Formaldehyde sensing using MQ138

[Joy Biswas]

Are there any links / docs available (other than datasheet) that demonstrates to calculate the ppm value of formaldehyde in air using MQ138. We have searched a lot of places but not quite successful.
[This is my 2nd post]

 

[BradtheRad]

Guess you know about this datasheet? It contains a graph of ppm vs resistance.

http://particle-sensor.com/immagini/MQ-138.pdf

If you do not trust the graph, then you can calibrate your meter against a known good instrument. (This assumes you have constructed a meter.)

Take both meters into a room where the air contains a measurable amount of the gas. Let them sit until their readings settle.

Adjust your meter so its reading matches the other.

 

[Joy Biswas]

Yes, we know about the datasheet.
A few points:
1) The datasheet does NOT mention formaldehyde
2) Level of HCHO beyond 0.1ppm itself is dangerous. I'm not sure how can you generate 200ppm (min. reading shown in graph) HCHO concentration. It is too much high. Keeping an open bottle of HCHO for 5 mins, in a glass chamber of 1 cu. meter (we made out of glass) gives a reading of 7-10ppm
3) From the datasheet graph we have generated the logarithmic function to solve for low values (0.01 to 10 ppm).
4) We do not have a HCHO measuring device with us, I wonder how that works.
5) The reading of this sensor MQ138 is too much jumping around, we have a 10-bit ADC encoding done on the raw value.

Henceforth, I asked is there any concrete document / that specifies the sensor characteristics in detail for these organic gases. I have written to the manufacturer of the sensor a week back. To a surprise they too don't have much clue.
Or if any reader in this forum has tried out with these sensors, so that he/she can share their views on it.

Thanks,
- JB -

 

[BradtheRad]

It appears the device is an all-purpose sensor, designed to detect molecules heavier than N2 or O2. (I'm guessing since I don't know how it works.) When you use it, the assumption is that other gases are absent except the one you are measuring.

The datasheet refers to 10 ppm as a minimum for detection. If it were reliable below that amount, I think the manufacturer would extend the graph below that point.

You are probably getting random noise readings, when you try to detect at the .1 ppm level.

There is a meter which claims to measure formaldehyde at the small levels you desire. It works via photodetector and a complicated process which involves a new tablet each time you take a sample. The tablet contains chemicals which react specifically with formaldehyde.

http://formaldehydedetector.com

If you can duplicate the chemical reaction, you might have your formaldehyde detector.

 

[FvM]

1) The datasheet does NOT mention formaldehyde

You statement doesn't make sense at first sight. Either the sensor is specified to be sensitive for formaldehyd or not. What makes you think it's suited to measure formaldehyde?

I have however seen different MQ-138 datasheets on the internet. Some specified the sensor working for formaldehyde.



Formaldehyde is in fact toxic, but you can prepare higher concentration calibration mixtures by evaporating formaladehyde solution (formol) quantitatively in an enclosed container.

 

[Forbix Semicon]

Hhmm.. seems like it check all heavy gases.
HCHO, C6H6, Toluene and what not.
We even see if we blow any of these in calculated quantities, the sensing values rise rapidly.

Can you tell me what are acceptable values of HCHO? I read somewhere its 0.1 ppm, true?
What values in normal houses, new and old ones?

Dear Brad, I never said the 138 does not checks HCHO. I said, the earlier link that you have provided does not mention HCHO.

Anyway, we will do more experimentation on these.

 

[Joy Biswas]

Finally, we are able to make things work.
Thanks for all your inputs.

- JB -

 

[sundavi]

Can you please share the solution to calculating ppm for Formaldehyde using this sensor? THanks