EC is a general means to measure nutrient concentration of reservoir. Since plants absorb nutrients as they transpire water, with a lesser EC reading your plants will have to transpire more water to have access to the same amount of nutrients as a higher EC reading. Thus, they will grow more slowly or "out of whack", so to speak. (leggy, pale leavers, etc.)

The current ions are what EC sensors detect. To translate PPM in EC advanced nutrients uses as conversation factor of 700(ppm/mS/m); simply divide the ppm value by 700 to convert ppm to EC.
But for the information about PPM and EC visit to Nutrient Calculator

@ggrizzman

am i correct to compare the idea of maintaining a correct EC to a correct diet? Kind of like EC is the amount of food we eat everyday?

Will 10 liters of solution with an EC of 2.0 yield the same result as 20liters of solution with an EC of 1.0 assuming only 2 plants are planted?

Nutrient uptake is a complex subject and is influenced a lot by the overall health of the plants. I think it would be neat if you could try it both ways and experiment for yourself. There are always too many variables to know certain what method is best for you.

Thanks Cole_Robbie,

I have read that the nutrient concentration in growing lettuce will largely depend also on air temps. The hotter the weather the lower the EC the better since plants transpire more.

Should I try to find out the optimum EC/temps for lettuce whereby the lettuce is drinking with the highest possible EC without it having nutrient toxicity?

You'd be better off using a set concentration and then adding water or nutrient to top off the tank as the liquid diminishes. just check the EC before "watering" and depending on if it has raised or lower, compensate with water or nutrient.