A TDS tester is the small handheld pen that reads a number when dipped in water, and what it actually measures is electrical conductivity, which it then converts to an estimate of total dissolved solids. That conversion is the key to using one sensibly, because it means the reading responds to every dissolved ion without distinguishing among them.
How the pen works
Two electrodes at the tip pass a small current through the water and measure how readily it flows. Dissolved ions carry current, so the more of them there are, the higher the conductivity. The meter multiplies that by a conversion factor to display a dissolved solids figure. Different manufacturers use slightly different factors, which is why two pens can disagree on the same sample, and why comparing readings between meters is less meaningful than tracking one meter over time.
The one job it does well
Checking a reverse osmosis membrane. Read the feed water, read the permeate, and the difference expressed as a proportion is the rejection rate, which is the single best indicator of membrane condition. Take a baseline at commissioning and check periodically, because a falling rejection rate reveals a failing or bypassing membrane long before the water tastes different. This is what these meters are for, it is genuinely useful, and it costs very little to do.
What it cannot tell you
What the dissolved solids actually are. Beneficial minerals count exactly as much as unwanted ones, so a mineral rich spring water reads high while being entirely desirable. It cannot give hardness, since the USGS defines that as calcium and magnesium specifically and a conductivity meter cannot isolate two ions from the rest. Softened water reads much the same as it did untreated, because sodium replaced calcium ion for ion, which regularly convinces owners a working softener has failed.
Using it properly
Rinse the tip in the sample before reading, so residue from the last measurement does not skew it. Immerse only to the marked depth. Let the reading settle rather than taking the first number. Temperature affects conductivity, so use a meter with temperature compensation or compare only samples at similar temperature. Calibrate against a known solution occasionally, since these drift. And treat a surprising result as a reason to commission a real analysis. Our softener cost guide covers what the hardness figure decides.
Questions people ask about what is a tds tester
What does a TDS tester actually measure?
Electrical conductivity, converted to an estimated dissolved solids figure. It responds to every dissolved ion without distinguishing them.
What should I use one for?
Checking reverse osmosis membrane health by comparing feed and permeate readings to get the rejection rate, tracked over time.
Why do two meters give different numbers?
Manufacturers use different conversion factors from conductivity to dissolved solids. Track one meter rather than comparing between them.
Can it tell me if my softener is working?
No. Sodium replaces calcium ion for ion, so conductivity barely changes and softened water reads much as it did untreated.