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I am creating an automated chemical component mixing system (DIY project). One of the features required is continuous monitoring of pH value in my main solution. I do not need it to be very precise, precision of 0.2 would be enough. Also, I want it to be as low maintenance as possible (cleaning the probe once a month would be great). pH of the solution will vary from $pH = 4$ to $pH = 7$.

So, the question is, which of this testing ways will require less maintenance of pH probe:

  1. Constant submersion in the solution.
  2. Once per hour dipping in the solution.
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The most common approach is to measure periodically.

pH glass electrodes (the most common ones) need to be "wet" in order to be used, so if you are going to remove it from the solution every hour you'll need to keep it submerged in the appropriate solution. Not only that, you need to wash it (preferably with the most pure water you can get) between measurements. Read section 2.2 of this monograph to understand why it needs to be "wet" and what solution to store it in. There's information on a lot of types of electrodes. This explains how to wash it. Another problem you might face with continuous measuring is not the electrode, but the meter. From that monograph:

The condition of the pH measuring assembly is subject to fluctuations over the course of time. Depending on the accuracy demands, a pH calibration should therefore be performed periodically, e.g. once a day.

You'd have to test and see what accuracy you get with your equipment.

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