Calibrating a Figaro TGS822 sensor, by drawing…

After consulting with the boys and girls at chemicalforums.com about how to produce an ethanol gas with a 300 ppm without having to buy a lot of fancy gear and thus finding out that it was more difficult then I initially thought I have reluctantly decided that I don’t think I will be able to pull it of. There for I have decided to work with what I got. What I got is air and a datasheet.

The data sheet provided by Figaro for the TGS822 only goes down to 50ppm, however the graph looks pretty logarithmic linear to me so I decided to add the 10-50ppm part my self making a bold assumption that it will be logarithmic linear in that interval as well.

From the data sheet we get the relation between RL and RS which is a voltage divider circuit.

Rs_vs_RlFrom the graph in the data sheet we can also see that the resistance of the sensor in air is RS (air) = R0 * 19.

If we combine these two facts we can express R0 as a relation of RS (air) and the value of RS (air) can be deduced by reading the voltage of the sensor and using the voltage divider formula.

RS (air) / 19 = R0 in my case RS (air) = 78kΩ. => R0 = 4105Ω

When we have R0 we can make a table to relate resistance (RS) to ppm by reading the scaling factor of RS/R0 from the graph for different gas concentrations.

Rs in air = 78000 Ro = 4105,26315789474
ppm Scaling factor Rs = Ro * Scaling factor
0 19 78000
10 15 61578,947368421
10 10 41052,6315789474
20 9 36947,3684210526
20 7 28736,8421052632
30 6 24631,5789473684
30 5,7 23400
40 4,7 19294,7368421053
50 4 16421,0526315789
60 3,5 14368,4210526316
70 3,2 13136,8421052632
80 3 12315,7894736842
90 2,7 11084,2105263158
100 2,5 10263,1578947368
150 2 8210,5263157895
200 1,6 6568,4210526316
300 1,2 4926,3157894737
400 0,9 3694,7368421053
500 0,75 3078,9473684211
600 0,67 2750,5263157895
700 0,58 2381,052631579
800 0,52 2134,7368421053
900 0,47 1929,4736842105
1000 0,4 1642,1052631579
2000 0,2 821,0526315789
3000 0,15 615,7894736842
4000 0,1 410,5263157895

The TGS822 sensor is affected by both temperatures and humidity and it should be complemented with a thermistor and hygrometer so that it is possible to compensate for temperature and humidity. I don’t have any thermistor or hygrometer yet but if we use the ”calculate R0 from RS (air)” every time we start the sensor then perhaps we will also compensate for temperature and humidity, this is something further experimenting will tell.

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