This is not for your tomatoes :P but it might work for your herb garden, or other plants that use less than 1000ppm
I found this as I was looking around for float valves lol... Speaking of if anyone happens to know where to get some for dirt cheap for me to install in my res please let me know and perhaps we can work something out.
This is a 2 site inline TDS meter that reads up to 999ppm at 1ppm accuracy.
I think it is impressive for it's price, though it has limited uses.. It can easy find a place for low nut tolerant plants of your nersery plants.
dual inline TDS meter
Not something everyone can use but for the price it is pretty impressive.