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So, I've been working on figuring out how exactly determine BAC from this little guy, and its been an interesting journey.

First I went through the Datasheet, which is all about the resistance ratio of the sensor element of a sample of fresh air (R0), and a the unknown (RS). Everything is about RS/R0. as that fraction gets smaller the measured alcohol goes higher.

I have two problems with the Rs/R0 approach. Firstly, R0 is not a stable number. The longer the heater is running, the higher the resistance, and it seemed logarithmic, over time, never approaching a single value.

My second issue with the Rs/R0 approach, is that there is not solid data in the data sheet(there is a graph) that maps a certain ratio to a parts per million (ppm) measurement.

So after I abandoned the Rs/R0 approach, I decided to use the standard law enforcement formula (BAC = breath mg/L * 0.21). I decided to do one measurement at an equivalent .08 BAC, and another data point at .16 and a third at .24. This actually produced the best, and most predictable and consistent results. I took a 1/4 tsp of 91% rubbing alcohol and mixed it into 35 oz of tap water (1000:1 mixture). Then I used a 5.55 Liter bottle I had, and after cleaning it out, would pour 1/2 tsp of the 1000:1 mixture in, and read that as .08 BAC. I would then add another 1/2 for .16, and a total of 1.5 tsp for .24

So, after all this, I thought I was spot on, and knew how to measure effectively a .08.

Not so fast.

The temperature of this sensor when measuring affects everything. And when you blow quickly on it, the temperature and the resulting reading goes lower than it should, and will actually spike when you stop, because it warms up and registers the alcohol that is still present. Conversely, if you open your mouth wide, and blow hot moist air, the reading is exaggerated in the other direction.

My next step is to build a T shaped straw, with the sensor not getting hit directly by the breath, therefore minimizing the cooling/heating effects of the breath.

As I learn more, I will add my comments below.