Breathalyzer

I was interested in how different sensors worked, and came across a series of gas sensors which were low cost.  I decided to buy the alcohol sensor ($7) and play around with it, by making a breathalyzer.  My breathalyzer is an Arduino microprocessor based unit, using a low cost MQ-3 sensor.   The sensor uses a tin dioxide layer to sense the presence of alcohol in the air, and changes resistance with increasing concentration.    I created a voltage divider with the alcohol sensor and measured the drop in voltage when sensing alcohol, compared to no alcohol.  The most difficult part of this project was calibrating the sensor.   Per the manufacturer’s data sheet, I created an alcohol/water mix of 0.4mg/l and used a load resistor in the voltage divider of 200K.  However, the sensor output also varies with changes in temperature, relative humidity and presence of other gases.   It also seems to vary with the amount of time you blow over the sensor.  To convert this gas concentration to a blood alcohol content (BAC) concentration, I used the conversion value that I found on Wikipedia, of 0.10 mg/L breath alcohol concentration = 0.021g/dL blood alcohol concentration.  My software continually averages the BAC readings and displays the 3 second peak BAC on the LCD display.  Although a good project, I don’t believe this is accurate enough without humidity and temperature compensation to be useful as a BAC monitoring device.