A smarter way to use electrochemical gas sensors
Perhaps the most promising near-term technology for accurately detecting many atmospheric pollutant gases within modest cost, power, and size constraints is the electrochemical sensor cell. Recent "echem" sensors from Alphasense in particular (-A4 and -B4 models, for example see CO-B4 datasheet) provide:
• impressively low noise and sensitivity down to the very low ppb range
• low cross-sensitivities to other gas species
• ridiculously linear response across a wide detection range
• 1% gain accuracy from the factory
• built-in temperature compensation mechanism that provides an offset error signal on a fourth "Aux" pin
All for less than $100 per unit in volume. Sounds fantastic, right? These are specs that usually match up with a $$$$ benchtop gas analyzer. Except...
How do you get a good signal out of this thing?
And here's the rub. It turns out to be somewhat tricky to actually achieve those specs in a real-world circuit. Not impossible, but it takes a fair amount of analog-type thinking and careful design considerations to make sure various noise and calibration error sources don't creep in and ruin all those sparkly numbers on the sensor's datasheet. I'll get into error budgets and noise sources in future Research Notes, but it's not hard to imagine that any attempt to measure parts per billion of anything requires a perhaps fanatical attention to these sorts of details.
And the sensors turn out to have something of their own personalities as well. Depending on their deployed environment, they may drift in gain and offset over time and temperature, and since they are typically gel-filled electrolytes exposed to air, they can dry out or become more saturated depending on moisture content in atmosphere. Oh well, here we are in the real world, what happened to that shiny datasheet again?