Juraj:
The raspberry pi runs a database that replicates to a server database, it records video through the raspicam, it runs a web server and interacts with a server RPi as well as other client RPis in the cluster (and I'm sure a few other things that I forgot). It also allows me to SSH in and do the development work directly in the environment.
Robin2: Thank you for the links, I will check them out.
ChrisTenone: You're correct, the EC sensor uses a solution as the resistor to test electrical conductivity which would throw off the pH sensor in the solution. They have to tested separately because it is using DC. The load sensors are for measuring mass on a different part of the project. I plan on using RPi to send instructions to the Arduino on which sensor should be turned on and for how long, then processing the results when received. The RPi won't run a GUI when deployed but I need it for development. But overall, this is a chemistry project, yes. Initially I was using an ADC breakout board for the RPi but accuracy is a problem and Arduino costs about the same as ADC.
Reading the sensors takes a few seconds to let them level out and ditch the first couple of readings. So my understanding is correct. My plan is to code the Arduino to return an array with the two load sensors first, then EC, then pH but until instructions are received via serial from RPi, the EC and pH sensor will remain off and return 0. The RPi operates on a schedule and when its time to record the EC or pH, it will send instructions to Arduino to turn the EC or pH sensor on and put the results into the return array. When the results in the array are greater than 0, that data will be stored in the database.
Thank you all for your insight.