Laser Gas Sensor

Thread Starter

LostTime77

Joined Dec 9, 2010
30
Hey all:

As the thread topic suggest, we are planning on doing some experiments with laser gas sensors. These are available on the market; however, for our application, they are bulky and have extra features that are unneeded. We want to make a sensor that is just the bare sensor. Weight and space is an issue with these large sensors. I apologize in advance if this has been asked before.

What we were thinking of doing is using a laser emitter that emits light at 760nm for oxygen. The laser would be shot through the gas sample and then detected on the other side by a photo diode. The photo diode would be able to measure light intensity or power. This laser is based on the fact that the oxygen in the gas would absorb light at 760nm. Depending on the input and output power of the sensor, you can measure oxygen concentration in the sample.

I read somewhere on these forums that modulating or shooting the laser out at a frequency rather than just switching it on / off and reading would allow the laser light to be separated from other sources of light? How does this work?

How does gas temperature affect the system?

As this is a new design, we need help with the theory of this sensor. I believe this is how commercial sensors do the detection, but I am not sure. In the end, we want to create a low power (milliwatt) high response time sensor that would be able to be used in our application. We understand the complexity of the issue and are willing to work with it. Keep in mind that we are not chemists and are engineers. We don't know the specifics of every little thing.

Let me reiterate the fact that we cannot use commercial sensors because of the following reasons: They are bulky as in their dimensions are in the several inches, sometimes feet. We need something that can fit in an extremely small space - a few inches. They are expensive: hundreds of dollars or more. A laser emitter and a photo diode could be had for tens of dollars or less. They consume lots of power, most of the time several watts. Our application is very power strained. We are balancing power with several devices, so the lower the power the better. We would like to consume milliwatts compared to watts.


Thanks,
LostTime77
 

Thread Starter

LostTime77

Joined Dec 9, 2010
30
Thank you for your suggestion. I don't do a ton with light type sensors. What we ideally want is something that has fast response times. Given your suggestion, the current selection is an LDR, a photo diode, a photo transistor, and a solar cell. From research, photo diodes are generally used to detect intensities or powers. A photo transistor is slow to respond. An LDR strikes me as another device that is slow to respond, but correct me if I am wrong. Not sure about a solar cell.

From your circuit, it seems like you used a photo diode to do the detection. How reliable is your laser pointer power detector and or accurate? I am just curious as I have never done this type of project before. We may need our device to be extremely sensitive as I am not sure how well gases actually do absorb light, or even how fast they absorb light.

LostTime
 

wayneh

Joined Sep 9, 2010
16,399
I think you're looking for a needle in a haystack, and therefore will need a high signal to noise ratio. Modulating your laser is one way to get a big boost in S/N. It's not the modulation per se, it's the filter you put on the detector that prevents passing signals that are not near the modulation frequency. This knocks out a huge portion of the noise. This is all simpler to implement than it sounds, and will be far easier than other measures. Also consider mechanical means of eliminating noise, for instance shielding ambient light.

How do you know you need a laser, as opposed to just the right LED?
 

BMorse

Joined Sep 26, 2009
2,675
Thank you for your suggestion. I don't do a ton with light type sensors. What we ideally want is something that has fast response times. Given your suggestion, the current selection is an LDR, a photo diode, a photo transistor, and a solar cell. From research, photo diodes are generally used to detect intensities or powers. A photo transistor is slow to respond. An LDR strikes me as another device that is slow to respond, but correct me if I am wrong. Not sure about a solar cell.

From your circuit, it seems like you used a photo diode to do the detection. How reliable is your laser pointer power detector and or accurate? I am just curious as I have never done this type of project before. We may need our device to be extremely sensitive as I am not sure how well gases actually do absorb light, or even how fast they absorb light.

LostTime

If your project needs some speed, then yes an LDR would have a very slow response time.... the circuit I posted was designed by someone else, but it had worked for what I needed it for..... as for the solar cell, this would work very well, solar cells are used in many laser barcode scanners to detect the reflections back from the laser...... in one of the threads on here called "Fast Horses" I designed a long range laser detector for another forum member, there are some circuits on there utilizing a tiny 1/8" x 1/8" solar cell to detect an off the shelf "laser level" type laser from a distance... but it does use some special laser optics.....
 

Thread Starter

LostTime77

Joined Dec 9, 2010
30
I think you're looking for a needle in a haystack, and therefore will need a high signal to noise ratio. Modulating your laser is one way to get a big boost in S/N. It's not the modulation per se, it's the filter you put on the detector that prevents passing signals that are not near the modulation frequency. This knocks out a huge portion of the noise. This is all simpler to implement than it sounds, and will be far easier than other measures. Also consider mechanical means of eliminating noise, for instance shielding ambient light.

How do you know you need a laser, as opposed to just the right LED?
I could use a tunable laser diode which are readily available. The problem is that the gas is going to be fairly hot, meaning close proximity will be a problem. This is where we had the idea of a laser since the laser could be shot from a short distance without seeing the effect of temperature. It might be possible to do something similar with a normal LED as well.
 
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