Water Knowledge

Resistivity

Summary: Resistivity tells you how strongly water resists the flow of electricity. Purity (fewer dissolved ions) means higher resistivity; more dissolved salts/minerals means lower resistivity. It’s the inverse of conductivity (EC) and is useful for tracking RO/DI performance and diagnosing corrosion/scale tendencies when paired with pH, alkalinity, hardness, and TDS.

Why homeowners should care

Resistivity is a quick purity check. If you use reverse osmosis (RO) or deionization (DI) for drinking water, aquariums, humidifiers, or specialty appliances, resistivity shows whether the system is producing low-ion water. Changes in resistivity can also hint at rising salinity (chloride/sodium) or treatment issues.

What resistivity really means

  • Units: megohm-centimeter (MΩ·cm) or kilohm-centimeter (kΩ·cm). High numbers = purer water.
  • Inverse of EC: Resistivity = 1 / conductivity. As EC rises (more ions), resistivity falls.
  • What drives it: Dissolved ions (chloride, sodium, sulfate, calcium, bicarbonate), temperature, and CO2 dissolution. RO/DI removes ions and raises resistivity.

When to test

  • Baseline your home’s water chemistry (pair with EC/TDS).
  • Before and after installing or servicing RO/DI systems.
  • When taste gets salty/flat or scale and corrosion change suddenly.
  • For aquariums/coffee/ice where consistent water quality matters.

How we test

HealthWaterLab measures resistivity and conductivity with calibrated lab probes, and can pair results with TDS, chloride, sodium, sulfate, hardness, alkalinity, and pH to explain what’s driving the number. For quick checks at home, many users rely on inline or handheld meters at the RO/DI tap.

How to collect a good sample (or meter reading)

  • Use the kitchen cold-water tap (or RO/DI faucet if checking treated water).
  • Run water long enough to get a fresh, stable sample; avoid touching the inside of bottles/caps.
  • For meters: rinse the cell with sample water, avoid air bubbles on the electrodes, and allow temperature to stabilize.
  • Record location, date/time, temperature, and whether softening or RO/DI is upstream.

How to read your result

  • High resistivity (e.g., ≥ 0.5–1.0 MΩ·cm at a home RO/DI tap): Very low ions—great for coffee/ice and many aquariums. Taste is clean; may seem “flat.”
  • Moderate resistivity: Typical of many groundwater supplies; taste usually fine.
  • Low resistivity: More dissolved salts—taste may be salty/flat, and scale risk rises with hardness. Consider RO for drinking/cooking and manage scale house-wide.

Practical homeowner steps

  • Trend it: Log resistivity with EC/TDS and temperature to spot seasonal or treatment changes.
  • Find the drivers: If resistivity falls, test chloride, sodium, sulfate, hardness, and alkalinity to see what’s increasing.
  • Protect appliances: Use RO for drinking/ice; pair with softening to reduce scale in heaters and dishwashers.
  • Maintain RO/DI: Replace carbon prefilters (protect membranes from chlorine), change RO membranes/DI resin when resistivity drops or TDS rises.

FAQ

How is resistivity different from conductivity and TDS?

Resistivity is the inverse of conductivity; both reflect total ions. TDS is a related mass estimate (ppm). None of these identify which ions—lab tests do.

What resistivity is “good” for home RO/DI water?

Higher is better. Many home RO systems produce low-TDS water that corresponds to hundreds of kΩ·cm up to ~1 MΩ·cm. Full DI can reach several MΩ·cm. Track your own baseline and watch for drops.

Why did my resistivity drop after a few months?

RO membranes and DI resin exhaust over time; carbon prefilters may be overdue. Rising salts from the source (e.g., winter road salt) can also lower resistivity.

Does temperature affect resistivity?

Yes. Warmer water lowers resistivity (raises conductivity). Many meters auto-compensate, but note temperature when trending.

Will a softener raise resistivity?

No. Softeners swap ions (hardness for sodium/potassium), leaving total ions similar. Resistivity changes little. RO/DI is what raises resistivity.

Can carbon filters change resistivity?

Not much. Carbon removes chlorine/organics but doesn’t remove salts. Use RO/DI to meaningfully raise resistivity (lower ions).

Why does my RO faucet show low resistivity right after I open it?

First water in the line may mix with storage-tank water or pick up ions from tubing. Let it run briefly; resistivity typically rises to the normal level.

Is high-resistivity water safe to drink?

Yes—RO/DI water is commonly used for drinking and cooking. Some people prefer a little mineral taste; a small remineralization cartridge can add it back.

Do I need lab resistivity or is a handheld meter enough?

For routine home checks, handheld/inline meters are fine. Use lab testing when you need to identify which ions are changing or to calibrate meters.

How often should I check resistivity?

Monthly for RO/DI maintenance, seasonally for private wells, and whenever taste/scale changes or after servicing filters/membranes.

Want help interpreting resistivity with EC/TDS? See kits and step-by-step guidance at HealthWaterLab.com.