Long story short: The tap water TDS = 400, I have access to RO water which i just tested at 0.00 TDS. I also have access to RO run off which TDS = 44. It will be a hassle of time to use the TDS supply. My factual TDS memory is fuzzy so im asking for some help recalling the facts about TDS. WHen I used to grow my tds was about 200 from faucet. You write that number down. Add nutes. The new number is how much nutes you added. Track TDS over the days and see if it raises or lower from plant intake. My question: I've no clue what the other 400 PPM of TDS is. Aren't plants particular about what they absorbed for the most part? Meaning i should worry because the plants will only intake the good nutes i add to the water. Or should i be worried about what's in the tap water and DEFINITELY use RO water since it's available to me at a cost of 1-2 hours everytime i need to refill? ps - TDS = total dissolve solids. RO = reverse osmosis water.
400 is on the high side, so the RO water (or even the runoff) would probably be preferable. Without some kind of analysis you're right that you can't really know what those 400ppm consist of. The one pitfall I frequently see with RO water is that cal/mag need is higher - I think a good portion of the tap number comes from calcium and you lose that with RO water - you may wind up needing an additional cal/mag supplement so you don't get a deficiency... My tap water comes from a well and usually comes in at around 80-100ppm. I just use that straight up and when I mix my nutrient solution I add 100 to their number to hit the target number. (EG if they suggest 1200 I shoot for 1300 at full strength).
There are a few different ways of expressing conductivity measurements that use different scales. TDS is expressed in ppm but in the 400 scale. PPM meters often use the 700 scale while conductivity measures in uMhos/cm or uSiemens/cm, or EC. A 400 TDS reading is actually 560 ppm on the 700 scale. It's important to know what scale your expected reading should be in. Most nutrient manufacturer's use the 700 scale or EC. If you are using a TDS meter then you'll need to convert the reading to the appropriate scale. But a 560 ppm source water is pretty high.
Ill have to look into my brand of meter at a later time since i do not feel like getting back up lol. My pen had good reviews and is not a bluelabs EC pen.