[Myka]

I like an inline TDS meter in one place - between the RO membrane and the DI resin so that I know how much TDS is going to the DI. This is a luxury though, and not needed. I like to use a handheld meter for product water. Make sure that any TDS meter that you buy is either self-calibrating when you dip it in calibration solution or it has manual calibration. All TDS meters are "factory calibrated" but do not buy a TDS meter that you can't calibrate afterwards because the calibration will wander off.

IF you buy a dual TDS meter, spend the extra few bucks to get the HM Digital DM2 as it has dual calibration where the DM1 has only one calibration screw for both probes which often do not read the same, so it is impossible to calibrate both probes properly. Or make sure whichever brand it is that if it has dual probes it also has dual calibration. The ones Greg linked to above are both the DM1.

I have fond that I can't rely solely on the color of color changing DI resin because I found the outside of the DI resin would all change color and I would dump it out to find the core of the DI resin still had plenty of color! So now when the color on the outside is all changed that's my red flag and I start testing product water until it shows 1 ppm, and then I change the DI resin. In my case, the DI resin often last another 4 months or so after the outside portion has changed color.

What is the TDS of your tap water? Phosphate level? Chlorine/chloramines?