TDS is Total Dissolved Solids. It's the reading given by the measuring instrument that shows how much nutrient is in solution.

Plants will use more water than nutrient so as water levels drop, TDS will rise and ph will fluctuate until you add more water so it's best to keep water levels constant as possible and adjust ph of the water before adding to the system to match it if it's ok or add ph adjustments if the reservoir needs ph adjustment.

so by doing so i can avoid flushing my system every two weeks ?

i got a PH checker already, what affordable TDS checker do you recommend ?

Some think so and others say to flush. I think if you can adjust you will be fine. The trouble is knowing exactly which nutrients are lacking and being able to replace just those. I think this is why a flush and change are recommended as different plants will remove different amounts of specific nutrients while leaving others so a change will replace all essential nutrients for optimal growth.

EC meters for TDS as far as I know do not show specific nutrient levels but just show the total dissolved solids. You would probably have to have expensive testing done to see exactly which nutrients are needing replaced.

Looks like the chemical companies have us over a barrel eh!

I'm learning too! Can't honestly recommend an affordable TDS checker. Perhaps someone else can. Read threads on EC Meters.

Truncheon is the EC meter to get. ~$100 USD. a bit high but reliable.
I only change solution every 4-6 weeks in veg and fruit cycle. less often with start ups.
Most recommendations of a two week change is for people who won't keep after their ph and EC levels.
you can buy kits to individually test which nutrients remain in solutions. kits to test the full spectrum run about $400 for I believe 30 or so runs.

thank you Charlie and Grizzman

so keeping the Ph and Ec stable will help spacing the nutrient drain.
Just did an ebay lookup, looks like the truncheon is available at $35 with shipping, i'll try that

thanks again

I read in a few places that there is a very good reason to replace the nutrient solution every two weeks. They all stated that (paraphrasing), "Even with a TDS/EC meter, you are only getting a sum of all nutrients. It is possible that the plants have completely consumed a single micro-nutrient and are now suffering from the lack of it. This will not show on the TDS meter."

I don't know if it is true or not though. Just food for thought.

Truncheon for $35? Makes me sick, I paid $135 at the hydro store :)

must be a fake, will let you know (dont want to make you sick though ^^)

Iliketoast
I have read that about dimished nutrients also and that is why you don't run the same nutrient solution indefinitely.
Here's my take on that. Micronutrients are needed in very small quantities, thus the micro in their name. generally, the amounts needed is less than is practically measured, so a surplus is put into the concentrate(for ease of measurement) so you know they're there. Plus as your plant grows you have to add extra nutrient , in addition to more water, to keep your EC correct, thereby replenishing your micro supply too.