Table of Water Resistivities

A. Resistivity was formerly measured in mhos, or "inverse ohms" but uses the term "Siemen" under the international system. It is usually called "conductivity" rather than resistivity. Distilled water ideally has a conductivity of zero, but practically has one or two microsiemens. Rain water, river water, and well water vary wildly depending on location, distance from headwaters, etc. You will find anything from 150 to 6,000, depending on location, although rainwater is usually at the low end. Potable water is usually less than 1500, and "good" water is less than 400 microsiemens. Seawater (Gulf of Mexico tidal)is around 40,000. Discharge water from a typical waste pre-treatment plant in a metal finishing shop ranges from 1,500 to 5,000, with plants using good water conservation practices toward the high end.

Paul Morkovsky
- Shiner, Texas, USA

A. Resistivity and conductivity are not the same thing, in fact they are opposites. 1 divided by resistivity gives you conductivity.

The mho (much more descriptive) or Siemen is the unit of conductivity.

Peter Hucker
- Scotland
July 26, 2023

Thanks Peter. But resistivity is not quite the same thing as resistance, so conductivitiy is not quite the same thing as conductance. The resistivity or conductivity of a material is a function of what kind of metal it is, such as copper, silver, steel, etc., and you can look up the resistivity of a material in a chart. But the resistance of a wire also depends on its length and cross sectional area.

R = r * L / A
ohms = ohm-cm * cm / cm²

And so, likewise:

C = c * A / L
mhos = mho/cm * cm² / cm

So the conductivity of water is often expressed in microsiemens per cm.
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey