Hello,

I am calibrating NO2 sensor where two factors, Gain and Resistor are important to determine Vout.
Which values should I choose for both of them for the purpose?
For now I am using gain = 1 and resistor = 1.

What should be the value of R0 in this case? Is it okay to use 2.2?

Thanks.

[1] Formula to calculate Rs given in the manual is:


Is it
or


[2] In e2V MiCS-2710 NO2 Sensor manual, I find a different formula to calculate Rs which is:


Which one should I use?

Thanks.

I would say just use the formula from the Gas manual.
The equation are simply derived by using the voltage divider law. So it depends on how the circuit of the Gas Board connects the NO2 sensor to power and to the variable load resistor.

Thanks AmroQuandour.

What about my other question?

Well the idea is as follows: -

1. You determine the "required" concentration you need for your application. For example, in my application I need to monitor NO2 concentrations of 0.5 ppm.

2. Now, the calibration process is as follows. You get a quotation from your local gas distributer or any company that supply gas and ask them to make you a mix of 0.5 ppm of NO2.
PPM is a volumetric unit. I.E - 1 ppm = 1 particle of NO2 in 10^6 particles in air.

3. You put your sensor inside a gas chamber and release the NO2 mixture into the gas chamber and observe the output of your sensor.. after a while you will notice that your sensor output will stabilize. At that point you know that the sensor is calibrated at 0.5 ppm of NO2. Using that voltage, you can measure the RS value and set that RS value as your R0 ...

4. Now, when you deploy your sensor network. your NO2 sensor voltage will increase if NO2 concentrations are lower than 0.5 ppm and will decrease if your NO2 concentration increases.

That's how you generally calibrate your sensor.

On the other hand if you need to measure at different concentrations, then you'll need to measure the output at different ppm levels (you also need a way to monitor how much ppm is in the gas chamber, so you will need another calibrated sensor). and then you have data for different concentrations.

---
Regards

Hi,

very good explanation, AmroQuandour (again :D, thank you very much). I just wanted to add that the load resistor for the tests that AmroQuandour mentioned may be taken from the graph and the air resistance value in the gases technical guide. This is an approximation for typical values, you may use it directly in your application but at the expense of an uncertainty in your measurement, if you need accuracy in the value obtained you should calibrate the sensor following the process described by AmroQuandour.

Regards.

Thanks AmroQuandour.
I am calibrating my sensors in the range of 600 ppb - 1 ppm. In my calibration, process is a bit complex. We vary gas concentration and first analyze it on NOx analyzer/FTIR and then based on observed concentration values, sensors are calibrated.



How can I derive RL from looking at the calibration graph provided in the sensor guide?
My sensors were initially giving values 0.0000000 in such a low concentration but worked when I increased RL to 3 from 1.
I also wanted to add that one of our node is not working perfectly. I will check it again with the second stage of calibration, and if it doesn't work well I will inform you.

Thanks.

I understand what you mean.
The value of RL is specified in the datasheet. So if the NO2 requires a minimum of 10 k ohms Load resistance, then you'd set RL= 10.
deriving RL itself does not affect your calibration process. I.E- it does not need to be calculated as per say in order to get accurate estimation of the concentration. You just have to use the RL value as specified in the datasheet.
Also note that the relationship between concentration and the Rs/R0 is in log10 scale. so you just have to use << Rs/R0 = b*10^m >> where b is constant .

Upload your code to see if something is wrong with that

Formula to calculate Rs given in the manual is:
Quote:
Rs=VcxRI/Vout-RI

Is it
Quote:
Rs = Vc*(RI/Vout-RI)

or
Quote:
Rs = (Vc*RI/Vout)-RI

Best,

Thanks.

Dear zeus2kx,

The right formula is:

RS = (Vc*RI/Vout) - RI

regards