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==pH-meter== Here's a basic tutorial on how common probes work - http://www.sensorland.com/HowPage037.html and this article is titled "building the simplest possible pH meter" - http://www.66pacific.com/ph/simplest_ph.aspx - at least once you have found a sensor; as with the thermocouple, you need an amplifier because of the millivolt signal. This led me to his source for sensors (http://www.americanmarineusa.com/ - they supply to the aquarium industry so also sell conductivity and dissolved oxygen meters) and their cost of $50 is comparable to others we have seen. $30 for a similar one at another aquarium supply joint http://www.bulkreefsupply.com/store/brs-brand-ph-probe.html, with BNC connector (http://www.practicalmaker.com/tutorials/bnc-sensor-shield-documentation). Maybe we can buy one of these instruments (or similar), break them open and figure out how to read the output from an arduino: *[http://www.heavydutysupplies.com/servlet/the-15/Checker,-HI-98103,-HI98103/Detail HANNA Instruments HI 98103] $55 *[http://www.amazon.com/Milwaukee-pH600-Portable-pH-meter/dp/B004CZ8632 Milwaukee pH600] $20 - doesn't look like it needs specific buffers for calibration, but the accuracy is probably not great. Maybe it's enough, though. *[http://www.google.com/products/catalog?hl=en&q=ph+electrode&sqi=2&cid=15011737823946485839&os=sellers# Google shopping results] approx. $40 upwards *[http://www.pulseinstruments.net/sotaphelectrode.aspx SOTA pH Electrode] $100 - expensive, but so so sweet: designed for continuous measurement, and comes with any kind of connector. You can also get little tester units, such as this [http://www.jencostore.com/ph-meter/ph-testers.html?price=1%2C100 Jenco 610 pH tester] for $30 - perhaps it could be hacked? There’s also the option of attempting to build one ourselves using [http://www.ph-meter.info/pH-meter-construction this] (or a similar) schematic with the Arduino instead of a voltmeter - not necessarily cheaper, although it’d certainly be both fun and informative. This site mentions the Ion Selective Field Effect Transistor (ISFET - see http://www.colorado.edu/MCEN/micronanobio/Homework/Homework_Nano-ScaleEngineering_3_2008_Solutions.pdf for more exotic applications) as a solid state alternative to traditional electrodes, but I can't find a source for the right/inexpensive one yet; they are said to be less accurate/sensitive, but that is something we are willing to consider. As soon as some one packages a sensor (e.g. http://www.microsens.ch/products/pdf/MSFET_datasheet%20.pdf) into a full blown "probe" the price goes way up. Bilding from scratch a pH meter: http://damien.douxchamps.net/elec/ph_meter/, and googling on "DIY pH sensor" will find you more meter schematics. Many of them are referenced here at the pHduino project site: http://code.google.com/p/phduino/
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