Good afternoon.
I can not get the formulas from the graphs of the following sensors:
- TGS4161 (CO2)
- TGS2444 (NH3)
- TGS2611 (CH4)
Would appreciate someone help me.
Thank you very much.

Hi CIMA,

we don't provide a formula for the gas sensors because there is a great variability among different sensors of the same type, in both resistance in air and sensibility. This means that a formula that would work with some sensors wouldn't work with many others.

For example, in the case you mention, the TGS4161 sensor, the electromotive force under 350ppm of gas may vary between 220mV and 490mV, and the variation of the electromotive force between 350ppm and 3500ppm may vary between 44mV and 72mV.

If we used the same formula with all of them, we would have a very high error in most of them. This error may be assumed in some applications (like detection applications, where monitoring the variation of the output is usually enough), but normally a specific calibration is needed for each sensor, so a general formula is not advisable.

Regards.

Ok, I agree with libelium-dev,

However, could you explain the steps needed to generate the formula

Hi mihussein,

you will need to find out the output voltage at 350ppm and at another point (such as 3500ppm or 1000ppm), so you can get the electromotive force variation. Then you can use both points to figure out the equation y = m*log(x) + b ; where y is the gas concentration, x the frequency and m and b the two constants of a linear relationship. You can take a look at this post by AmroQuandour, where he did the job for the CO sensor (but be careful, since both frequency and concentration are in logarithmic scale for this sensor):

viewtopic.php?f=15&t=2447

You may get a better accuracy by getting more concentration values and linearly approximating the different stretches, but you can get a good result just with one.

Regards.

I got it, many thanks

now I have programing problem. How can you use 10^(x) in java environment

Hi mihussein,

if you mean the Waspmote IDE, you can use the function pow(), like this:



Regards.

I have one question..from the graph of both TGS2442 & TGS4161 sensor what I got is x axis representing the gas concentration and in case of TGS2442 y axis is RS/R0 and for TGS4161 y axis is the difference in voltage between 350ppm of CO2 and x ppm of CO2

pls let me know my understanding is correct or not..

Hi abhi123,

yes, that is completely correct.

Regards.

In case of TGS2442 both x & y and for TGS4161 x axis is in logarithm scale. just want to confirm whether for both the cases logarithm scale base is 10 or not?

Hi abhi123,

that's right, those are the axis in logarithmic scale, all of them in base 10.

Regards.