sensor fundamentals

[George Yu]

Hi everyone,

Just want to get people here to understand some limitations of the
sensors mentioned here.

1. cross sensitivity

most of these metal oxide sensor readings are not specific to any
single gas. So if you take a CO sensor, and breath alcohol or water
vapor on it, the sensor will read a high CO concentration.

2. sensor stability

as the weather conditions such as temperature or humidity changes, a
CO or NOx metal oxide sensor will drift significantly. The only to way
minimize that is to build a temperature and humidity offset
calibration curve.

3. sensor uniformity

for metal oxide sensors, the manufacturing process makes it very
difficult to make two sensor to have exactly the same response. The
gas concentration reading could be +/-30% in variations.

4. power consumption

metal oxide sensors uses an internal heater to heat the sensing
element so it require large amount of power. It is difficult to use
only batteries to supply power for the sensors.

[Chris Nafis]

So do you know more about the fuel cell Electrochemical type of CO monitors?

According to http://en.wikipedia.org/wiki/Carbon_monoxide_detector
"This is a type of fuel cell that instead of being designed to produce
power, is designed to produce a current that is precisely related to the
amount of the target gas (in this case carbon monoxide) in the atmosphere.
Measurement of the current gives a measure of the concentration of carbon
monoxide in the atmosphere. Essentially the electrochemical cell consists of
a container, 2 electrodes, connection wires and an electrolyte - typically
sulfuric acid. Carbon monoxide is oxidized at one electrode to carbon
dioxide while oxygen is consumed at the other electrode. For carbon monoxide
detection, the electrochemical cell has advantages over other technologies
in that it has a highly accurate and linear output to carbon monoxide
concentration, requires minimal power as it is operated at room temperature,
and has a long lifetime (typically commercial available cells now have
lifetimes of 5 years or greater)."

My concern is that non of these sensors (both seminconductor or
Electrochemical) seem to be sensitive enough to measure street level CO
(0<8ppm).

[George Yu]

oh, btw, electrochemical sensor can easily measure CO from 0 to 400ppm
at 0.1ppm resolution

we had done a lot of those gas flow experiments in the lab to compare
with experimental sensors.

-George

On Mar 29, 11:11 pm, "Chris Nafis" <na...@nycap.rr.com> wrote:
> So do you know more about the fuel cell Electrochemical type of CO monitors?
>

> According tohttp://en.wikipedia.org/wiki/Carbon_monoxide_detector

[Nafis]

According to Kidde their sensor is electrochemical:
http://www.kidde.com/utcfs/ws-384/Assets/900-0076%20Sheet.pdf
It looks pretty straight-forward to interface/hack the unit. It can
measure
concentrated CO levels as low as 11ppm and up to 999 PPM
It has a 40F-100F and 5-95% Relative humidity range
You can pick them up for $40 at your local big box building supply
store.

[joe saavedra]

George, you are right on!

We have been prototyping with a range of sensors, the majority of which are in fact metal-oxide.

However, as the kickstarter is gaining more and more momentum, we're looking at incorporating at least one electrochemical sensor. 

concerning the stability, this is precisely the reason we are also measuring temperature and humidity. The data pushed to Pachube will be raw, but we'll be writing applications that compensate for the drift caused by temp/humidity fluctuation.

The sensor boxes will both be powered by a 1A power supply. You are right the metal oxide sensors SUCK current, so mobile with batteries is not part of this design.  A lot of people are doing great things in this space, and I really believe mobile is the future, but this design will be stationary.

As we start to see which options are possible, we'll be posting on here, so please keep the feedback coming!

Thanks,

joe

[George Yu]

I am actually putting together a handheld bluetooth pollution sensor
for iOS and android called Node. It was recently funded on Kickstarter

My plan is to either source electrochemical sensors and combine into a
single sensor module or get the sensor manufactures to work with me to
design a pollution sensor. I think such module would be bring a lot of
value to this project.

gas sensors is my technical specialty (linkedin reference:
http://www.linkedin.com/profile/view?id=40836988&trk=tab_pro)

-George

[Nafis]

Great work George!

You should put your early sensor ideas out as a Arduino Shield so we can start getting some experience with them :-)

[George Yu]

my Node device is essentially an arduino device. You can program it
via Arduino IDE.

