Carlos Neves has documented an open software and open hardware pH meter based on the Arduino system. Roughly speaking, the pH level of a liquid determines the acidity or basic/alkaline levels of a liquid - and in some situations the value is very important. For example, certain types of fish require water that falls within a set range of pH levels. Carlos has designed and published plans for a custom Arduino shield that, in his words:
It seems inferior and a little bit more expensive compared with a pH meter bench instrument. However, it is interfaceable, programmable, expansible, and, the more important, open and free!
Kudos to Carlos, and here is an example of his work the pHduino:
As always, you can achieve a lot with the Arduino system. By using freely available design tools for schematic capture and PCB layout, anyone can design their own shields and share the work with others. However for first prototypes, we have a range of Arduino prototyping shields to suit standard and Mega-type boards - and also shorter ones that don't bump into large Ethernet or USB sockets.
However to get started on your own phduino, and more information and the design files, code etc. - visit the phduino github page.