The inlines are not truly temperature compensated so can be significantly off unless your water temperature and air temperature are exactly the same. What they don't tell you is there is no temp probe in the water, its a little metal piece inside the fat part of the probe behind the little rectangular window or opening so it sees only air temperature not water temperature. How often are they the same?
A good handheld like the HM Digital TDS-3, TDS-4TM, COM-100, AP-1 or AP-2 will run circles around their own dual inlines since they are all ATC, automatic temperature compensated. I have two dual inlines and never even turn them on anymore, I use the COM-100 exclusively.
The other issue with the inline is it is dedicated to two spots but you need 3 TDS readings to troubleshoot your RO/DI system. You need tap water TDS, RO only TDS before the DI and final RO/DI TDS. The first two are used to calculate your RO rejection rate which is critical since the RO does 90-98% of the work and the DI is only a final polishing of what it misses. Pay attention to the tap and RO only too so you know how your membrane is doing.
