TDS Meter Calibration

[mike31154]

Since I have both a handheld device and a dual TDS meter directly hooked into my RO/DI system, I decided to do a little comparison test to see what my tap water reading is. The result was a little surprising and not too sure what to do about it now.

The handheld meter (cheapo from WalMart) showed close to 220. I've always been a tiny bit skeptical of this one due to its low price and the fact that the longer I keep the thing immersed, the more the TDS tends to creep up. It goes up very slowly mind you, but it does rise. It has and auto shutoff feature designed to conserve the battery so I'll sometimes turn it back on several times, always with the result that the TDS rises very slowly. Only have the patience for about 5 minutes of this, but I've seen it go from 216 to above 220 doing this.

First time today I decided to pop both probes of the Dual Inline TDS meter on the RO/DI out of the T fittings and test the same glass of water I just tested with the handheld. Both the IN and OUT probes were virtually identical reading 290, a slight fluctuation to 291, back down to 290 and then rock steady, no change.

Only difference other than price that is obvious between the handheld and Dual Inline is that the handheld actually has a visible temperature compensation probe right along side the two metal measuring probes. No idea if the Dual has any kind of internal temperature compensation, but I doubt it since the temperature probe would only be effective if immersed as is the case with the handheld.

Anyone have similar experience or thoughts on this phenom, other than 'chuck the cheapo handheld!'?? If anyone in Vernon on the Kal Lake water source could provide their tap water TDS readings that would be awesome. Thanks in advance.