Using Your TDS Tester

This is a simple but very reliable tester that measures the Total Dissolved Solids (TDS) in water. It gives a reading in parts per million (ppm) of all the dissolved solids, good or bad. Dissolved solids in water are mainly its mineral content. If your tap water, for example, has a TDS reading of 250 parts per million, that means that the total content of the minerals dissolved in the water make up 250 parts per million of the total volume.

For practical purposes, TDS measurement is a very effective way to test you reverse osmosis unit’s performance.

Measure the TDS of your tap water, then measure the product water of your reverse osmosis unit for comparison. The RO water should be about 1/10 or less the reading of the tap water. In other words, if the tap water reads 250, the reverse osmosis water should read around 25 or less. What you are measuring is the performance of the reverse osmosis membrane. As long as you get a respectable TDS reading, the membrane does not need to be replaced. Typically, reverse osmosis membranes last three years or longer.

The reading you get with the TDS tester has nothing to do with the carbon filters. They are replaced periodically, and there is no way to test their performance.  The only exception to this is that if the prefilter of the RO unit is so clogged that it is restricting flow to the membrane,  TDS performance of the membrane will be improved by changing the prefilter.  The membrane will perform better if it gets better pressure.

Follow the instructions that come with the tester, but generally you can disregard the section on Calibration. For general use purposes, calibration is unnecessary.

The number one tip I can give you as an experienced user of TDS testers is, don’t forget to turn it off. It should last forever if you don’t forget to turn it off. Should you need to replace the batteries, you can usually find hearing aid batteries that will work, or you can get them from Pure Water Products. Some testers now come with an auto-shutoff feature that turns the tester off if you forget to.  This is a great feature to have.

Important: If you are testing a new or unit, always wait until it has run for a few days before testing. It takes quite some time for the newness to wear off the system. The meter can "see" particles that you eye can't and you'll get an unreliable reading unless you allow the unit to break in.

If you are servicing the unit, take your test before changing the carbon filters. New carbon filters will corrupt the reading considerably and the reading will be much too high. The meter can "see" the manufacturing residue from the carbon filters.

To operate:

1. Remove the cap.

2. Turn the tester on.

3. Insert the tip of the tester (the end where the cap was) into the water. A half inch or so is plenty. You'll ruin the meter if you submerge it too deeply.

4. Read the numbers on the display.

Some testers have special features, such as temperature tests and a "hold" button.  The literature that comes with the tester will explain.