Many people think that a PPM meter gives a definitive reading of the
purity of water. This is not true. A ppm meter can only detect those
dissolved impurities that raise the electrical conductivity of the
water. So a meter can only determine the ppm of the dissolved NON-
ORGANIC impurities (e.g. dissolved salts) because only these
impurities raise the conductivity. It cannot detect the dissolved
ORGANIC impurities* that may also be in the water (e.g. sugar or
rotted leaf litter). Nor can a ppm meter detect the microscopic solid
particles that may be in the water (i.e. the grit). So a cup of
distilled water that contains a tablespoon of sugar and finely
powdered glass may display a ppm reading of near zero even though its
obviously not pure.
So, when using a ppm meter to measure water purity, you have to know
if the dissolved organic impurities and the fine particles have been
removed along with the salts. The only way to remove all these
impurities is with a high quality reverse osmosis/deionisation system
or with careful steam distillation.
The shortcomings of meters mentioned here should especially be
considered when measuring the purity of rain water. Although the rain
water may initially be pure, it can become significantly contaminated
by organic impurities in the gutters and the tank. Although these
impurities are not detectable by the meter they can have quite a
detrimental effect on colloidal silver production.
(* One qualification to the comment above about the 'non
conductivity' of organic impurities concerns the presence of acid.
Any acid raises the electrical conductivity of water and can induce a
completely meaningless reading on a ppm meter. While organic
impurities usually do not register on a ppm meter, a few drops of,
say, acidic lemon juice in a glass of water will produce readings in
the 100's.)
David
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