Amazing article on making your own Ketone sensor. Any members good at DIY computing?
Excerpt below. Read the full article at JenLabs (http://jenslabs.com/2013/06/06/ketosense-an-arduino-based-ketosis-detector/)
Introduction
The instrument's purpose is to detect the amount of acetone gas that is present in a persons breath.

On a very high level, the instrument consists of an electrochemical gas sensor that is sensitive to various types of gases, a combined hygrometer/thermometer and an Arduino Uno board. The electrochemical gas sensor and hygrometer/thermometer has been placed in a plastic case that serves as a ”gas chamber” that you blow into to collect the gas sample. If there is gas present in the air, the resistance of the sensor will decrease. The decrease of resistance is dependent on three factors; gas concentration, humidity and temperature. If these values are known, it is possible to calculate the gas concentration in PPM (parts per million) using the data sheet provided by the manufacturer.