very high as I made some french fries 2 hours ago.

[epierre]

@Heinz so bad I don't have a MQ135 running anymore to compare, here are the MH-Z14 curves I have near my desk.

[epierre]

@bhavika said:

IN MiCS 2614 , sensitivity factor is defined by "Rs at 100 ppb of O3 divided by Rs at 50 ppb of O3". From where you measured Rs at 100 ppb or 50 ppb.???

this is just to tell you how much a double concentration scales up, here they just say it is somehow linear at ratio 2

[Rasenheizung]

@Heinz very nice work (wie heisst du im FHEM Forum?)

Does someone have the values a MQ-07 needs to perform properly? I rally don't know how to gather the right values from the description....

Thank you

[epierre]

@Rasenheizung if you base yourself on my template here https://github.com/empierre/arduino/blob/master/AirQuality-Multiple_Gas_Sensor1_4.ino

H2
1.3 50
0.8 100
0.28 400
0.16 1000
0.05 4000

y = 73.59123879 x-1.355617483
Residual Sum of Squares: rss = 87393.44418
H2Curve[3] = {73.5912, -1.355617};

CO
1.6 50
1 100
0.39 400
0.21 1000
0.09 4000

y = 99.27731641 x-1.513643469
Residual Sum of Squares: rss = 43168.87424
COCurve[3] = {99.27731, -1.51364};

CO is a dangerous gas, the MQ7 is very sensitive to many things and no calibration too... for a serious work please think about using the MICS-6814 that is calibrated, includes 3 gases out of the box, and is very low consumption (can run on battery !)
https://github.com/empierre/arduino/blob/master/AirQuality-CO-NO2-NH3.ino

[bhavika]

What is the pre-heating time for TGS2442???

[epierre]

@bhavika said:

What is the pre-heating time for TGS2442???

from datasheet: "the sensor’s resistance reaches to 90% of its final value in less than one minute"

[bhavika]

So what is the conditioning period for testing in it??? It is two days.. What does it mean??

[epierre]

@bhavika said:

So what is the conditioning period for testing in it??? It is two days.. What does it mean??

on this kind maybe to have it powered for a full two days to burn out the production "dusts" that may remain in.

those sensors are not good science, see the correlation of gases they are reported for in the datasheet

[epierre]

Hello,

a very sensitive HCHO module you can see a demo here: http://www.seeedstudio.com/recipe/379-a-handheld-hcho-detector.html?utm_source=EDM&utm_medium=EDM&utm_campaign=EDM20150930

https://github.com/empierre/arduino/blob/master/AirQuality-HCHO.ino

[paqor]

I have a question for the MQ135. Do not always be above 500 ppm, the measured values? I also get values of 0. I suspect that it is due to the conversion. The values vary greatly. Is that normal? I get the values with an ordinary power supply.

[paqor]

This post is deleted!

[Heinz]

Looks strange, could you supply your code?
Btw the sensor draws a lot of current, the power supply should be able to deliver 1A. Did you burn in the sensor for 12-24 hours?

[paqor]

I have used the code from epiere. Baking I have not done. I use different power supplies (USB, Samsung and a large controllable power supply). I sometimes feel that contacts do not function properly. But the sensor draws but only about 100 mA = 0.1 A? Then it seems to be on branding. I have three new sensors. Mostly we read that it is only an attempt here is to measure the CO2 content. When I check the values with a measuring instrument with which agree the details here https://olimex.wordpress.com/2015/05/26/experimenting-with-gas-sensors-and-arduino/ Branding should work outdoors? Can you show me a chart or values?

[paqor]

This post is deleted!

[Heinz]

I used this library:
https://github.com/GeorgK/MQ135

Together with this description:
https://hackaday.io/project/3475-sniffing-trinket/log/12363-mq135-arduino-library

Simple to use and very good explanations.

[paqor]

I've read about. It sounds interesting.

I stand a little hard with the development. Is there a skit for MQ135.h and MYSENSORS?

I will test this afternoon.

[Heinz]

Hi @paqor,
I wrote a little sketch which you could use as a start, it uses the MQ135 library.
I also used a timer library, but this could be replaced by a simple sleep, too.

RZERO should be changed when calibrating the sensor, this can be done at compile-time or during runtime by sending the value to the sensor via the gateway.

Unfortunately I could not test the sketch yet, but it should help you getting started.
If you have a second sensor which provides humidity and temperature then you can also get the corrected ppm from the library,...

good luck!

