On 8 September 2011 10:36, cdb wrote: > =A0I've been looking at the specifications for a domestic water heater an= d in > the sales bumph they specify the minimum water resistivity required (2K79= 5 > x inches or 1K1 x cm's). I've never seen such a specification before for > this kind of water heater nor the unit resistivity per area/length. I'm assuming this is to ensure shorts don't occur across the heating coil. Most manufacturers will probably just assume this is safe. > I can find no documents on the interrent apart from those referring to > measurement devices for analytical purposes. Electrical resistivity (or conductivity^-1) varies as a function of salt concentration and temperature--two things that are not absolute in domestic water supplies. NB: I refer to "salt" here as any ionic salt, not just table salt (NaCl). I found some information at http://www.fao.org/docrep/T0667E/t0667e05.htm concerning the conductivity of water at various concentrations of NaCl at 25 deg C. Regards, Brendan --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .