Summary for water resistivity table Discussion
Water resistivity is discussed here as the inverse of conductivity, and the thread notes that conductivity is the term more commonly used. In response to Bob Coe’s request for typical values, Peter explains that distilled water ideally has zero conductivity but in practice measures about 1–2 microsiemens. He says rain, river, and well water vary widely by location and can range from about 150 to 6,000 microsiemens, with rainwater usually on the low end. Potable water is usually under 1,500 microsiemens, and “good” water is under 400 microsiemens. Seawater in the Gulf of Mexico is around 40,000 microsiemens. He also notes that discharge water from a typical metal finishing waste pre-treatment plant ranges from 1,500 to 5,000 microsiemens, with better conservation practices tending toward the higher end. He adds a brief clarification that resistivity and conductivity are related but not the same as resistance and conductance.
Q. With regard to water resistivity: Realizing that water resistivity is a function of water purity, can someone provide me with a table of typical values of water resistivities (in ohm-cm) for pure water, potable water, sea water, well water, etc – or any other type of summary of information along these lines.
Thank you!
Bob Coe
– Westminster, Colorado
2003
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
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