/*
MQ135 MySensor
A0      white
D0      black
GND     brown
5V      red
*/

#include <SPI.h>
#include <MySensor.h>
#include <MQ135.h>
#include "Timer.h"

Timer timer;

// 30 seconds
#define TEMP_UPDATE_INTERVAL 30000

// MQ135
#define CHILD_ID_CO2 0
#define CHILD_ID_R0 1

#define CO2_SENSOR_ANALOG_PIN 0

/// Calibration resistance at atmospheric CO2 level
#define RZERO 300.0
#define EEPROM_R0 0

MQ135 gasSensor = MQ135(CO2_SENSOR_ANALOG_PIN, RZERO);
int lastC02;
float lastR0;

//-----------------------------------------------------------------------------
// MySensor
MySensor gw;
MyMessage msgCO2(CHILD_ID_CO2, V_VAR1);
MyMessage msgR0(CHILD_ID_R0, V_VAR1);

//-----------------------------------------------------------------------------
void setup()
{
    gw.begin(incomingMessage, AUTO, true);
    gw.sendSketchInfo("MQ135 Sensor", "1.0");
    gw.present(CHILD_ID_CO2, S_AIR_QUALITY);
    gw.present(CHILD_ID_R0, S_CUSTOM);
    
    uint8_t R02 = gw.loadState(EEPROM_R0);

    // get R0 from EEPROM
    float R0 = R02 * 2;

	// do a plausibility check
    if (R0 > 1.0 && R0 < 400.0) 
    {
        Serial.print(F("Setting R0 from EEPROM: "));
    }
    else
    {
        Serial.print(F("Setting default R0: "));
        R0 = RZERO;
    }

    Serial.print(R0);
    Serial.println(F(""));

    gasSensor.setR0(R0);
    
    timer.every(TEMP_UPDATE_INTERVAL, timerHandler);
}


bool MQ135Changed()
{
    bool changed = false;

    lastR0 = gasSensor.getRZero();
    Serial.print(F("R0: "));
    Serial.println(lastR0);

    {
        float ppm = gasSensor.getPPM();

        Serial.print(F("CO2 ppm: "));
        Serial.print(ppm);


        int roundedPpm = (int)ppm;
        Serial.print(F(" --> "));
        Serial.println(roundedPpm);

        if (roundedPpm != lastC02)
        {
            lastC02 = roundedPpm;
            changed = true;
        }
    }

    return changed;
}


void timerHandler()
{
    bool airQualityChanged = MQ135Changed();
    if (airQualityChanged)
    {
        gw.send(msgCO2.set(lastC02));
        gw.send(msgR0.set(lastR0, 2));
    }
}


void loop()
{
    gw.process();
    timer.update();
}


// Gets the R0 value from the gw passes ot to the lib and stores it into the EEPROM.
void incomingMessage(const MyMessage& message)
{
    Serial.println(F("Incoming Message:"));

    if (message.isAck())
    {
        Serial.println(F("This is an ack from gateway"));
    }

    uint8_t sensor = message.sensor;
    if (sensor == CHILD_ID_R0)
    {
        float R0 = message.getFloat();

        Serial.print(F("Incoming R0: "));
        Serial.print(R0);
        Serial.println(F(""));

        gw.saveState(EEPROM_R0, (uint8_t)(R0/2));
        gasSensor.setR0(R0);
        gw.send(msgR0.set(R0, 2));
    }
}

[paqor]

I have compiled from various sources themselves something. The result is https://gleisnetze.de/2015/12/25/das-erste-kleine-programm/ described herein. The most plausible values I received with the MQ135.h library. For that I have then added the transfer after MySensors / FHEM.

[bhavika]

Hello

In MiCS 2614 or in any gas sensor. what should be the value of R0... Because in datasheet they mentioned that value is measured in ambient condition. Any default value for that??

[epierre]

@bhavika said:

Hello

In MiCS 2614 or in any gas sensor. what should be the value of R0... Because in datasheet they mentioned that value is measured in ambient condition. Any default value for that??

The datasheed propose the "clean air calibration": "Sensing resistance in air R0 is measured under controlled ambient conditions, i.e. synthetic air at 23 ±5°C and 50 ± 10% RH. Sampling test."
so you should go outside and mesure the defautl R0 value in your environment.

[Heinz]

MQ-135 with 10k results in a R0 of around 360 in my environment. So you could start experimenting with values around 300-500. I had to use a voltage regulator to get reliable values from the sensor.

