On Wed, 2 Jun 2004 07:41:15 +0100, "Ian White, G3SEK"
wrote:

Roger Halstead wrote:
The guys who operated the heat exchangers used to
complain about that highly corrosive liquid... but they were talking
about the water.

De-ionized water wants ions and it'll take them from any available
metal. It's particularly corrosive with brass. We had a series of
induction heating units that used high power RF. The tubes and coils
were cooled with de ionized water. A brass hose barb would crumble in
your hand after only a years exposure to the di water.

Vicious stuff, that water.

The added problem with tube power amplifiers is that there's usually
high voltage around. DI water is a good insulator, but if there are
enough ions in solution the leakage current will cause electrolysis...
which dissolves more metal ions, and then the whole system goes rapidly
downhill.

I designed and built the monitoring system for the plant. to directly
measure "resistivity" in ohms per cubic centimeter. As far as I know,
they are still using the same monitor over 30 years later.

According to an article by N6CA (which was in the ARRL Handbooks for
some years) the secret is to monitor the DC leakage current, and change
the water when the current starts to creep upward. Modern plastic
components help a lot, by minimizing the total area of exposed metal.

You actually either measure or convert to ohms per cubic centimeter.
Typically the water is changed out over time, rather than letting the
resistivity drop and then change. They might dump a hundred gallons
a day, a week or what ever and replace with fresh.

Of course there was the time some one dumped a small salt shaker in
the drain. A spoon full of salt is enough to render many thousands of
gallons out of spec. Actually it shut down that part of the
operation.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com