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'[EE] Caphograph design'
2006\07\03@090702 by Russell McMahon

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I wish to build a Capnograph.
Don't know what that is?
I didn't either until yesterday - even though I wanted one.
It's an instrument to measure exhaled CO2 content.
Exhaled CO2 content is typically in the range 2% - 10.%
Y.CO2.M.V.

Real ones cost real money.
Most straight forwards method is measuring absorption at about 4.2 -
4.3 nm where CO2 has an absorption peak.
This usually involves producing light which includes this wavelength
and then filtering it with a suitable filter. These are commonly
enough avail;able due to their use in Automotive gas analysers, and,
not surprisingly, in Capnographs. Such filters are nastily
expensive - typically hundreds of dollars. This is a 'discretionary'
project and while such a cost is bearable it would be much better if a
cheaper solution was available.

One possible solution is the use of a diffraction grating or possibly
a prism.

Other people may have alternative innovative suggestions for
addressing the 4.2 nm line approach OR may have alternative methods. I
want a potentially "breath by breath" response and ideally electronic
logging so chemical means are unlikely to be useful.

I can imagine a "colour wheel" approach where CO2 is absorbed on a
substrate causing eg colour change, read by a sensor and then
regenerated by eg heating, all on a rotating wheel, might provide a
viable real time solution. How one achieves such a colour or other
change I don't yet know.

Pushing that a little further, exhaled air could be bubbled through a
solution causing transparency change and light occlusion and the
solution subsequently regenerated. Much messier than I would like, but
such "idea starters" may lead to other things.

FWIW the End Tidal value (the CO2 level at the end of exhalation)  is
an amazingly sensitive and valuable indicator for many medical
situations. ET value gives far faster response than Pulse Oximiter
blood oxygen readings to oxygenation changes (one breath versus a
minute plus), is a sadly accurate indicator of resuscitation prospects
and a good indicator of whether resuscitation is working (reading at
20 minutes resus gives near certain indication of death/life
prospects). It's invaluable as an intubation indicator and monitor for
patients during critical transfers.




       Russell McMahon



2006\07\03@091949 by Michael Rigby-Jones

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>-----Original Message-----
>From: spam_OUTpiclist-bouncesTakeThisOuTspammit.edu [.....piclist-bouncesKILLspamspam@spam@mit.edu]
>Sent: 03 July 2006 13:59
>To: PIC List
>Subject: [EE] Caphograph design
>
>
>Pushing that a little further, exhaled air could be bubbled
>through a solution causing transparency change and light
>occlusion and the solution subsequently regenerated. Much
>messier than I would like, but such "idea starters" may lead
>to other things.

This might be totaly off the mark, but instead of measuring CO2 content could you perhaps measure oxygen content, which presumably will be the inverse of the CO2?  Heated, wideband exhaust gas oxygen sensors are readily available though wether they will work outside of the heat of burnt exhaust gasses I don't know.  If not maybe mix exhaled breath with a fuel gas and measure combustion temperature (using some kind of catalyst rather than open flame..e.g. the stuff use in gas powered soldering irons).

As I say, probably not even worth the $0.02 these posts are traditonaly valued at...

Regards

Mike

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2006\07\03@170655 by James Newtons Massmind

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And may we ask WHY you want this? I'm trying to see the application.

---
James.



{Quote hidden}

> -

2006\07\03@172846 by Mauricio Jancic

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Hi,
       I have developed a product that detects CO and I think the sensor
can also detect CO2. the sensor is like the ones at http://www.citytech.com, but a
lot cheaper. I cant disclose the brand nor any detail, sorry, but that link
might be of help.

       Regards,

Mauricio Jancic
Janso Desarrollos
http://www.janso.com.ar
EraseMEinfospam_OUTspamTakeThisOuTjanso.com.ar
(54) 11-4542-3519


> {Original Message removed}

2006\07\03@203424 by Denny Esterline

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What about mass / specific gravity / density? Sure, breath is a messy soup with various gasses and moisture and such, but most of those things seem considerably easier to measure.

Just a thought.

-Dk

{Original Message removed}

2006\07\03@222516 by Russell McMahon

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> What about mass / specific gravity / density? Sure, breath is a
> messy soup with various gasses and moisture and such, but most of
> those things seem considerably easier to measure.

Gotta admit I thought of that and then left it out of the post to
think about as it seemed too good ;-).

Mass is low but doable.
Water vapour needs removing.
Air mass is 1.3 grams per litre and this increases by about 0.5% per
1% of CO2 or about 50 micrograms per litre per percent of CO2. A
balance system would be easy enough to trial.Direct strain gauge
weighing or (name escapes me) electronic balancing against a voice
coil or ...

Also moment of inertia type approach or ...



