Understanding metal oxide semiconductor gas sensors' datasheet.

Question

I just need some help understanding how a circuit that I already built works.

I am using the the Figaro TGS 2201 air quality sensor. The datasheet for it can be found here: tgs 2201 datasheet from manufacturer

Currently, my sensor setup is as follows:

I am not using R2 but only using R1, which makes the air sensor targeted at gasoline exhaust rather than diesel. My R1 is equal to 10k ohms, as specified as the minimum value in the datasheet.

From my understanding, as the concentration of the gas that the sensor is targeted for increases, the conductivity across pin 3 on the sensor increases proportionally (logarithmically) to the concentration of the gas in the air.

I have a micro-controller read the DC voltage from pin 3, and i measure between 420 and 410 milivolts.

In order to translate this value into parts per million (concentration of the gas), the data sheet provides the following equation for calculating Rs:

Rs = (Vc - VRL)/VRL *RL

How do i measure RL and VRL? I assume that Vc is 5 volts because that is what my voltage regulator outputs.

Furthermore, in order to obtain Ro, do i need to run a control, and record RL in clean air and assign Ro to that value?

Answer 1

How do i measure R_L and V_RL?

They're noted on page 2 of the datasheet you linked. R_L is your 10k resistor. V_RL is the voltage developed across it. V_C is the sensor's supply voltage (note that the heater (V_H) must be 5V +/-5%, but V_C can be up to 15V max per the datasheet).

Furthermore, in order to obtain Ro, do i need to run a control, and record RL in clean air and assign Ro to that value?

The datasheet defines R_o as "Sensor resistance in clean air". So if you require R_o, it would appear that you need a sensor exposed to clean air. I'd suggest chatting with the manufacturer as to whether your application requires a) continuous operation of a clean air reference sensor, b) if it's just a one-time calibration step, or c) if it can be done empirically.

Answer 2

I've designed for similar sensors from e2v (NO2 and CO) in connection with the Air Quality Egg project. For that project I actually implemented a feedback control loop to control the heater voltage to keep it within the specified power output (schematics here and here). These sensors change resistance in the presence of different concentrations of gas so long as the surface temperature is well regulated. They are also known to have strong correlation with temperature and humidity, so watch out for that.

You need to measure V_RL1 and V_RL2 with your ADC. From those measurements, and knowing the load resistances (R_L1 and R_L2) and high-side voltage, you can infer the resistance of each sensor using the standard voltage divider equation described in the datasheet. These sensors also tend to have a fairly broad range of resistance that they can vary over, so I also implemented a 'configurable' low side resistance as well to handle different orders of magnitude in resistance. This requires some firmware.

Going from there to gas concentration requires a bit of creativity. In the Air Quality Egg, I stored a tabulation of the response curves that relate RS/R0 to concentration and interpolated between the values. If you care a lot about absolute accuracy, you need to store R0 in a mutable location (like EEPROM), and calibrate by some procedure.

I wrote up a decent hardware design description here that describes the circuitry I implemented if you're interested. And the firmware for the shield is located here if you want to browse the code that runs the hardware.

Answer 3

Standard Test condition is given at 200K load for clean air. Spec is given as RM1 ~ 0.1 MΩ to 2 MΩ for clean air (after climatizing)

And sensitivity as 12±8 for 0.3 ppm NOx for Rs ratio to RM2 of clean air. However graph shows sensitivity of near worst case = 5 at 0.3ppm

So basically resistance goes up with NOx and thus voltage goes down across VR2 and voltage from RL to RM1 ratio is easy to calculate.

Since using 5V for both sensor and heater, tolerance of 2% prevails.

Side note: I do not know for sure, but if you are not using low sulpher diesel fuel in the USA as they use in Europe, you may get false readings from lowering the resistance of the sensor. Anyone know?