1

I am using figaro tgs gas sensor (2600, 2602 and 2620) to sense breath from human. What I obtained is the output voltage. However, I would like to convert the voltage to concentration in ppm? Since these gas sensors can sense more than one gas. Is there any way?? Thanks.

1

I would like to convert the voltage to concentration in ppm?

Is there any way??

the datasheet for those devices would have the answers for you. Generally, you read the datasheet BEFORE deciding on using them or not. not that otherway around.

1

This is one of the datasheet. I have calibrated it using the clean air factor in datasheet and got the Rs/R0 ratio. However, from there, I not sure how to find the concentration for each gas.

  • The conditioning is 7 days. Turn it on with all wires connected and wait 7 days. Then you have to calibrate it with a known concentration of the gas that you want to measure. You might also use the data from the datasheet, for example from the "sensitivity characteristics" graph. A single sensor detects multiple gasses, and there is no way to tell which gas it is. If you combine more than one sensor, you might distinguish a few different gasses, but that is not in the datasheet. I think you have to use a known concentration of a gas and collect the data from all sensors. – Jot Dec 4 '17 at 16:06
  • Thanks for the reply. I refer to jenslabs.com/category/electronics/ketosis-detector and sandboxelectronics.com/?p=165 For the first link, I not really sure where the value got to convert from resistance ratio to ppm...The second link use the gradient in the datasheet for each line and generate the ppm value from ratio got. Can it be use? – Blurman Dec 4 '17 at 16:56
  • The TGS2600, TGS2602 and TGS2620 are not designed to breath on. The TGS822 (in the first link) is. In the datasheet is a graph of ppm versus Rs/Ro. The most common (and also cheapest and poor quality) are the "mq" gas sensors. You can find examples for ppm when you search for: arduino mq gas sensor ppm – Jot Dec 4 '17 at 20:55
  • But 2600, 2602 and 2620 also show the Rs/Ro versus ppm graph. I can obtain the Rs/Ro ratio, will it means if I use the same method shown in the link I can get the concentration as well? – Blurman Dec 5 '17 at 0:50
  • I think that is specific for the TGS822. Every sensor might have the same shape of the curve, but for the TGS822, the calculation is split into two parts. A calculation for the resistance below 3600Ω and a calculation above that. I don't know if the 3600Ω can be used for other sensors. The overall accuracy is not very good, perhaps a simple calculation will do. – Jot Dec 5 '17 at 1:34
0

If you want to measure concentrations of individual gases you would have to separate them and detect them separately using a chromatography column. In the case of gases this is known as gas chromatography (GC) and for liquids it's LC, with the most popular one being HPLC (high performance liquid chromatography, or sometimes called high pressure liquid chromatography). This is actually a really fun thing to do as a project and I stumbled on this question because I want to make my own GC using the sensor you mentioned.

The principle is fairly simple. Most modern GCs have a carrier gas, usually an inert gas like nitrogen or helium, at a steady flow rate and it passes through a column. You inject your sample into this carrier gas flow and then your sample also passes through that column. Most columns are packed with a material like silica gel or long alkanes (usually around 18 carbon atoms). Silica has more affinity for polar compounds (like alcohols) while alkanes have more affinity for non-polar compounds like butane, hexane, etc. If you have a mixture of compounds, like in human breath or lighter fuel, they pass through that column at different rates depending on affinity for your stationary phase (the column) and also size of the molecules (heavier molecules like hexane pass through it more slowly than the lighter propane just due to size). At the end of the column you get the gases coming out separately, depending on the quality of your separation with the ones being less retained by the column coming out first.

A small problem with this is that the separation takes time and can take up a lot of time depending on conditions. Most GCs operate at about 200 degrees Celcius to increase the speed but also to make sure that everything actually is in the gas phase.

For a hobby project I just want to do it at ambient temperature first using lighter gas (which I know is a gas at room temperature) and for a column I will use bentonite clay, which they used back in the old days before silica and it's cheap. Kitty litter is made from bentonite clay as well so it's easily available. As a carrier gas just normal air using for example an aquarium pump or other small membrane pump.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.