Hello Joshua...

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pH:
Here in India, good quality glass pH sensors (or electrodes) 
are about Rs. 1000-1200 (~US$25). They haven't any 
temperature compensation built-in but then since you are 
measuring temperature, you could do the compensation in 
software. These sensors behave like very weak cells, 
producing about 60mV per pH away from 7pH.

In a 7.0pH solution the electrode will produce 0V. So:
In an 8.0 pH solution, the sensor = -59.7mV
In a 6.0 pH solution, the sensor = +59.7mV
and so on...

Read the sensor using a high input impedance JFET Op-Amp 
like the LF356. Another quad op-amp to level shift the pH 
readings to something readable by a PIC (0.5-4.5V <=> 
0-14pH). To calibrate the pH reader, you get 4.0, 7.0, 9.2, 
10.0 pH tablets or solutions in all educational supply 
shops. Try: www.carolina.com or www.wardsci.com if in US.
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Conductivity:
Get small two small plates of SS316L, 10mmx10mm. Coat one 
side of each with something non-conductive electrically, 
like  epoxy. Face the non-coated sides 1mm apart and dip in 
water/solutions. You get a 2-electrode home-made 
conductivity sensor.

Now check out this site:
http://members.tele2.nl/rsetteur/aquarium/karel/colorie/english.htm
http://tinyurl.com/yey6e8x
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Dissolved Oxygen:
Unfortunately, this is going to be expensive as sensors 
themselves are quite expensive. There are two kinds of 
'chemical' DO sensors: Polarographic and Galvanic. I suggest 
galvanic. They output current in microamperes, which can be 
read by an instrumentation amplifier like the INA111. The 
electronics is pretty straightforward after this.
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Temperature:
RTD, thermocouple, silicon, thermistors, lots of choice and 
circuits easily available on Google.
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Hope this helps,
Mohit Mahajan.

Joshua Shriver wrote:
> I'm starting a project to create an outdoor environmental recorder as a
> hobby. Looking for water sensor recommendations that can be used with PIC's
> for data gathering and interfacing with a SBC.
> 
> Prefer to keep the costs down as much as possible; so far the few sensors
> I've found (pH, temperature, conductivity, dissolved oxygen) have been
> extremely expensive ($300-700 each).
> 
> Any tips or ideas? Might be to expensive for a hobby project.
> 
> -Josh
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