Does anyone have a vi for a pH probe with automatic temp. compensation?

[CBoggs]

I am using a standard pH probe with a RTD sensor to record the pH of a solution. Since pH values are temperature dependent, I need to automatically compensate for different temperatures. Not just read the pH values. I am using LabVIEW 6.1 with a 6024E card.

[JRA]

You could always just hook up a thermocouple to the 6024E and take temperature measurements from that. You will probably need some signal conditioning if using a thermocouple since the voltages are so small. This article I found on NI's website should help you get started. You might also want to contact your local NI engineer. They're really helpful at setting up these types of applications.

[shoneill]

Please bear in mind that the input impedence of the 6024E card is (in terms of pH-measurement) quite low. Normal pH-Meters on the market have input impedences in the Teraohm range. Using a 6024E without conditioning the signal first can result in a shortened lifetime of the electrode, and sub-nernstian (below 98%) slopes.

Also bear in mind that you need to find the isotherm point of the electrode to accurately compensate for temperature (the pH where the electrode has the same voltage output independent of temperature).

You also cannot compensate the temperature dependence of your sample using this approach. Two things change with temperature. The response of the electrode changes with a given temperature, and the actual pH of the sample changes also. Y
ou can correct the changes from the electrode, but the sample has an unknown behaviour and thus cannot be corrected. Any correct pH measurement is always accompanied by a temperature value i.e. pH 6.5 at 24.3°C.

Shane.