CO Sensor

Jack Crenshaw

I see that Parallax now has a neat CO sensor. It's interesting to me because, years ago, a friend and I were trying to build a CO detector for RVs and mobile homes. We found a raw sensor that put out a voltage proportional (or at least depending on) CO level.

The problem we had was that the voltage also depended on a lot of other things, such as age, temperature, humidity, pressure, and the presence of other chemicals in the air. We couldn't figure a way to calibrate the thing and guarantee its accuracy over the whole range of conditions. And, clearly, if it gets the wrong answer and people die, you are in deep trouble.

That was a lot of years ago. I'm wondering: Has the technology improved enough so that calibration is not such a problem anymore?

Jack

CHIPKEN

The CO sensor does require you to set the alarm limit from some suitable CO source. A butane hair curler would be a good source. The sensor itself does change with time and needs to be checked. The sensor used is a type MQ-7 and has been around for awhile. There is some·data in the MQ-7 specification found on the web.
Chuck

Tracy Allen

I ran across this one recently..
www.nano-proprietary.com/TechnologyPlatforms/GMOS.asp
which claims to dispense with the heater and to have other desirable characteristics such as low interference from other gases and wide operating temperature range. However, as with many things that turn up in NASA Tech Briefs, the development is currently on hold, not available, and it was being funded by the USAF for CO hazard detection in cockpits. Too bad. Something like that would be a great substitute for the fussy electrochemical cells currently used in household CO detectors.

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Tracy Allen
www.emesystems.com

SRLM

I wonder: can you replace the CO sensor with one of these? It looks like they have the same footprint, and I'd assume that they all output all the same type of data.

Professorwiz

I am unimpressed with the amount of "Technical" data they provide on the web site at futuretec

Tracy Allen

SRLM,
Even though the packages look very similar, the electrical outputs and signal conditioning required by the sensors on the Futurelec page are quite different. For example, the CO2 detector requires a very high input impedance amplifier, similar to a pH probe amplifier, while the alcohol sensor requires a differential bridge. The CO2 CO detector at Futurelec is the MQ-7, which is the exact element that is on the module sold by Parallax. The Parallax module includes a dual op-amp/comparator, and also a transistor circuit to switch from low to high current drive, as required uniquely by the CO sensor. All of those gas sensors at Futurelec have a heater that draws in the neighborhood of 100 mA that has to be active all the time to achieve the best results.

Much of this technology was initially developed by Figaro Sensor, and the technical information on their web site is worth perusing if you want more detail of how these work.

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Tracy Allen
www.emesystems.com

Post Edited (Tracy Allen) : 3/16/2009 10:02:13 PM GMT

Chris Savage

Tracy is correct…you definitely don’t want to just switch out the sensor itself. The CO unit varies its element resistance in proportion to the amount of CO gas detected and helps form a resistor divider which changes the voltage available at that point. The heater plays a big role in this. The manufacturer of the CO (MQ-7) Module specifies the heater be cycled between 5V and 1.4V depending on sense or purge phase.

Now, take the CO2 sensor…it (as Tracy mentioned) requires a high-impedance op-amp to amplify the voltage produced by it. This voltage changes in proportion to CO2 Gas exposed. Also not the heater on this module must be at 6V steady during sensing. So there are some big differences. Take care.

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Chris Savage
Parallax Engineering