Req.: Schematics for chemical meters

[maiasj]

Hi all,
I'm needing to build pH, Eh and condutivity meters for a laboratory in my town. Searching the net I found various projects but I need some more professional. Does anyone have schematics of comercial meters for me to use as a base to build my owms?
Thank you for any help.

[kcwcc]

The meter circuitry is usually quite trivial since most of the sensors output mv/xx signals...you need only set span and range...I would bet the schematics you have found would be suitable...but, reallistically, I bought a fully compensated, 2 point calibratable ph/Temp probe for $30 that is stable and accurate to 0.1pH and 0.1C...it's pretty tough to beat that price...a pH probe with a BNC will cost you $20....is there something special you need to do with these?

[maiasj]

Thank you for your post. I already did some simple meters and it works good. Now I'm thinking to make something better. The problem is that I live in Brazil. The pH probe here costs about $70. A good pH meter, $400. So I think is worth to make my own. 
     

[omni]

Perhaps it might look easier to make your own, but if you are going to use it to ensure quality of substances that are consumable by people (food grade) then I would definitely rely on a commercial product for that kind of work.

At least you should have a laboratory level equipment as a second measurement to check against when building and calibrating your own...

Just my 2c.

[kcwcc]

Calibration is a good point...calibration solutions for pH and TDS require distilled water and a very accurate scale..a single point temperature calibration can be performed with an ice slurry (distilled water please) in an insulated container that is stirred constantly.

[rinderpest]

As kcwcc said the meter side is fairly trivial, you just need some opamps to condition the signal and a simple meter to display it, either an analogue voltmeter or an LED one - try a 7107 from Microchip.
http://www.electronics-lab.com/projects/test/014/index.html
That one above is very easy to make, the main 7107 chip is free on Microchip sample program.
Its the sensors that are going to be tricky, the pH one anyway.
http://www.66pacific.com/ph/simplest_ph.aspx
Here is a simple one, you just need the probe. They use a conductor in a glass rod, not so easy to make.http://en.wikipedia.org/wiki/Glass_electrode

EC tho is easier. You are basicly measuring the resistance of the solution, but you must use AC or otherwise you will get electroplating or electrolysis caused by passing DC through a conductive solution.
If you set up opamps to compare the amptitude of the AC signal before going through the solution and afterwards the difference should be the EC. You will need a solution of known EC to calibrate it against.
Easiest way is find a friend on the net or elsewhere, get him to add say 1 gram of salt to one liter of distilled water and measure. Then add another gram, measure. Then another etc. Then you have a calibration curve to work against.
Good luck.

[kcwcc]

FWIW:
I have bought decent pH probes from these guys:
http://www.aqua-digital.com
they're an aquarist supplier but they sell probes and calibration solutions...fast shipping, good packaging and good prices.