What is ZERO TDS? Is it real??
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I thought I would comment on the common misnomer "ZERO TDS". Persons stating ZERO TDS I assume are referring to Zero Indicated TDS. What's the difference you ask? Well, quite a bit when you are really talking about ultrapure water. Ultrapure water is about 18.2 mohms or about 0.05 microseimens. Most Reefers are after such ultrapure water in our experience. Thus, if there were such a thing as "ZERO TDS", it might be more appropriately measured around 0.05 micro siemens. Unfortunately, most (not all) inexpensive TDS meters can resolve at best about 5 micro seimens. That is several decades less sensitivity than required to be stating so called "ZERO TDS".
While 5 micro siemens conductivity (at an affordable price and rugged instrument easily operated and maintained) is very reasonable for for break-through detection to signal cartridge replacement required, it is clearly not sufficient to measure or crow about having ZERO TDS. Thus, postings where one has "measured" a system or component performance and state they have "ZERO TDS" should be careful to quantify the accuracy of the instrument they are using to make such a claim. Hopefully this will shed some light on... zero is not always zero.
For most uses, a hand held COM-100 is an excellent instrument, and being portable you can find many places to use the instrument. For in line usage for membrane rejection and Di maintenance, the HM Digital DM-1 dual probe in line meters are more than adequate.
bruce