CO Sensor
Jack Crenshaw
Posts: 46
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
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
Comments
Chuck
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
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
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