Sensing CO2, O2, CO not MG / MQ sensors

Hi - first a quick explaination of 'where I am coming from' :-

I recently had a quote for the replacement and re-calibration of the CO2 and CO sensors in my Anton Sprint 2000xt - £350 - and the sensors are only good for 2 years - so another £350 in 2 years time !

What is the Anton ? It is a portable combustion gas analyzer - the type used by gas/oil boiler service engineers.

I'm not one of them. My intention is non commercial. I want CO2, CO, and O2 measurements to support a home based combustion project – think furnace – but not heating water or air – its for testing a modified solid fuel burner. ( for my home central heating one day - maybe ? )

Opening up the Anton to get the details of the sensors and lots of Google searches later - I determined the sensors were supplied by citytech.com .

So - a potential project for my arduino ........ if I can find sensors to suit !

Problem : In other posts the MG ( CO2 ) and MQ ( CO ) type sensors have been addressed - the circuitry looks daunting ! and the CO2 reading wont go high enough ( 10000ppm = 1% ) - I need to aim for O2=21%

Reading posts for Lambda sensors - they look scaryierly differcult and some differcult cicuitry - I dont need sensors that tolerate high temperature / pressure changes - as I cool the sample gas and dry it before measurement. Also - they dont appear to be accurate enough for my application - but hard to discern specific numbers for gases detected.

The MG /MQ sensors look pretty cheap - No prices from Citytech - But there is another company - sensorsdirect.co.uk - there sensors look a different technology than the MG /MQ types ( no heaters ? ) and the reading range is considerably wider

O2 http://www.sensorsdirect.co.uk/userimages/procart5.htm

CO : http://www.sensorsdirect.co.uk/userimages/procart6.htm

The data for the above is limited - but since they are equilvents for citytech sensors ( not for Anton though ! ) - then the sensordirect links should be read in conjungtion with:-

O2 http://www.citytech.com/PDF-Datasheets/4ox1.pdf ( 25% - 2 pin )

CO http://www.citytech.com/PDF-Datasheets/4cf.pdf ( 1500ppm – 3 pin active ) http://www.citytech.com/PDF-Datasheets/7ef.pdf ( 1000 ppm – 3 pin active )

these low impedence devices - have such a low output though !

so I don’t get CO2 directly – but it is inferable by chemistry of combustion equation – which can be entered into arduino sketch

But Look : The “7 series” ( below ) has a bigger output 4 – 20 mA dc, and Power supply 10 – 35Vdc

O2 http://www.citytech.com/PDF-Datasheets/t7oxv.pdf

For the electical boffins here : Question : Is there sufficient information in the CityTech pdfs to deduce the support circuitry ? What questions do I need to ask citytech ? ( assuming they will sell to me - as i am not an oem )

thanks for reading

Fixador: Is there sufficient information in the CityTech pdfs to deduce the support circuitry ?

In my opinion, no not quite. It seems odd that these are specified as current sources -- 4 mA to 20 mA for example -- without specifying a load circuit that this should feed into. I'm also a little iffy on the temperature range they are designed to operate in. I saw ranges to to 50C and for your application looking at combustion gasses, I would think you would want temperatures on the order of 700C to 1000C.

What questions do I need to ask citytech ? ( assuming they will sell to me - as i am not an oem )

A design note giving sample application circuits is what you want.

I think the specs are borderline -- if you have a working instrument you could work out the remaining details, probably. Also if the sensors were cheap enough to take a chance on, you could likely determine the nitty details experimentally.

It seems odd that these are specified as current sources – 4 mA to 20 mA for example

This is the industry standard current loop specification.

Interesting. I have never run into analogue current loops before. This was interesting, and could be useful for the O/P http://www.bapihvac.com/CatalogPDFs/I_App_Notes/Understanding_Current_Loops.pdf

Regarding the O/P's needs, I think I would tend to look at an automotive O2 sensor as these are designed for continuous operation in high temperature combustion gasses. Heated ones are more resistant to fouling by condensate. A waste-oil or waste-wood burner would be pretty messy and foul any O2 sensor rather quickly.