I designed and built a hydroponics system with pH logging and I am trying to understand why pH measurements in the nutrient reservoir vary ~ ±0.2 pH while measurements in a separate bottle of probe storage solution only vary ~ ±0.01 pH. The nutrient solution is in a 3.5 gallon plastic pail, as shown below. The pH probe is the one with a blue top. The black probe measures EC and removing it has no effect on the pH noise in the reservoir.
The plot below shows the logged pH. The abrupt change is when the probe is moved from the nutrient solution to the storage solution. Note the relatively large noise in the nutrient reservoir and the nearly imperceptible noise in the storage solution.
In the plot,the pH is measured every 5 minutes at the start of a spray cycle, when the pump is off and has not been running for the last ~4.5 minutes. Note that similar noise occurs when measuring at a rate of 1 Hz. During the storage solution measurements, the probe tip is in a separate O-ring sealed bottle. The bottle is sitting at the spot where the pH probe would normally be inserted into the reservoir.
What are potential causes of the pH noise in the nutrient solution?