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Guest · Posted: Sat Oct 04, 2008 5:18 pm Post subject: Gas sensor

I want to realize a gas sensor for a small room: i>ve found that
absorption peak for methane is 3.3um and InAs photodiode have that
peak. But i have no idea how to realize it: what type of laser diode
is used?Where i can put it?
Thanks for any suggestion

Bob May · Guest

You can also use a infrared lamp and a narrowband dicroic filter to seperate
out the wavelength.

--
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net

Skywise · Guest

lionelgreenstreet@gmail.com wrote in news:4063459e-393a-43bb-98db-
a72911f9df68@u65g2000hsc.googlegroups.com:

[quote]I want to realize a gas sensor for a small room: i>ve found that
absorption peak for methane is 3.3um and InAs photodiode have that
peak. But i have no idea how to realize it: what type of laser diode
is used?Where i can put it?
Thanks for any suggestion
[/quote]
Fart detector?

Brian
--
http://www.skywise711.com - Lasers, Seismology, Astronomy, Skepticism
Seismic FAQ: http://www.skywise711.com/SeismicFAQ/SeismicFAQ.html
Quake "predictions": http://www.skywise711.com/quakes/EQDB/index.html
Sed quis custodiet ipsos Custodes?

Richard J Kinch · Guest

Skywise writes:

[quote]F**t detector?
[/quote]
Don>t laugh.

I had an RV with a propane leak detector that howled like a banshee. These
are designed to age into *increasing* sensitivity as a safety feature. No
kidding, this thing would start wailing if you so much as expelled a bit of
natural hydrocarbon gas in the vicinity. It was the source of much
hilarity. Middle of the night, family fast asleep, then "*BEEP* *BEEP*
*BEEP* WHO F**TED!?"

Iain Mackay · Guest

Most commercial IR HC gas detectors currently use two IR channels - one as
the measurement at around 3.3um , the other as a reference, normally around
3.0um. (The main problem we see with this technology is water interference
which can interfere with the 3.3um channel giving false positives.)

With the exception of open path gas detectors, most systems use a filtered
broad band source eg, a small flashing incandescent bulb.

I>m not aware of any reasonable priced detectors that use a laser at 3.3um -
current las-based gas detectors use cheap telecom lasers and tend to
operate around 1.5um using (weak) overtone absorption features.

There are some research facilities using, for example, difference frequency
generation to reach 3.3um but this is currently expensive and cumbersome.

Unless the objective is simply to roll your own, you can buy one, two and
three band HC gas sensor pyros 'off the shelf' from a number of places.

HTH
IM

<lionelgreenstreet@gmail.com> wrote in message
news:4063459e-393a-43bb-98db-a72911f9df68@u65g2000hsc.googlegroups.com...
[quote]I want to realize a gas sensor for a small room: i>ve found that
absorption peak for methane is 3.3um and InAs photodiode have that
peak. But i have no idea how to realize it: what type of laser diode
is used?Where i can put it?
Thanks for any suggestion[/quote]

Charles Manoras · Guest

"Skywise" wrote

[quote]I want to realize a gas sensor for a small room: i>ve found that
absorption peak for methane is 3.3um and InAs photodiode have that
peak. But i have no idea how to realize it: what type of laser diode
is used? Where i can put it?
Thanks for any suggestion

Fart detector?
[/quote]
Maybe.

But what about H2S hydrogen sulfide then?
Another gas generally associated with the above emanation?

It is true however that while CH4 methane is odorless, H2S hydrogen sulfide
is not.

Therefore your own nose is an adequate detector.

The verb to realize as used here instead of to make / to build / to put
together/ to assemble etc. betrays the fact that the OP must be French IMO.
:-)

DM · Guest

Richard J Kinch wrote:
[quote]Skywise writes:

F**t detector?

Don>t laugh.

I had an RV with a propane leak detector that howled like a banshee. These
are designed to age into *increasing* sensitivity as a safety feature. No
kidding, this thing would start wailing if you so much as expelled a bit of
natural hydrocarbon gas in the vicinity. It was the source of much
hilarity. Middle of the night, family fast asleep, then "*BEEP* *BEEP*
*BEEP* WHO F**TED!?"
[/quote]
Apparently only about 30% of the population have the ability (?) to fart
methane, and even then the concentration is pretty low.

I>ve made a few fairly sensitive laser based methane detectors in my time,
and a few guys in the lab have tested them out but we>ve yet to pick up any
reliable fart detection.

cheers

David

Charles Manoras · Guest

"DM" wrote

[quote]Apparently only about 30% of the population have the ability (?) to fart
methane, and even then the concentration is pretty low.

