TDS, EC and temperature: what the numbers mean (and what they don't)
October 25, 2026 · 4 min read
The AquaScanPro pen has three readings. Each one describes a general property of the sample. None of them, alone or together, tells you whether water is safe to drink, and none of them identifies a specific substance.
EC: electrical conductivity
EC is how easily the sample carries an electrical current, shown in microsiemens per centimeter (µS/cm). Pure water conducts very little. Dissolved mineral ions, such as calcium, magnesium, sodium or bicarbonate, make it conduct more. EC is the actual measurement the pen makes.
What it doesn't see: substances that don't carry a charge, which includes many organic compounds, and anything that isn't dissolved, like bacteria or particles.
TDS: total dissolved solids
TDS, shown in parts per million (ppm), is an estimate of dissolved solids, calculated from conductivity with a conversion factor. That's why two different meters can show different TDS numbers for the same glass, and why it's worth comparing readings from the same pen.
A TDS number is not a list of what's in the water. A low number doesn't prove water is clean or safe, and a higher number isn't automatically bad: much of it is often ordinary minerals.
Temperature
The pen also shows the sample temperature, in °C or °F. Conductivity changes with temperature, so writing it down with each reading helps you compare one day with another fairly.
Units matter
Always write down which mode you used. A 120 ppm TDS reading and a 120 µS/cm EC reading are not the same thing. Compare ppm with ppm and µS/cm with µS/cm.
What these readings can't tell you
- Whether water is safe to drink
- Whether lead, bacteria, PFAS, pesticides, nitrates or other specific contaminants are present
- When a filter needs replacing
For those questions, follow local water advisories and use appropriate laboratory testing. Your utility's annual water quality report (in the US, the Consumer Confidence Report) is a good place to start.