       RM


2006\07\03@224050 by Russell McMahon

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> This might be totaly off the mark, but instead of measuring CO2
> content could you perhaps measure oxygen content, which presumably
> will be the inverse of the CO2?  Heated, wideband exhaust gas oxygen
> sensors are readily available though wether they will work outside
> of the heat of burnt exhaust gasses I don't know.  If not maybe mix
> exhaled breath with a fuel gas and measure combustion temperature
> (using some kind of catalyst rather than open flame..e.g. the stuff
> use in gas powered soldering irons).
>
> As I say, probably not even worth the $0.02 these posts are
> traditonaly valued at...


Aaagh.
Sounds totally obvious and totally brilliant to me.
I have easy access to O2 sensor at a reasonable if not marvellous
price.

Given the nature of the combustor involved (eg you and I and ...) the
other gases liable to be present can probably be well enough
quantified to make that an adequately acceptable approach.

I'd say that was one of the better 2 cents you had there. So obvious
I'm embarrassed :-)



       Russell McMahon


2006\07\03@225802 by Matthew Miller

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On Tue, Jul 04, 2006 at 12:59:11AM +1200, Russell McMahon wrote:
> I wish to build a Capnograph.
> Don't know what that is?
> I didn't either until yesterday - even though I wanted one.
> It's an instrument to measure exhaled CO2 content.
> Exhaled CO2 content is typically in the range 2% - 10.%
> Y.CO2.M.V.

Russell, could you use the quantitative lab method? Pass the exhaled air
through concentrated sulfuric acid, to remove the H20, and then through dry
NaOH. Then measure the before and after  mass difference of the NaOH
container. Is this method used any longer? Is there a better method to use
to make this measurement?

Take care.

Matthew

--
Necessity is the plea for every infringement of human freedom. It is the
argument of tyrants; it is the creed of slaves.
                                                 -- William Pitt, 1783

2006\07\03@230638 by dbwood

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How about IR gas absorption sensors? The thermopile devices (from
Perkin-Elmer) are a little of the costly side, but I've been able to
design some very accurate sensors using them.

Douglas Wood


{Original Message removed}

2006\07\04@015256 by Jesse Lackey

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I recently used this CO2 sensor:

Figaro TGS4161
http://www.figarosensor.com

to detect when someone breathes on it.  It worked well!  I was able to
tweak things to make it quite sensitive.  I did: sensor -> sensor buffer
opamp -> PIC A/D, and wrote some code to watch the flow of numbers and
detect a fast-moving trend.

I think they are around $40, the client bought them.

Jesse


Russell McMahon wrote:

{Quote hidden}

2006\07\04@142850 by Peter

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On Mon, 3 Jul 2006, Mauricio Jancic wrote:

> Hi,
>        I have developed a product that detects CO and I think the sensor
> can also detect CO2. the sensor is like the ones at http://www.citytech.com, but a
> lot cheaper. I cant disclose the brand nor any detail, sorry, but that link
> might be of help.

CO detectors are catalytic (CO is flammable). CO2 is not. The CO
detectors output CO2 ;-)

Peter

2006\07\04@143233 by Peter

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On Tue, 4 Jul 2006, Russell McMahon wrote:

>> What about mass / specific gravity / density? Sure, breath is a messy soup
>> with various gasses and moisture and such, but most of those things seem
>> considerably easier to measure.
>
> Gotta admit I thought of that and then left it out of the post to think about
> as it seemed too good ;-).
>
> Mass is low but doable.
> Water vapour needs removing.
> Air mass is 1.3 grams per litre and this increases by about 0.5% per 1% of
> CO2 or about 50 micrograms per litre per percent of CO2. A balance system
> would be easy enough to trial.Direct strain gauge weighing or (name escapes
> me) electronic balancing against a voice coil or ...
>
> Also moment of inertia type approach or ...

Could you use a luft cell sensor ? Does it have to be small ?

Peter

2006\07\05@092045 by Vasile Surducan

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4.2nm or 4.2um ? CO2 is absorbing in the near IR or in the far UV ?

greetings,
Vasile

On 7/3/06, Russell McMahon <apptechspamspam_OUTparadise.net.nz> wrote:
{Quote hidden}

> -

2006\07\05@100919 by David VanHorn

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On 7/5/06, Vasile Surducan <@spam@piclist9KILLspamspamgmail.com> wrote:
>
> 4.2nm or 4.2um ? CO2 is absorbing in the near IR or in the far UV ?


um, near IR

2006\07\05@113358 by Russell McMahon

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> 4.2nm or 4.2um ? CO2 is absorbing in the near IR or in the far UV ?

What's 3 orders of magnitude between friends ? :-)

As you imply, its 4.2 um - deepish infrared.


       Russell

2006\07\06@020244 by Vasile Surducan

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On 7/5/06, Russell McMahon <KILLspamapptechKILLspamspamparadise.net.nz> wrote:
> > 4.2nm or 4.2um ? CO2 is absorbing in the near IR or in the far UV ?
>
> What's 3 orders of magnitude between friends ? :-)

Indeed right,  does it matter if the prism is made by glass or by NaCl
or BaTio3 ?
the 3 orders of magnitude is reflected only in price...
:)

cheers,
Vasile

>
> As you imply, its 4.2 um - deepish infrared.
>
>
>        Russell
>
> -

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