I've bought this sensor module board to link to an Arduino nano: http://www.ebay.com/itm/For-Arduino-Liquid-PH-Value-Detection-detect-Sensor-Module-Monitoring-Controller/321764233326

I assumed the two screws were for calibration (low point + slope), but it seems the operation is different and I can't figure out how. I've got two reference liquids for pH of 4.0 and 7.0. When I put the probe in either one, I get readings. Turning one of the two screws seems to offset the reading so for instance I had pH 4 = 900 and pH 7 = 1000 (0 - 1023 range). Turning one of two screws would lower the value a bit, but it never gets below 500 or so.

So what I can do is measure two points (4.0 and 7.0) and their respective measurement values. That way I could calculate points in between, but as far as I know pH is not a linear scale, but logarithmic. If I get this wrong, the reading might seem right but only show 4.0 and 7.0 correct and not the rest of the values (I don't have 10.0 reference fluid at the moment).

Now onto the second screw, that one at first seemed to do nothing. But then I noticed that it sets some kind of threshold value, turning a LED on the board on and off. That LED seems to be linked to the value being read, but it's further purpose? Calibration? No idea.

Ok so best case someone has an equal module and wants to detail how it works. Otherwise maybe some of you have some bright ideas on what I could try in order to get this to work the way I want it to :)

Update 09-11-2015 Got feedback from the seller. She sent me the following ZIP: http://1drv.ms/1MSk1DX. It contains some files in Chinese but one had sourcecode in it. Tried it on my Arduino but unfortunately the values still don't make much sense. Depending on how I turn the knob on the print, I get values between 9 and 17 for a liquid that's in real life 7.0. I didn't try but doubt that doing a -7 offset is sensible.

The code itself is pretty straightforward, taking a 6-center average of ten consecutive measures. The interesting lines are these:

 avgValue = 0;
 for (int i=2;i<8;i++)       // take the average value of 6 center sample
   avgValue+=buf[i]; 

 float phValue = (float)avgValue*5.0/1024/6; // convert the analog into millivolt
 phValue=3.5*phValue+Offset;

Maybe someone can make sense of this, because I can't. I don't understand the 5.0 multiplication and the 3.5 after that one. Where do these come from?

Anyway, when I dip the probe into the storage liquid, my normal meter plummets to pH 0.5 whilst the software is giving me 17 (which is a higher value than the one I got from reading my 7.0 liquid). So either my board is broken or this sample code is useless.

Schematics can be found in the ZIP file by the way.