Having a clean EC probe helps to guarantee accurate measurements.
Dirty, or improperly clean probes, can result in erratic and inaccurate readings. Not only can this cause an issue with measurements, but also with calibrating. Remember, just because a probe looks clean, it does not mean that it is.
General Cleaning Procedure
Always rinse probes with purified water before storage, after storage, and in between measurements. Water such as tap water can introduce different ionic contaminants to an EC probe, making it unsuitable as rinse water. For a more thorough cleaning, a cloth and warm water with a soapy surfactant can be used. After washing with a surfactant, make sure to thoroughly rinse the probe clean with purified water. Always beware of solvents of any sort when cleaning a two electrode probe or a four ring probe. Many solvents are not chemically compatible and can harm your probe (incompatibility of solvents with the plastic bodies/housing can result in weakening, embrittlement, and other severe damage).
More Cleaning Tips
For a more in depth cleaning, some four ring probes can be disassembled for cleaning. The outer plastic sheath can be removed in order to clean off build up and residues from the internal stem. Either purified water or warm water/surfactant mixtures followed by a purified water rinse can be used. Allow the pieces to dry, and then reassemble.