[Victor Aprea]

George,

Your Node project sounds really cool, congrats! Please could you provide some links to datasheets and distributors for your specific sensors you guys are using so that we can evaluate their applicability to our the Air Quality Egg?

Many Thanks,

Vic

--

Victor Aprea // Wicked Device

victor...@wickeddevice.com

M: 607.745.1515

T: twitter.com/vicatcu/

[George Yu]

Victor,

Thank you.

I think Alphasense is a pretty good source, but they don't sell in
small quantities. I would recommend people to look at their OEM sensor
products and data sheets. It will give you some insight into
electrochemical sensor limitations.

My overall recommendation is to stay away from metal oxide sensors for
air quality measurements. Try the Alphasense sensors and use their
circuit board to acquire data.

I am also negotiating with a few sensor manufactures to partner with
them to produce a pollution sensor array with industrial resolution
but at consumer prices.

-George


On Mar 31, 11:57 pm, Victor Aprea <victor.ap...@wickeddevice.com>
wrote:

> victor.ap...@wickeddevice.com
> M: 607.745.1515
> T: twitter.com/vicatcu/

[subreptice]

On 30 mar, 04:29, George Yu <yu.geo...@gmail.com> wrote:
> Hi everyone,
...

> 2. sensor stability
>
> as the weather conditions such as temperature or humidity changes, a
> CO or NOx metal oxide sensor will drift significantly. The only to way
> minimize that is to build a temperature and humidity offset
> calibration curve.
>

Hello.
In the same way, I wonder about wind influence ;

bonsoir
la mobilité des masses d'air qui vont être échantillonnées puis
mesurées, ont aussi une influence non ?
Est-ce que c'est intéressant que les quality eggs prévus pour
l'extérieur donnent des indications sur la vitesse et la direction du
vent ?

[Nafis]

Vic,

I noticed a 2008 Zilog reference design for a CO Detector (http://
www.zilog.com/docs/z8encorexp/appnotes/an0225.pdf) that uses the Sixth
Sense ECO-Sure (2e) CO sensor (http://www.euro-gasman.com/PDFs/ECO-
Sure.pdf)

FYI, Chris

[Luis Fraguada]

As a reference, I am linking to the Waspmote Gas sensor board PDF.

Here are the sensors they use:

CO: Figaro TGS2442
CO2: Figaro TGS4161
O2: Figaro SK-25
NO2: e2v MiC-2710
NH3: Figaro TGS2444
CH4: Figaro TGS2611
C4H10: Figaro TGS2610
CH3CH2OH: Figaro TGS2600
C6H5CH3: Figaro TGS2602
Solvent Vapors: Figaro TGS2620
O3: e2v MiCS-2610
VOC's: e2v MiCS-5521

On Friday, March 30, 2012 4:29:32 AM UTC+2, George Yu wrote:

[nicox]

Hi all,


And what about "calibration" of these sensors ? These ones need to be
calibrate regularly,to give relevant datas, and it costs a lot, so you
need a laboratory to do that on a monthly or weekly basis I suppose

Any idea to manage that ?

On Apr 9, 7:34 pm, Luis Fraguada <fragu...@gmail.com> wrote:
> As a reference, I am linking to the Waspmote Gas sensor board PDF<http://www.libelium.com/documentation/waspmote/gases-sensor-board_eng...>.

[Dirk Swart]

Hi Luis,

Any idea what a system using this board costs?

Cheers
Dirk


Follow me on Twitter now: dswart

[Ed Borden]

Super expensive, as it's marketed I believe as a "development
platform" and is therefore extensible, etc. But knowing what actual
sensors they are using is super-helpful.

[Dirk Swart]

Absolutely.

D

Follow me on Twitter now: dswart

[Luis Fraguada]

Hello Ed, Dirk,
Libelium was very kind to sponsor a workshop we were putting on with 5 waspmote units with gas sensor boards equiped with CO, CO2, NO2, Temp and Hum.

Pricing for the waspmote units was +1000€ (for a 5 unit pack with Xbees).  Pricing for the Gas board with the sensors was +500€ (for a 5 unit pack).  On top of that you would need the rechargeable batteries (6600 mA/h) , the GPS module (+300€ for pack of 5), and the gateway XBee unit (+100€).  We had the prices as part of the agreement in case anything went wrong or we were not able to return the equipment in its exact state.  Pretty standard agreement.

