During the weekend I have managed to put together a first very early prototype of my ”Ketosense”. It does register when a person is blowing in to the ”gas chamber” where the sensor is located and something that feels encouraging is that my wife gets significantly higher readings then me. Since I’m still a carbohydrate junky and she is not that is exactly what we want. However we do have some hurdles to cross before this thing is useful.
- Calibrate the sensor
- Determine sensor characteristics for gas concentrations between 0-50 ppm
- Build a better moth piece
Calibrating the sensor
Calibrating the sensor is the major thing that needs to be done. In the sensor data sheet there is stated exactly how the resistance of the sensor behaves at different gas concentrations but everything relates to one calibration point, the sensor resistance Rs = Ro at 300 ppm of ethanol. Based on this value we can calculate the relation between sensor resistance and gas concentration for all other gas concentrations. Further in the data sheet it is stated that Rs at 300 ppm of ethanol is between 1-10 kΩ and that is quite a wide range and I don’t want to make a generalization of 5k before I even tried to calibrate it. Right now I don’t rely have a clue how to create a 300 ppm ethanol gas mix but hey, thats just another problem to solve.
Sensor characteristics for gas concentrations between 0-50 ppm
The datasheet for the TGS822 doesn’t have any data for how the sensor behaves at low gas concentrations between 0-50 ppm. In the graph displaying the Rs/Ro relation it looks like the function for the sensor resistance follows a Log-linear model so if I can determine the Ro (Rs at 300 ppm ethanol) of my sensor and also have the Rs for < 10 ppm of gas, which I presume is the ethanol content of normal room air, then I should be able to deduce some info for how the sensor should behave in that range as well.
I quickly understood that to get a good reading you needed to give the sensor some time to take the reading. This means we have to trap the gas around the sensor in some sort of chamber for a while to get a good reading. Right now I have a plastic cup with a tube and some tape over the opening and it does the job. However it is actually to air tight and I have to remove the tape from the opening to vent out the gas after getting a reading for the sensor to be able to reset it self. Another issue with the cup design is the condensation. Breath has quite a high moisture level and after just a few readings with this moth piece you start so see condensation on the inside of the cup and readings from the same person are different from time to time which they obviously shouldn’t be. Both humidity and temperature affects the sensor resistance so I am thinking of getting a humidity/temperature and add that so I can compensate for those factors but I believe that a better design of the moth piece can be just as or even more effective.