One issue with the dual inlines is they are not truly temperature compensated so can have issues with accuracy. Unless your air temperature and water temperaure are exactly the same they will never be accurate since their probes sense air not water. I much prefer a good handheld from the same manufacturer, HM Digital, such as the TDS-3, TDS-4TM or AP-1. All are around $25, are ATC temperature compensated and all have a built in digital thermometer function so serve dual purpose. Best of all, for a RO/DI you need to know 3 TDS measurements not just two so the handheld can get them all while the inline is limited to two points. You need to know the tap TDS and RO only TDS to calculate the membrane rejection rate or removal efficiency, the condition of the RO membrane, and the RO/DI TDS to monitor the DI resin condition. With the handheld you can also test your ATO storage, bottled water, LFS water, your buddies water etc. Very versatile and very accurate.
I have two of the dual inlines and never evene turn them on anymore unless I am looking for a rough guess, they never agree with either my COM-100 or AP-1 handhelds which I trust.
