Carlos from Brazil has designed a pH meter using the Arduino. Although not available for sale yet (unless someone can help him get a full version of Eagle), all the code and associated files are available for download.
pHduino is a open hardware and open software for pH measuments using a glass electrode sensor for hydrogen ion and a Arduino board.
This pHmeter can be powered by a power supply and display the data using a LCD. Also, you can control it and power it up by USB port. The offset and slope can be adjust manually by potentiometers or by an auto calibration code (not implemented, yet). There is a temperature sensor for temperature compensation.
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