I have an MG-811 sensor on a SanboxElectronics board that has been working perfectly for months. Its been in storage for the past month or two. I've come back to it and found that its output is now backwards and too low. The support circuitry is working fine. Heater voltage is 6.03V, and it gets warm, so the element isn't blown. Amplifier gain is about 8.5
Ambient sensor voltage starts at 27mV, When I gently exhale over the sensor, voltage rises to 40mV When I stop exhaling, the voltage drops rapidly to 17mV and then slowly rises back to 27mV
My software is setup with the following calibration points: float v400ppm = 1; //voltage for low CO2 float v40000ppm = 0.25; //voltage for high CO2
This gave a useful, if not precise, ppm reading, prior to this failure. Now, however the result is meaningless.
The sensor was placed indirectly across the exhaust of a generator. There is no visible sign of damage to the sensor, nor is it dirty from the exhaust.
Has anyone else had this problem, or a similar problem with the MG-811?
I would like to find out: 1) what is the nature of the failure (can the sensor be reconditioned or repaired)? 2) what caused the failure (can I avoid a repeat failure)?
Looking at the working principal, the voltage is the result of a reaction, and so the sensor has a limited lifespan. Did it die of old age? I've had it for about a year now and it has had 100-500 hours of use with various CO2 levels, including higher concentrations than its rated maximum output. It would also have experienced elevated temperatures from the exhaust. Perhaps the high CO2 and temperatures accelerated the reaction and reduced the lifespan. In which case can the electrodes/electrolyte be regenerated by applying a reverse voltage of similar order to its output?
Working Principle Sensor adopt solid electrolyte cell Princip le，It is composed by the following solid cells： Air，Au|NASICON|| carbonate|Au, air，CO2 When the sensor exposed to CO2，the following electrodes reaction occurs： Cathodic reaction：2Li + + CO2 + 1/2O2 + 2e - = Li2CO3 Anodic reaction：2Na+ + 1/2O2 + 2e- = Na2O Overall chemical reaction：Li2CO3 + 2Na + = Na2O + 2Li + + CO2 The Electromotive force（EMF） result from the above electrode reaction, accord with according to Nernst’s equation:： EMF = Ec - (R x T) / (2F) ln (P(CO2)) P(CO2)—CO2--- partial Pressure Ec—Constant Volume R—Gas Constant volume T— Absolute Temperature （K）F—Faraday constant
This may all be over the tops of most peoples heads on this forum, but this also seems to be where most people are talking about this sensor (albeit mostly to complain that the sainsmart board doesn't work).
Any contribution welcome, thanks.