[epierre]

a little update on the MICS-6814:
NO2: sensible
CO: flat
NH3 flat

For the HCHO:

sorry for the flat part, some arduino issues at this time

[mfalkvidd]

Welcome @betthorn, great to have you here.

[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);

[tantt2810]

@epierre Hi you,
I think the Rs/Ro is not RL. In the datasheet RL=5kOhm. As I know it is a resistance on board Arduino, and we set it 5kOhm. Is it right? I also don't know why we set it equal 5kOhm. =))
And why 20000 is in "float Rs = 20000 * ( 5.00 - Vrl) / Vrl ; // Ohm" ?
Please help me clearly. Thank you so much !!!

[tantt2810]

Hello,
I have read the sensitivity characteristics curve in datasheet of MQ135 and I don't know AL R, 3/4, 1/4* and +- mean? Can you help me, please !!!
NH4 represent for NH3, doesn't it ?
Thank you so much !!!

[tantt2810]

Hi everyone,
I have an CO2 MG811 sensor. I don't know where tcm pin connect with ?
Please help me. Thank you so much !!!!

[TFPOK1]

Hello, I'm working with an MQ-136 sensor, and am interested in measuring H2S and/or SO2 concentrations. I've got a simple sketch written which gives me a fairly consistent value on the serial monitor, but I've tried to run the multiple gas sensor sketch both complete and I also tried picking apart the sketch just to include MQ-136 related variables and algorithms, but I can't get it to work. It's showing too many errors to list here. Does anyone have simple conversions from the serial outputs to usable values for various gases that this sensor can measure?

[Joel Pérez]

hello, how i read co2 ppm with my mq135?
helpme please

[epierre]

@tantt2810 said:

Hi everyone,
I have an CO2 MG811 sensor. I don't know where tcm pin connect with ?
Please help me. Thank you so much !!!!

this is an analogic sensor, so VCC/Gnd to power it, and readings on analogic Aout

[epierre]

@tantt2810 sorry for not being more responsive, I left MQ/TGS behind me but I've not kept all my notes on mesuring this and that, that would require me to search for it again. But I appreciate if you can spot me some float/type error conversion in sketches !

[epierre]

I was a bit disappointed when a project I contributed to on Kickstarter showed the inside of their boxes, I wonder how they will manage to satisfy customers with calibration... or their home made index as netatmo did

[moskovskiy82]

Used the air quality script for MQ7 but throwed out all the conversions and calibrations. Just reading the plain input from the analogue pin.
I targeted this as a smoke detector but it seems this sensor highly depends on the humidity of the surrounding air. Are there any existing formulas to correct for the humidity of the surounding air?

And on one sensor i have some irratical data as shown below (blue). I wonder if it's a power supply (black and green is DHT) or a broken sensor?

[epierre]

@moskovskiy82 well looks like a strange sensor... try averaging several samples/// always remember to have it running several days before measuring for there are chemical left from china factories...

[wreiner]

Hey all,

I've got a question regarding the calibration function in https://github.com/empierre/arduino/blob/master/AirQuality-Multiple_Gas_Sensor1_4.ino :

$ grep MQCalibr *
AirQuality-Multiple_Gas_Sensor1_4.ino:  Ro0 = MQCalibration(MQ2_SENSOR,10,RL0,SmokeCurve);
AirQuality-Multiple_Gas_Sensor1_4.ino:  Ro1 = MQCalibration(MQ6_SENSOR,10,RL1,LPGCurve);
AirQuality-Multiple_Gas_Sensor1_4.ino:  Ro2 = MQCalibration(MQ131_SENSOR,10,RL2,O3Curve);
AirQuality-Multiple_Gas_Sensor1_4.ino:  Ro3 = MQCalibration(TGS2600_SENSOR,10,RL3,C2H5OH_terCurve);
AirQuality-Multiple_Gas_Sensor1_4.ino:  Ro4 = MQCalibration(MQ135_SENSOR,10,RL4,CO_secCurve);
AirQuality-Multiple_Gas_Sensor1_4.ino:  Ro6 = MQCalibration(TGS2602_SENSOR,1,RL6,C7H8Curve);
....
AirQuality-Multiple_Gas_Sensor1_4.ino:float MQCalibration(int mq_pin, double ppm, double rl_value,float *pcurve )

Th function MQCalibration should calibrate the sensor in clean air, the calibration function uses the PPM concentration of the gas in a clean air environment.
You already use the appropriate value for i.e. CO2 (around 399 PPM) in the MQ135 sketch - but why are you using those obscure values in the calibration (1, 10) here?

