Anyone with an RODI unit needs a TDS meter to determine if and when their filters need to be changed, and if the unit is working properly. I got my TDS meter off of Ebay probably about 5 years ago for . It dawned on me last week that I have never bothered to calibrate it! I should know better -\- when I use them for work I have to calibrate them before and after every job. \r\n\r\nI bought the correct NaCl calibration fluid off of Ebay (.95).\r\n[url=http://cgi.ebay.com/HM-Digital-342ppm-TDS-NaCl-Calibration-Solution-1x3oz_W0QQitemZ370311918054QQcmdZViewItemQQptZSmall_Kitchen_Appliances_US?hash=item56384f71e6]HM Digital 342ppm TDS/NaCl Calibration Solution, 1x3oz - eBay (item 370311918054 end time Feb-24-10 18:04:21 PST)[/url]\r\n\r\nMy TDS was off by about 30 ppm -\- luckily it was 30 ppm too high, not too low, so my water was actually cleaner than I had been thinking.\r\n\r\nI haven't changed my RODI filters in nearly two years, and the output from it is still less than 10 ppm. Yay! \r\n\r\nDon't forget to calibrate all your instruments!