Question:
Silly question, but can anyone tell me what the relationship is betweenuS/cm and Mohm when measuring the quality of ultra-pure water ?

I've looked through several text books and they all describe either oneor the other, but not how to convert one to the other !

Answer: uS/cm is a measure of electrical conductivity over a specifieddisttance, and megohm is a measure of electrical resistance.The conductivity is the reciprocal of the resistance, but youalso have to define the distance to be able to compare the units.

In general, centimetres are used, and 1uS/cm would be the equivalentelectrical conductance of water with an electrical resistance of1Megohm/cm. Ultra-pure water is usually called 18.3 Megohm/cm or0.055 uS/cm.

the above units were taken from the Millipore brocure that came with ourMilli-Q system, and were also used in the 1982 Millipore catalog I had handy.

I've just looked at the latest catalogues we have at work for water systemsfrom Millipore ( 1997 ), and the section on water purity standards uses thefollowing headings ( p.213-4 )" Electrical Conductivity ( uS/cm at 25C ) " and " Electrical Resistivity ( megohms x cm at 25C ) ".

A check of the latest Barnstead " Laboratory Water Purification Systems "catalogue that we have ( received 1997 ) uses the following for ASTM water" Electrical Conductivity, max., uS/cm at 298K ( 25C ) "" Electrical Resistivity, min. Mohm x cm at 298K ( 25C ) ".It also uses for CAP/NCCLS water" Specific Conductance ( Micromhos, max. ) "" Specific Resistance ( Megohm, min. ) "

No mention of volume, electrode cross-sectional area, or applied voltage, so Iwould suggest that using " cm " is the convention when discussing laboratorywater purification systems. I accept that it may not be completely correct todo so, but I suspect that the specifications include details of the method tobe used, and that will standardise the testing parameters - including thecross-sectional area and applied voltage.

One good thing that did emerge from checking is that the specification forASTM Type 1 water appently now includes a new "plus" category that has a minimum electrical resistivity of 18.2 Mohm x cm at 25C, which I'llcheck out and include in the FAQ.