MG-811 sensor output inverted

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.

I'd suspect wrong wiring of the module, please check.

Also electrostatic discharge might have damaged the module while plugging it in/out.

Thanks for your reply. Both good points to check.
The module was previously working and hasn’t been taken out of circuit.
The board has test pads and the first thing I did was check for correct Vcc and Vheater.
I’ve checked input vs output voltage on the amplifier and verified the gain is 8.5.
The sensor’s temperature seems about right also: hot to the touch, but not painfully so.

Reading some other threads on the MG-811, I was reminded that the sensor needs to be run in for many hours before it gives correct readings, so I’ve decided try doing that again. The output from the sensor has risen from 27mV to 36mV in the last 6 hours, so I may be onto something here… or I’ve just got my head too close whilst measuring.
Ahh… but now it goes up and then down with sustained exhalation, so my earlier observation of dropping below ambient voltage after ceasing to exhale may have been a delayed response.

I shall leave it overnight and see what happens.

As a point of reference, after first running in this particular module, it’s output voltage at ambient CO2 levels was around 1.0v, and dropped to 0.25v when saturated with exhaled air, corresponding to sensor voltages of 118mV and 29mV respectively.

I know this thread is long gone but I'm wondering, have you ever compared your CO2 readings with a good NDIR reader like Telaire for instance? What are you basing your "precision" on.

As far as I know these eletrochemical sensors are very, very bad.