PH Probe connected to Photon

I purchased a PH Probe from ebay that has a BNC connector and a PC board. I found the following site that describes the pinout:

I am connecting 3.3v and ground to the PH Probe, and also the output to my A3 pin. When reading the values, I am seeing 4088-4087 constantly.

The probe I am using is the:

Based on the first link I provided, I assume that I should be seeing a value of 511 to represent a PH of 7.

Any thoughts as to what I might be doing wrong?

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Made some progress and thought I would share my results.

First, this sensor needs 5v for its input. I decided to connect the sensor to my Vcc line on the Photon which is outputting 4.8v. Next, I mixed up some PH Probe calibration powder and began testing. I plotted all of the values that the ADC was reading vs the calibration ph levels. I then used some good old algebra to find the slope intercept formula and we were set. I realize this is not accurate, but it is close enough for my needs.

In short, I am using the following code:
(analogvalue*-0.00443349753695 + 20.5438423645));

Yours will differ based on the values you measure.

Hope this helps!


Thanks for letting us know you solved it!

Use this. for more accurate results.

//If you short out the prob, you'll get a nuteral PH (7)
// PH_step = (voltage@PH7 - voltage@PH4) / (PH7 - PH4)
// PH_probe = PH7 - ((voltage@PH7 - voltage@probe) / PH_step)
pHValue = 7.00 + ((2.5 - voltage) / 0.19); // Based off my numbers, yours may vary.

You can use this to calibrate your step periodically to maintain accuracy. You can also add the temperature in you calculation to get even more accurate results. My solution is always between 69-71F so no big change in the results.