Dear Jim,
pure water will take metal ions of all metal surfaces it come in contact with,
to build a solution, so it will corrode metal!
I have repaired professional dummy loads a few years ago:
I do not see any reason for more than a gallon of oil for a 1-2kW dummy.
I would use a cars heat exchanger from a cars heating system, a small
pump and a fan to cool the oil continously. So one could build a small
1-2kW-dummy for 100% duty cycle. Such a small system can be housed
in a second case, protecting the concrete floor...
Purified water and oil have a HIGH relative dielectric factor. Therefore the
distance between the RF-film resistor and the wall of the conductive container
which encloses the resistor and the oil has to be relatively W I D E.
Otherwise the high stray capacity will cause a bad VSWR of your dummy
on the higher bands
73 de Andy DH5AK
----- Original Nachricht ----
Von: Jim Thomson <jim.thom@telus.net>
An: amps@contesting.com
Datum: 29.11.2011 15:05
Betreff: [Amps] Can distilled water be used for a dummy load ??
> My hb 50 ohm dummy load consists of 4 x 200 ohm globars in parallel
> [type SP, glass bodied
> and suitable for oil immersion]. Each resistor is 1" diam x 12" long.
> [275 W CCS in air]
> Each resistor resides inside it's own 2.875" OD x sched 40 Aluminum
> tube.
> All 4 x thick walled AL tubes are heliarc welded.
>
> I heard some place that instead of xfmr oil, that distilled water can be
> used instead?
> The same glass bodied type SP resistors, [50 ohm, high wattage] are used
> in water cooled loads,
> the type that has the garden hose on one end, etc.
>
> Seems to me that distilled water will conduct heat a lot better than xfmr
> oil. The question is,
> will the use of distilled water in a 7 gallon metal container, with
> resistors immersed present any
> swr problems ? If I remember correctly, swr was high on heath cantenna
> loads if no oil used at all,
> and dropped to dead flat once the oil was poured in. There was also a
> similar 50 ohm load in the
> old arrl books, same resistor as I'm using but it was a single 50 ohm unit,
> and not 4 x 200 ohms in parallel.
>
> I would prefer not to use xfmr oil if at all possible. It's a little
> tougher to get a hold of these days..and not cheap.
> My main concern however is something leaks. Water on the concrete floor in
> the shop is one thing, but 7 x
> gals of xfmr oil is a disaster. Can you add rust inhibitor's or glycol
> to the distilled water ??
> Do I even have to use distilled water, or will clean tap water work ?
>
> A 50/50 mix of water/glycol will result in a 265 deg F boiling point..BUT
> the heat transfer is no
> where as good. I'm using a new 7 x gal paint can for this project, the 4 x
> resistors and the welded AL
> tube assy, hang from the top lid.
>
> Thanks... Jim VE7RF
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