Thanks in advance!

[epierre]

@wreiner said:

Th function MQCalibration should calibrate the sensor in clean air, the calibration function uses the PPM concentration of the gas in a clean air environment.
You already use the appropriate value for i.e. CO2 (around 399 PPM) in the MQ135 sketch - but why are you using those obscure values in the calibration (1, 10) here?

Well, I didn't know the concentration in clean air so I had to put in a value even random... I took the hypothesis there are very few in the air so low value.

[tantt2810]

@epierre
Thank you. Your mean is Vcc->5V, GND->GND, Dout->Digital pin, Aout->Analog pin. Is it right?
But I don't know the tcm pin connect with ? I have read datasheet but I don't find any info about tcm pin. Please help me. Thank you so much !!!

[alexsh1]

@epierre

Readings of my MQ135 are going up and down like a yo-yo. Not sure how to read them really?
What do you think?

[bhavika]

I am confused with MiCS 4514 sensor. What I had done is :

using MiCS quick start evaluation board, measured ADC value
calculated Vout and from that calculated R0.
Rs/R0 concentration gives me high value, which is not possible, My data is as follows
clean air file is used for the purpose of R0 value
https://drive.google.com/file/d/0B8sF8a6FHoseWjhWOHRoYzIxakk/view?usp=sharing
and Polluted file is used to calculate the actual pollution
https://drive.google.com/file/d/0B8sF8a6FHoseR1BnclhCUHdibEU/view?usp=sharing

I am confused. Please help me

[epierre]

@tantt2810 said:

@epierre
Thank you. Your mean is Vcc->5V, GND->GND, Dout->Digital pin, Aout->Analog pin. Is it right?
But I don't know the tcm pin connect with ? I have read datasheet but I don't find any info about tcm pin. Please help me. Thank you so much !!!

do not use the digital pin !!! it only reacts with the potentiometer in on/off mode, so useless... you need to power the device with Vcc/Gnd, and read the output with Aout (A stands for analog). The last one forget it.

[epierre]

@alexsh1 is that raw ADC output or with a calculation ? normaly this should go up or down but not in sucha drastic way

[alexsh1]

@epierre

This is a value gw.send(msg.set(MQ135_DEFAULTPPM+(int)ceil(valAIQ))) reported to Domoticz.
It does not make any sense to me at all.

[tantt2810]

@epierre
Thanks you so much !!! Good job, Good boy =)).

[epierre]

@alexsh1 said:

@epierre

This is a value gw.send(msg.set(MQ135_DEFAULTPPM+(int)ceil(valAIQ))) reported to Domoticz.
It does not make any sense to me at all.

same for mle, sometime it is good to have two piece to compare their behavior... I have 3 o them ...

[alexsh1]

@epierre do you think it is a faulty sensor though?

[tantt2810]

Hello,

Can I use pinMode(digitalpin, OUTPUT) to turn off sensor? Or Is there any way to wirte a function that turn off sensor?

I want to remote it on web to turn on or off it.

Thank you so much !!!

[supersjimmie]

For those who are trying to use an MQ135 module from e-bay or other sellers, on a small pcb with a chip, LED and potmeter:
It took me a while until I figured out that the little chip has a very big influence on the value that you read on the A-out pin.
Only after removing the connection between the A-out pin and the little chip, I got normal measurements.

I also replaced the small resistor marked "102" with a 10k resistor, to make measurements more compatible with code that uses a 10k pull down resistor. But that was already a known issue, found on many other places.

[epierre]

@tantt2810 said:

Hello,

Can I use pinMode(digitalpin, OUTPUT) to turn off sensor? Or Is there any way to wirte a function that turn off sensor?

I want to remote it on web to turn on or off it.

Thank you so much !!!

well no, powering is through Vcc/GND, so you whould use a switch on this. Digital pins do not gives enough power for such sensors.

[epierre]

@alexsh1 said:

@epierre do you think it is a faulty sensor though?

coud be, hard to know without possibility to compare to another one.

[alexsh1]

@epierre

This is what I discovered. When nobody is around the sensor 'settles' on a particular value. However, when there is anyone around in the room, the sensor reading goes up and down - see the snapshot attached. Any idea what the problem may be?

