Hi!
I am working on a project and I would like to know if it is possible to determine a gas concentration that is flowing inside a tube with a laser and a receptor.
Is it possible? Or is there any other way to do this without disturbing the flow?
Thanks for the attention!
If the detector puts-out a voltage, yes. The Arduino has a 10-bit analog-to-digital converter (it reads 0-1023) with a range of 0-5V, or optionally 0-1.1V, or you can supply your own voltage-reference.
You'd probably need a amplifier, but of course we don't know anything about your detector.
If your asking for help with your laser-detector, you may not find experts here... That's a chemistry/science question... Personally, I have no idea if this will work, and if it does work it would depend on the type of gas and the concentration.
What gas?
What concentration?
What accuracy?
The gas is a mix of air + cooking oil. The main objective is to compare the concentration of the cooking oil before and after passing threw a filter. It is not necessary to have the best accuracy, just enough to see the difference between the two phases.
Thank you!
There is no detector for cooking oil.
Detectors for fine particulate matter (by light scattering) could be used, but they are expensive.
FranciscoMota:
Hi!I am working on a project and I would like to know if it is possible to determine a gas concentration that is flowing inside a tube with a laser and a receptor.
Is it possible? Or is there any other way to do this without disturbing the flow?
<...>
Anything is possible is you can describe the physics/chemistry and if there are sensors for the process. Building one's own sensor is possible, but this forum generally does not engage in speculation. This is where experimentation mu st fall into your role.
If you tell us that a laser pointed across a volume of air to a type of sensor will produce a voltage output on the sensor ranging from , then we can tell you that the Arduino has the ability to monitor analog voltages between 0V and the Vref (generally 5V but can be lower ... such as using the internal 1.1V source.)
There are also software techniques to get that 2^10 analog step up by 1 or sometimes even 2 more order of magnitude... but there are trade-offs.
I would think that cooking oil that is emitted as a vapor and pulled through an exhaust would be sensitive to a particular wavelength. A photocell that is sensitive to that same particular wavelength would seem to be a reasonable approach to measuring average concentration across an area of the duct. However, I suspect the flow of air and particulates would not be uniform across a rectangular (or circular) exhaust pipe due to turbulent air flow through the duct... suggesting a series of sensors may be required.
Ray
So, the real issue is the sensor and I have no clue...
If you try to do it optically, my fear is that the oil will accumulate around the sensor (or around the glass, etc.) and that would foul-up your measurements... In fact, that might happen with any kind of sensor.
If you don't need continuous measurements the best solution might be to use a filter (or use the existing filter) and weigh it (with a precision lab balance) before & after to see how much oil you "catch".
And of course, oil is not a gas. 
There are gas & particulate components of burned oil and there may be gas components of evaporated/boiled oil.
Well, I saw in some papers that the diode laser would be the best one. The mix is air + burned oil so it would be on gas phase.
I want to know how can I determine the energy that is leaving the laser and the energy received on a photoresistor or any other kind of receiver so I can compare.
I appreciate your reviews and doubts and I will try to analyze it in order to see if it is feasible.
If you have any suggestion or other options please tell me, it will only help.
Some info on how to make light scattering detectors.
Hi,
Your detection process depends on if you are using scatter, as has been mentioned, or absorption?
Absorption uses the filtering characteristics of the medium you are measuring.
If all you are using is Air and Cooking Oil you will have to do some experimentation, and if you need quantitative results some method of calibration.
Tom... 
Why do you want this? Do you want to make sure your filter is working? Do you want to monitor the filter to know when it is time to replace it? Do you want to use it in different scenarios such as different speed of gas or transparence of the tubes?
I am not sure if you need a laser - common LED may work as good as laser diode. You may try to use LEDs of different colors and different detectors (photoresistor, another LED, photodiode) possibly with (instrumental) op-amps and see if there is a difference between dirty gas before the filter and cleaned gas after it (or ambient air). But unless you have vast knowledge you cannot be sure what are you in fact measuring - particles of the oil? CO2? Some strange byproducts of burning? O2? A bit better may be comparing gas after new and exhausted filter...
I need to know exactly the gas concentration. I think I will use Tunable diode laser absorption spectroscopy, do you think it is feasible? Using a diode laser and a photodiode to read the laser beam after it pass across the gas.
I am a chemistry graduate so may be able to help on the science side...but firstly..
This could be done. I wouldn't expect amazing accuracy nor precision. The resolution may be pretty good though (although useless).
You will need to do the following to form a spectrum:
A: A blank reading. A tube with the carrier gas an none of the target gas.
B: A calibration curve. A set of KNOWN concentrations of the gas you wish to measure.
C: A semi-wide spectrum light source of some form...ideally one that has a lot of its wavelength that is not absorbed by usual gasses in air (H20, N2,O2,CO2 mainly) but is absorbed strongly by your target gas. This will help with your sensitivity (signal:noise)...
Here is an example I have drawn up in paint...hope it helps...
Seeming a "oil" has a lot of C-H alkyl bonds, the 3000 cm-1 waveband would probably be of interest to you.
3000cm-1 = ~ 3400nm
(10E6 / wavenumber in cm-1 = wavelength in nm).
There are mid-IR diodes available for this.
The main objective is to compare the concentration of the cooking oil before and after passing threw a filter.
Simply weighing the filter before and after the test should give you some idea of how good the filter is.