Jim and all,
I've spent a lot of time for the past two years with high power liquid-cooled
loads. The globar resistor assembly sounds neat.
The popular high power resistor loads that have water hose connections (Bird
and Altronic at least) use a film resistor on ceramic core, not a bulk
carborundum element.
They run water through the center as well as around the outside of the element.
Altronics makes many of the elements for the other mfrs.
These resistors are made to run with tap water typically, as the R element is
50 ohms and it shunts the effective Z of the package with even slightly
conductive water (tap water).
Distilled or deionized water is tough on end contacts over time, so Altronics
doesn't recommend that practice. I have a 150 kW water cooled load running with
a pulsed amplifier, and it connected
to DI (deionized) loop out of necessity, not preference. I keep the flow very
low through it when it isn't powered, so as to not leave the harsh water in it
and then later flush this slug of water into my loop.
Same loop runs through the plate circuits of two tubes, so needs to be > 2
Megohm-cm resistivity to prevent > 0.5 mA through the fat water hoses.
You should be able to cool your globar load with tap water, but a check with a
network analyzer or antenna analyzer would be prudent. If you don't care about
VHF and above, then it will be easier.
It also helps to check the Z when it is hot, which, in my case, was simulated
with a New Mexico chili pot and a kW heating element along with a small
circulating pump.
Specific Heat of water is 0.997 BTU/lb/degF at 100F. Mineral oil is about 0.44.
50/50 Ethylene glycol/water is 0.855.
For very high power, I cannot use resistor loads period and resort to water
column loads, where the water is doped with a weak ionic solution to make it
appear as a lossy medium, and the length
of the pipe is such that the reflected power at chosen frequency is very low.
These are used for 3.125, 6.125 and 9.375 inch diameter coaxial loads. I've
been testing
QRO for months now at 210 kW average, 2.1 MW peak, pulsed, with about 70 gpm of
ionic solution flowing through the column, 50 deg C outlet temperature!
I wish I could use some of this waste power to heat my home but its a long way
from the work QTH.
73
John
K5PRO
> Date: Tue, 29 Nov 2011 06:05:34 -0800
> From: "Jim Thomson" <jim.thom@telus.net>
> Subject: [Amps] Can distilled water be used for a dummy load ??
> To: <amps@contesting.com>
> Message-ID: <60CEF2A432F54AA8AF17B9B978498671@JimboPC>
> Content-Type: text/plain; charset="iso-8859-1"
>
> 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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