From our FAQ's:
In this hobby we measure Total Dissolved Solids in parts per million, or "ppm." We often try to measure TDS down near 0 ppm. Because this TDS level is so low, we have to keep in mind the sensitivity of the meter used to measure it, and the technique used to measure the tds.
Nearly any contamination in the sample container will cause an erroneous TDS measurement. Some plastic containers are difficult to get absolutely clean, and although they appear clean, they are not. An easy standard approach is to use a drinking glass as a sample container - use one right out of the dishwasher. Obviously, keep your fingers away from the inside surface of the glass.
Calibrate your meter. Use a calibration fluid generally in the range of the tds measurements you'll be taking. Some meters require a specific tds calibration fluid (e.g., 800 ppm), regardless of the tds levels in your samples.
Be careful with how you take your samples. Let's say you intend to measure the TDS in your 1) DI water, 2) RO water, and 3) tap water. Start with the cleanest of the three - the DI water. After letting the system run for a sufficient period of time that you are sure the tds levels have stabilized (to assure you are not measuring tds creep water), rinse the sample container two or three times with the water you intend to sample, and then fill the sample container with sufficient DI water to take a reading.
Now on to the RO water. The water we are interested in here is the permeate – i.e., the water that has been purified by the RO membrane – not the waste water. Make sure you understand which is which before taking the sample. Most RODI systems other than Buckeye Systems are not plumbed to facilitate taking a sample of the permeate. If that is the case, you’ll need to unhook some tubing – likely where the tubing attaches to the “in” port on the DI housing in order to take this sample. This is inconvenient for many people, and we find that people never do it. They report only the tap water TDS and the DI water TDS. Contact Buckeye if you need guidance regarding installing a couple of extra fittings and tubing to facilitate measuring the TDS of the RO water (permeate). When you take the sample, follow the same procedure described above – use a clean sample container, assure you are not measuring TDS creep water, rinse with the permeate several times before taking the sample, and use a calibrated meter.
Use the same approach to collect and measure your tap water as well.
Russ
