PH diifferences within flood table??

Ok people I am mystified. I am using an ebb/flood table that is 6' long and 2.5' wide. I've got about 100 liters of hydroton in it. It takes about 10-12 gallons of water to fill it to the point where it almost overflows. I keep 20 gallons in my rez. 6 plants in net cups in the table.

Now... When I fill the table with water and take PH readings in the water/hydroton I get 5.8 (my rez PH) on the downhill portion of the table (where the nutrients flow in), about 6.1 in the middle of the table and 7.1 !!!!! in the last foot or so of the table. (the upper part i.e. last to fill, first to drain). Or the life of me, I can't figure it out.


The water in this part of the table (upper) is quite a bit warmer than the other portions as when the water enters the table it is filling the lower portion first and I believe that as the water starts flowing tot he upper portion of the table its flowing through more hydroton that has been warmed to room temperature resulting in warm water.


So I thought that perhaps its the temperature of the water that is spiking ph and maybe I need a temp compensating meter. Did some reading however and the PH of a nutrient solution CHANGES with the temperature so it doesn't matter if your probe has temp compensation as it is reading the true PH. and to my dismay, with increasing temps, PH should go DOWN not up.


Any thoughts? are salts or something accumulating in the higher portions of my rez?

BTW. here is a very excellent article on PH ....
http://www.growersunderground.com/pH_Manifesto.pdf

did you prewash your hydroton? Though I find your situation a bit out of the ordinary, this could explain it.

I originally did not prewash but I subsequently pulled the majority of it out and did soak it in 5.5 solution. Took a day and a half and LOTS of ph down to keep it around 5.5

It is quite strange.

you dont need to fill a whole table with hydroton. just put those sob's in some rockwool cubes.

Or you can just fill up square pots with hydroton and put your plants in there. Then you can move things around.

Anyways... I dont know if its been said yet. But you need a bigger reservoir for a flood table that big. I used 20 gallons back when I was running a little 2'x3' tray.
You are going to want like 50 gallons. That'll give it more buffering capacity and you shouldn't have to worry about pH as much.

also.
you gotta wash your hydroton!!!!! I use hot water, but I've read about people soaking it in vinegar.
I'm gonna try soaking with vinegar then rinsing with water next time around.

As an informational update; I installed another inlet on the flood table on the high side of the table where my ph problems were. Problem solved. I think it was a combination of both the hydroton being continually soaked in proper ph solution and filling the entire table at once, not just from one end.