pH should always be checked after getting the EC into the optimum range.1:

pH and EC

When monitoring the EC concentration, be sure to subtract the base EC of your source water from the level detected by your sensor.1

Measuring Electrical Conductivity

Using an EC Meter

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Zeroing an EC Meter

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Cleaning n EC Meter

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  1. Electrical Conductivity and pH Guide for Hydroponics ↩2