Again, this was a very generous thing they set up for us, and it made the workshop really interesting as we also had some of our DIY sensor kits.  It was a real pleasure to work with a more 'pro' kit as the waspmote, but the cost would have prevented us from utilizing them were they not sponsored.

[Dirk Swart]

Very interesting. Thanks.

[Ricardo Piedrahita]

Hello,
It was great to meet a lot of you at EcoHack this weekend, and I wanted to follow up on some of these topics.  We have been working on low cost monitoring and are developing a portable device and Arduino shield ( maqs.pbworks.com ).
We have a good in-lab calibration system, but as our system grows we are hamstrung by an inability to deploy low cost calibration systems.  It seems that a low cost calibration system is a difficult problem, but one that many of us have a great interest in solving.  I don't know exactly what such a system should look like, but I wanted to discuss possible designs.  Requirements may be different for other groups, but our ideal system would provide clean air (zero grade or better), control temperature and RH, and expose the sensors to sources of VOCs, ozone, carbon monoxide, CO2, and NO2.  I think clean air, T and RH control, and VOCs are quite doable, but the others get pretty tricky because we either have to use low-cost flow control for gases (if we use bought certified gas standards), or precisely generate the pollutants we are interested in.  We aren't doing particulates, but that might be on other people's minds as well.
Any help would be greatly appreciated

As a side topic, we have been playing with electrochemical sensors, but our amplification has been quite noisy, can the electrochemical sensor people here provide any insight or designs to help with this?!
Thanks,
Ricardo

[nicox]

Hi Ricardo,

I'm working on an environmental project too, I'm only in the first steps, and my major problems are calibration, and find the right sensors techno. Can I share with you what I'm finding ? your comments are welcome !

For the calibration issue, I think we actually can :

- Do it in a lab :  expensive, we'll have to ask our users send periodically their boxes, a nightmare !

- Do it ourselves : build/buy our own chamber (like this one , Gas test box from Figaro http://www.figaro.co.jp/en/data/pdf/20091110170331_5.pdf , or this one http://www.grainger.com/Grainger/BACHARACH-Calibration-Kit-6T135), may be less expensive (not really sure), but still have to check the AQE boxes every month !

- Find sensors that don't (never !) need calibration : forget that, don't found any, it's still a dream for many researchers !

- Find sensors that provide long time between each calibration session : electrochemical seems great (see these products http://www.cairpol.com/index.php?option=com_content&view=article&id=68&Itemid=155&lang=en ) , but still need to calibrate "one day" ! what about infrared sensors : don't really know what we can do with : reliable ?

- Many studies were done in many universities and research labs about "online sensors calibration" and many theories wrote about that : if each boxes can reach the Net, the Net can reach these to apply some dedicated processes, we can imagine that, due to temp and humidity that can act on some well-known sensors , we can have a linear degradation of the sense capacities, and so calculate these values and remotly apply the corrections , what do you think about that ?

We have to use the fact that all these boxes are reachable by many others, not always easy to find a reference box in our area to calibrate our own, so I think this is a good idea, we have many scientists in the AQE projects, I'm certainly not the first to think about this possibility !

Nicolas

[Nestor Tiglao]

Hi Ricardo,

As far as calibration is concerned, have you looked at the smart sensor interface standard, IEEE 1451?  The concept is incorporate transducer electronic data sheets (TEDS) to calibrate the sensors and make them plug-and-play.  The TEDS can reside in a memory chip or access through the network, i.e., Virtual TEDS.

Nestor

[Nestor Tiglao]

Perhaps, such virtual TEDS can be hosted on Pachube, for example.

[Ricardo Piedrahita]

Hi Nestor,
That sounds really cool, I will check it out!  Have you worked with TEDS before, or have you heard of it being adopted in other fields?
Thanks,
Ricardo

[Ricardo Piedrahita]

Hi Nicolas,

I agree with your assessment of the options, for some cases lab calibration is good, but not for a network this big! 
That cairpol sensor looks really cool, have you used one, or do you know if it is available? 
We are thinking about how to build a low-cost chamber, with some 'medium' cost reference sensors.  The chambers you suggested might make a nice starting point for this type of project.

Thanks,
Ricardo

[Nestor Tiglao]

Hi Ricardo,

I just came across this standard quite recently.  I think someone in this mailing list mentioned about it also.

Best regards,
Nestor