This means that if you wish to grow using an EC of 1.2, you'll have to be very careful and add nutrient concentrate until your truncheon starts to blink between 1.2 and 1.4 with there being a greater number of blinks at the 1.2 number than at the 1.4. If you use another method (other than a truncheon) to figure out your EC, you're on your own!

I tried using 100% pure water from R/O and had to add a LOT of concentrated nutrient to the water to get it to 1.2, but when I compared it to the amount added when I used tap water, it turned out to be nearly the same.

This leads me to believe that water tainted with calcium (as in my case) is actually beneficial to plants’ growth and nurture. Since we KNOW growing plants require more than just water and calcium, it is my conclusion that adding nutrient concentrate to tap water is just a effective as adding it to pure water with the exception that when using tap water, there is the ADDITIONAL BENEFIT of extra calcium.

Tomatoes, by the way, love a high calcium level.

Hope this helps someone.

I would just like to add that it's beneficial to take an EC reading of your tap water and ADD that value to your target EC. Example: If tap water EC=0.4 and your target=1.2, then your new target will be 1.6.
That way you actually add the tap water minerals (Ca and Mg) to your hydro nutrients.

I make up a huge batch of nutrient when I make it (like about 30 gallons!). Here's what I did:

First, I used tap water and added nutrient concentrate (home-made from compost tea using anerobically-digested chicken manure, which is brewed using tap water also). I filled the big plastic tub I use to store the stuff in with tap water and added nutrient concentrate until the EC reading was between 1.2 and 1.4 measuring things with a Bluelab Truncheon and assuring that the 1.2 light blinked more than the 1.4, indicating the reading is closer to 1.2 than 1.4.

I used up all but about a gallon of this batch and made up another only this batch used R/O water for the concentrate brew as well as the total mixture brew. I drew off 2 gallons of R/O every day from my home R/O unit to enable my having enough water to do it. I stored the water in a plastic trash can with a poly-bag liner and the lid on.

I noticed that when the EC reading was finally at 1.2, having started at zero conductivity, I had added the same amount per gallon of nutrient concentrate (1200 ml / gallon) as when I was using the tap water.

My storage bin has a fairly-large fish tank pump and a 1-foot-long air stone, which sits at the bottom and blows bubbles through the stuff 24/7. Ii do this to give my plants grown in static containers filled with perlite a little oxygen "boost" when I replenish their reservoir puddles as they transpire the water inside their 10" x 20" plastic trays sitting on the concrete floor of my greenhouse.

I've only done one batch each way, so maybe before you go whole-hog on this, either wait until I do more research or do your own using smaller water volumes. I dunno.

Actually, I intend on using tap water from here on in, so I can get the added benefit of the dissolved calcium and magnesium, which the R/O trashes.

I was as amazed as you are that each type water took approximately the same amount of nutrient concentrate. Maybe it's magic!