I>ve made a few fairly sensitive laser based methane detectors in my time,
and a few guys in the lab have tested them out but we>ve yet to pick up
any reliable fart detection.
[/quote]
A long time ago a medical student told me that he had witnessed a mishap
in the OR, the surgeon was operating on a poor individual who had done
something quite stupid but possibly enjoyable (at least for him) for a
while.

He had inserted a bottle in his rectum and there it had broken!

(My father an MD was fond of saying after many years on the job that the
weirdest things will and do occur and that some patients were still capable
of surprising him.)

The surgeon was using an electric scalpel (in french "thermocautere")
in the area (supposedly to avoid excessive bleeding) and a small explosion
did occur.

He attributed the explosion to the gaseous content of the above mentioned
cavity, although there could be several other explanations (more plausible)
but med students are med students...

Maybe he was pulling my leg.

I was skeptical but he assured me that some of his friends were capable
of blowing out candles in a "certain manner" while at the same producing
a fairly lengthy flame.

Methane being a so-called green house gas the topic bears watching! :-)

Richard J Kinch · Guest

Charles Manoras writes:

[quote]Maybe he was pulling my leg.
[/quote]
Yes, considering the explosive limits of methane in air, a ratio not to be
found in the gut, thankfully. Insufflation would have applied CO2.

Guest · Posted: Wed Oct 08, 2008 2:04 pm Post subject: Re: Gas sensor

Thanks for the infos,
but i don>t understand very well how all system works...
1)if i use a laser diode or a flashing incandescent bulb, i pilote it
with pulsed current, is it right?
2)if gas is not present, light emitted by source is captured by
photodetector or not? If not captured, where it arrives?
3)How can i distinguish between radiation emitted by source and
radiation absorbed and reemitted by gas?
thanks and excuse me for all these questions

Charles Manoras · Guest

"Richard J Kinch" <kinch@truetex.com> wrote

[quote]Charles Manoras writes:

Maybe he was pulling my leg.
[/quote]
I always thought that it might have been related to some carelessness during
the anesthesia, not an uncommon occurence then.

[quote]Yes, considering the explosive limits of methane in air, a ratio not to be
found in the gut, thankfully.
[/quote]
The topic was studied by none other than Benjamin Franklin.

http://en.wikipedia.org/wiki/Fart_Proudly

But he did not study the composition, the science of the time did not permit
it.

Bob May · Guest

The reason for pulsing the source is to mak a signal that you can recognize.
Doint this allows you to read a signal in the midst of a huge base level.
This is used all of the time and is a standard method of detecting signal
from the ambient light.

--
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net

Guest · Posted: Thu Oct 09, 2008 6:09 am Post subject: Re: Gas sensor

Ok, i>ll try to understand with an example....
i have a photodetector and a pulsing source....between them i can find
gas that i should measure
1)if gas no present:
nothing is absorbed by gas (it is not present), so all intensity made
by source is captured by photodetector
2)if gas is present:
a fraction of radiation is absorbed by gas, the remaining part is
absorbed by photodetector
So if i know quantity absorbed by gas (baecuase i have previously
measured all intensity source and stored its value), i can know gas
concentration
Is this right?

DM · Guest

lionelgreenstreet@gmail.com wrote:
[quote]Ok, i>ll try to understand with an example....
i have a photodetector and a pulsing source....between them i can find
gas that i should measure
1)if gas no present:
nothing is absorbed by gas (it is not present), so all intensity made
by source is captured by photodetector
2)if gas is present:
a fraction of radiation is absorbed by gas, the remaining part is
absorbed by photodetector
So if i know quantity absorbed by gas (baecuase i have previously
measured all intensity source and stored its value), i can know gas
concentration
Is this right?

[/quote]
You have roughly got the right idea.

What happens in reality (for broadband IR absorption) is that you use two
receivers one with a filter that corresponds to the wavelength region where
the absorption band is and another that is removed from the absorption
region that acts as your reference signal.

As the light source for both the reference and measurand is the same the
reference signal is used to compensate for any changes in the source
intensity, dirty optics, etc.

If using a laser diode based system then the measurement techniques are a
little bit different.

cheers

David

Guest · Posted: Thu Oct 09, 2008 8:18 pm Post subject: Re: Gas sensor

Thanks for your answer....
[quote]
What happens in reality (for broadband IR absorption) is that you use two
receivers one with a filter that corresponds to the wavelength region where
the absorption band is and another that is removed from the absorption
region that acts as your reference signal.

As the light source for both the reference and measurand is the same the
reference signal is used to compensate for any changes in the source
intensity, dirty optics, etc.

If using a laser diode based system then the measurement techniques are a
little bit different.
[/quote]
If i use a laser diode the reference signal isn>t so important,right?
I think that laser diode makes a more
predictable signal....or not?
In general how much time is present between radiation absorption by
gas and radiation re-emission?
Thanks


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