NEVER put a wet bulb pH meter in distilled or RO water! You run the risk of trashing the instrument.It may not be perfect and as cheap as others but this ph pen is a paper weight next to it when using distilled or R/O
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Remember that pH is not a function of the water itself but rather the acidity of the materials dissolved IN the water. If there's nothing but water, there's nothing to give it a pH value.
Some say that pure water is pH 7 but this is misleading; pH 7 is neutral. Pure water is not really "neutral" as much as it's "absent."
Therefore, don't bother messing with the pH value of distilled or RO water, because the pH probe needs something dissolved in the water to keep the osmotic pressure in line.
It's recommended to use a storage solution or the pH 4.0 calibration solution to store the probe in. Read the instructions that came with it.