[epierre]

@alexsh1 hard to say, could be something moving inside if someone makes vibration around and thus this strange reaction... the down curve at the end is correct, the in between not

[Elfnoir]

Hi,
Can you check the reception level?
I had same behaviour with an old temperature/humidity module; need to move it to another place in the room, and works perfectly now

[epierre]

@Elfnoir said:

; need to move it to another place in the room, and works perfectly now

temp/hum are "less" sensitive on environment, here the sensors heats up to 40-60° (or more) so temp/hum is not the most impacting (I can be wrong anyway

[alexsh1]

@epierre I wonder if you looked at this sensor Figaro TGS8100:

http://www.figarosensor.com/products/entry/tgs8100.html

Looks like it is perfect for a low-powered platform.

[epierre]

@alexsh1 said:

@epierre I wonder if you looked at this sensor Figaro TGS8100:

http://www.figarosensor.com/products/entry/tgs8100.html

Looks like it is perfect for a low-powered platform.

as a replacement for MQ in low power yes could be... the bad part is how to get one ?

[alexsh1]

@epierre said:
the bad part is how to get one ?

How about http://www.soselectronic.com/?str=371&artnum=170939&name=figaro-tgs8100 ?

[Vir]

Hi, It all make sense, I am just confused that in your code once your multiply and once you divide...

case CO:
{
if(ratio1 < 0.01) ratio1 = 0.01;
if(ratio1 > 3) ratio1 = 3;
//c = pow(10, 0.6) / pow(ratio1, 1.2);
** c = pow(ratio1, -1.179)4.385; //mod by jack*
break;
}
case NO2:
{
if(ratio2 < 0.07) ratio2 = 0.07;
if(ratio2 > 40) ratio2 = 40;
//c = ratio2 / pow(10, 0.8);
** c = pow(ratio2, 1.007)/6.855; //mod by jack**
break;
}
SO, what is the actual formula?

Vir

@epierre said:

I'm now testing the MICS-6814 (3 sensors in one) given for :
Carbon monoxide CO 1 -1000ppm
**Nitrogen dioxide NO2 0.05 –10ppm **
Ethanol C2H5OH 10 –500ppm
Hydrogen H2 1 –1000ppm
Ammonia NH3 1 –500ppm
Methane CH4 >1000ppm
Propane C3H8 >1000ppm
Iso-butane C4H10 >1000ppm

Datasheet maionly speaks on CO, NO2 and NH3:
http://www.seeedstudio.com/wiki/images/1/10/MiCS-6814_Datasheet.pdf

Here is are scripts:
http://www.seeedstudio.com/wiki/Grove_-_Multichannel_Gas_Sensor

http://www.seeedstudio.com/wiki/images/1/10/MiCS-6814_Datasheet.pdf

Some readings:

The concentration of NH3 is 0.99 ppm
The concentration of CO is 1.20 ppm
The concentration of NO2 is 0.15 ppm
The concentration of C3H8 is 1000.04 ppm
The concentration of C4H10 is 999.98 ppm
The concentration of CH4 is 2991.14 ppm
The concentration of H2 is 1.09 ppm
The concentration of C2H5OH is 1.40 ppm

I guess I'll make a script soon...

[Vir]

What does 37143 represents, Rs? and -3.178 is a second value from your regression method, but what about the first value?
Can you elaborate on ppm line, please
Vir

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);

[Vir]

@bhavika said:

I am confused with MiCS 4514 sensor. What I had done is :

using MiCS quick start evaluation board, measured ADC value
calculated Vout and from that calculated R0.
Rs/R0 concentration gives me high value, which is not possible, My data is as follows
clean air file is used for the purpose of R0 value
https://drive.google.com/file/d/0B8sF8a6FHoseWjhWOHRoYzIxakk/view?usp=sharing
and Polluted file is used to calculate the actual pollution
https://drive.google.com/file/d/0B8sF8a6FHoseR1BnclhCUHdibEU/view?usp=sharing

I am confused. Please help me

I am confused by the formula you are using, could you please explain?
Pollution NO2 = (POWER((RS/R0)*(1/POWER(10,0.8068)),(1/1.01)))

Pollution CO = POWER((RS/R0)*(1/POWER(10 , 0.5476)),(1/-0.8497)))

is the actual formula like this?:

pollution = POWER((ratio *(1/POWER(10, curve[0])),(1/[curve[1])))

This is different than any other formulas discussed in this thread...
Vir

[Vir]

so does 37164 represents the scaling factor and -3.178 the exponent?
Vir
@Vir said:

What does 37143 represents, Rs? and -3.178 is a second value from your regression method, but what about the first value?
Can you elaborate on ppm line, please
Vir

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);