Hello, I'd like to detect carbon monoxide with this sensor, I read the technical data sheet and a few documents about other similar projects, I read that I've "to train" the sensor to let him understand the normal Rs value in clean air but is not clear to me how I can set the sensor to detect only carbon monoxide or how I can understand if an alarm is related to carbon monoxide, CH4 or LPG.
Any help will be appreciated.
Paolo
You cannot configure it, for CO2 you need other sensors e.g. MTP40F which is afaik dedicated for CO2. In the pro segment (more expensive) you might want to look at the COZIR.
"Training" is likely 48h preheat time in clean air to get reference value
If you build your circuit with bare sensor, you probably can tune it to only sense CO by adjusting the internal heating voltage to 1,5V. But that datasheet is quite bad...
If you are after a MQ-9 module, you need to find out how that circuit is working. If it's cycling between high and low voltage or if there is adjustment for it.
It is simply not possible with a single MQ series sensor, as each responds to more than one gas. However, an interesting approach is to use several different ones together, as described in this Electronic Nose project.
Completely not, but neither responses are equal. CO and LPG have quite different (inverse) response for high and low voltage heating cycles.
Hello Kmin, according to data sheet it seems that it is cycling between high and low voltage: "standard measuring circuit of
MQ-9 sensitive components consists of 2 parts. one
is heating circuit having time control function (the
high voltage and the low voltage work
circularly )."
Furthermore in chapter Sensitivity Adjustement is written:
"The sensitivity adjusting program:
a. Connect the sensor to the application circuit.
b. Turn on the power, keep time of preheating through electricity is over 48 hours.
c. Adjust the load resistance RL until you get a signal value which is respond to a certain
carbon monoxide concentration at the end point of 90 seconds.
d. Adjust the another load resistance RL until you get a signal value which is respond to a
CH4 or LPG concentration at the end point of 60 seconds ."
I didn't understand this point. I mean, or you set RL to a value valid for carbon monoxide or to another value dedicated to CH4 or LPG.
Am I missing anything?
More less, like that.
I wrote, the datasheet is little bit chinese...
Anyway you didn't mention if you are dealing with bare sensor or module?
These sorts of instructions might be useful if you already know for certain which gas will present.
However, the sensors are unstable in the long run, as you will learn by experimentation, and need frequent recalibration.
If you really want to detect and/or measure CO concentration in the presence of other volatiles, you need a completely different type of detector. Fortunately, you can buy them.
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Firstly, let me say that this is hypothetical only. You wouldn't actually use these in a real world application beyond learning how MOS sensors and circuits work, or for fun.
To answer your question: you don't have to. Consider your usage environment. What are your sources of CO, potentially (gas dryer, stove, fireplace, hot water heater, etc) and are those also sources of CH4/natural gas?
Do you have LPG there as well, and in what?
Point is, if the sensor is alarming for CO, you have to respond to it. If it's alarming for LPG or CH4, you've got problems there, too, but the response would be the same in a real-world application: exit the building and call the fire department, touch nothing except what's needed to make egress.
Finally, these sensors are not to be used in life safety applications. They are simply not fit for these duties.
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I agree there are much better options than very old generation mos sensors.
If you work for example in car repair shop, you need to keep on eye CO levels, not to call fire department if you detect some.
Fair, but I would say that no, you need proper, listed equipment to constantly monitor for CO in the garage, which should never be an issue if the garage ventilation system is sufficient and in good working order and connected to the CO monitoring devices.
In your example, the MQ-9 is absolutely not a listed device; therefore, in a general sense, my info is sound.
Whenever I see threads in this forum that have anything to do with air monitoring for in potentially toxic/and or explosive atmospheres, I am compelled to suggest to people that they proceed very carefully with whatever they are building.
For reference (at least in my jurisdiction):
https://www.buildingcode.online/829.html
Also worth reviewing:
https://www.health.harvard.edu/a_to_z/carbon-monoxide-poisoning-a-to-z
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I agree absolutely, only approved devices for commercial activity!
But you know, home carage shop for tuning.. Diy food smoking device... Garden smoke sauna... Better something than nothing.
Anyway for complexity of calibration and life expactancy of this sensor, I would invest in ndir sensor or some ready built detector.