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.

By measuring the amount of breath acetone gas present in a persons breath, it should be possible to determine wether or not a person is in a state called ketosis. Ketosis is, very simplified, when the body runs on fat rather then carbohydrates. If you eat a ketogenic diet, your ketone levels will be elevated. Currently, the only way to monitor your ketone levels are either to take a blood based test or a urine based test, with test strips that changes color depending on the amount of ketons that are present. Both methods work fine but the blood method is very expensive, about $20 per test, and the urine method is not very precise. Being the curious type of person, and getting inspired by a podcast where Steve Gibson was talking about building an instrument like this, I decided to give it a shot myself.

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.