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I'm building a CO2 measurement unit via Arduino and I'm the phase of choosing the sensors. I've found out this IR sensor that, thanks to an emitter with WL of 4.26um is able to output a voltage proportional to the concentration in air of CO2. To calibrate the circuit I must use a titrated gas (% well known) and measure the voltage in output from the sensor.

My problem now is that I can't understand, with this measure done, how associate further measurement with the % of the gas.

Is this linear? Does it depend on the sensor (the distance between the LED and the sensor is fixed)?

I've never used IR or Gas sensors so, this is my first experience with them and I'm pretty disoriented

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  • \$\begingroup\$ What emitter are you using that produces 4300 nm? What bandwidth of light does it produce. I ask these questions first in order to understand your method and whether you plan to also measure 3900 nm (no CO2 effect). Pyreos do a 2 ch version BTW. \$\endgroup\$ – Andy aka Jul 11 at 11:51
  • \$\begingroup\$ I'm using an IR Led Emitter hamamatsu.com/resources/pdf/ssd/l13201_series_kled1069e.pdf We're still figuring out how to measure the no CO2 state. \$\endgroup\$ – NicoCaldo Jul 11 at 12:25
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The L13201 emitter is something I've looked at because it emits right on the sensitive wavelength for CO2 i.e. 4300 nm and it also has significant emissions at 3900 nm where CO2 has no effect: -

enter image description here

Vertical blue line added by me is the sensitivity at 3900 nm and the red line is about 4300 nm.

So, if you chose the Pyreos dual sensor of the type that has one channel sensitive to 3900 nm and the other channel at 4300 nm, you get a reference channel and a measurement channel all from one light source namely the L13201.

It's important to get the reference channel because that channel is unaffected by CO2 and can be used to stabilize the light output either by feedback or post compensation of the signals (in hardware but more likely software).

That reference channel is also subject to the same signal deteriorating factors that the CO2 channel suffers from such as contaminants such as water vapour or some other environmental effect. See below for the general idea: -

enter image description here

At 3900 nm there are no real signal perturbations due to any of the gases commonly associated with this part of the spectrum so it's "useful".

My problem now is that I can't understand, with this measure done, how associate further measurement with the % of the gas.

The ratio of the measured signal to the reference signal is the approach taken by several CO2 sensor manufacturers such as City Technology and SGX Sensortech to name a few that I've come across. That ratio as it drops, is the measure of CO2 present in the sampled gas.

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  • \$\begingroup\$ yeah, you are right. Pyreos gave me an Excel file with, given the Gas and Ref signal, an algorithm to calculate the %. Of course, you need to know at least 2 points with % and V out first. Thanks for the graphics tho, super useful. \$\endgroup\$ – NicoCaldo Jul 11 at 14:05
  • \$\begingroup\$ @NicoCaldo price is a big problem with the L13201 - what have you been quoted BTW? \$\endgroup\$ – Andy aka Jul 11 at 14:09
  • \$\begingroup\$ 27€ each one. Yeah, very expensive for an IR LED \$\endgroup\$ – NicoCaldo Jul 11 at 14:27
  • \$\begingroup\$ I got quoted a lot more from Hamamatsu in the UK - you got a better price than me - did you deal with Hamamatsu directly or through a dealer? \$\endgroup\$ – Andy aka Jul 11 at 14:45
  • \$\begingroup\$ Don't know. It wasn't my task to ask for the prize \$\endgroup\$ – NicoCaldo Jul 11 at 14:50
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I have done some work in this area, and it is not quite as simple as one might assume. We use Vaisala sensors, which are rather expensive.

At the very least you will need two reference gases with different % of CO2. At the simplest they could be pure air scrubbed clean of CO2 by bubbling through a NaOH solution, and a gas with a known % of CO2. Perhaps even 100%.

To determine whether the sensor is linear you will need a third measurement with a different % of CO2.

We usually use N2 as a carrier gas and mass flow controllers to do mixing in custom apparatus. None of it cheap. I would be interested to know how you get on with this project.

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  • \$\begingroup\$ So the fastest solution would be to ask the company that produces the sensor if they have a response curve of the sensor itself. If not, I've to do myself as you described with, at least, 3 measure (one could be a vacuum measurement where I'm sure the gas concentration is 0%). The project came out because it's very difficult to find a gas measurement station with specific gas (CO, H-C, SO2, NO, NO2) and, thanks to that sensor I'm trying to build one myself. \$\endgroup\$ – NicoCaldo Jul 11 at 8:59
  • \$\begingroup\$ @NicoCaldo Yes. I am actually working with a couple of PhD student at Imperial College London on similar stuff. It's going to take them 2 years. The other problem you might encounter is the sensor's sensitivity to water vapor. Where are you based? \$\endgroup\$ – Dirk Bruere Jul 11 at 9:43
  • \$\begingroup\$ So, I actually got from the sensors' company an Excel file that calculates and draws the output characteristic of the sensor from their algorithm (they provided the algorithm also). Don't know about sensor's sensitivity to water vapor but Pyreos states that their sensors have an S/N of 10k. Btw I'm based in Italy (Vicenza) \$\endgroup\$ – NicoCaldo Jul 11 at 10:28
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    \$\begingroup\$ @NicoCaldo Well, my best suggestion is that if you are not already at a university you find one near you where you can talk to people using these types of sensors. I assume you are looking at combustion products, so it's a Chem Eng dept. You might also be able to use their facilities rather than buy in talks of gas yourself \$\endgroup\$ – Dirk Bruere Jul 11 at 10:34
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    \$\begingroup\$ @NicoCaldo One more suggestion - maximize the path length of the light through the gas. There are two ways to do this. Either run the light and gas through a long glass tube with aluminized exterior. Or run the gas through a cell with the light bouncing between two mirrors \$\endgroup\$ – Dirk Bruere Jul 11 at 10:50

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