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I'm adding PH to my list of waterquality measurements. I've figured out how to get some cheap PH sensors to work, but a friend mentioned that they start giving misleading measurements and need to be recalibrated. Can anyone tell me why that might be true and is there a way to avoid that ? Thanks[/i]

I really have no clue but if I had to guess I'd say it's because of mineral build up.

Yes its true.

The PH probes and the meters come with several different settings.

First they have a temperature compensation knob on the meter, often a zeroing knob, and the PH probe it self must be cleaned, saturated in a neutral solution, and or at the least just washed out often in pure distilled water.

If left alone the settings will drift.

Darn. And it was such a good plan without that bit of reality. Thanks. Guess that will just have to be external where they can get to it.

Thanks again.

An alternate method would require you to pump chemical reagents and a sample into a tube then use an optical color sensor to read the results. This seems somewhat complicated and still needs to be serviced to replenish the chemicals but the results won't drift.

However, such apparatus could also be used to chemically measure other things like chlorine, nitrates etc (see fish tank testing kits for the complete list of possible tests)

Heres a little something on automated water sampling including PH
http://ri.water.usgs.gov/automon/rwgwmr.pdf

I like accuracy, so as to clean, neutralize and calibrate the probe to a known buffer standard.

I want to run a test so the results could be reproduced over and over.

firstly If all I need is a garden variety meter to do this then ok, so it depends on what you are doing. having said this

if continuious is part this scheme then I lean toward different thinking.

setting up a test to compare your sample to known solutions or even compare between two very close solutions having three individual probes lets say A, B, C where B is your sample now your scale is between A and C

I might want to look at a 4 inch meter rather than digital readout probably the rate of change for me is of interest on a analog meter