EC

What @Papa_T said although if you are pure organics you dont really need to worry about EC and ppm as long as you are following the recommendations from manufacturer.

Or you can use both PPM and EC as two point reference measurements to make sure your water is dialled in to what the manufacturer recommends for their nutrient line.

Sorry more so for trying to PH reverse osmosis water there aren’t enough particles to make the pen function properly. RO would have a really low EC number as there are no particles in the water.

It’s a measurement reading. As you add nutrients to your water the EC (electric charge) become greater. The more the particles in the water the better it can hold a charge. So when you dunk your PPM/EC meter in the water it puts a slight charge in the water to get a reading. So this is why you’ll notice that trying to PH reverse osmosis water is next to impossible because the water is so pure that the electric meter can't function properly. So if a nutrient line says 2.9 EC for a specific mix. It’s what you should shoot for. Honestly I don’t mess with EC I go more by PPM which is a different measurement for what’s in the water. Hope that helps you. Cheers.